Synchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation. These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields. This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments. Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters. Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated. In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation. Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken. The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy. The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses. In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations. A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields.

The role of cross field transport and volume recombination are of vital importance for a satisfactory understanding of the plasma edge in magnetically confined devices such as a Tokamak. Plasma fluctuations may travel cross field with significant velocities and play a central role in plasma transport. Cross field transport has been seen to be anomalous in most devices under a very broad range of experimental conditions. In recent years a clear indication of the relation between fluctuation, cross field particle transport and recombination has been reported.The University of Manchester Linear System (the ULS) has been used to observe the Balmer emission of the recombining plasma interacting with a dense neutral Hydrogen gas. The ULS is a device made of a cylindrical vacuum vessel 1.5 m long and 15 cm in radius. The plasma is formed in a separate chamber by a duoplasmatron source in the Demirkhanov configuration; the arc current was limited to 15 A and the potential drop was 100 V. The device is surrounded by a linear solenoid which was used to magnetize the plasma. The highest magnetic field was .1 T. Typical electron temperature in the device spans .1 to 10 eV, and the density 1. E+16 to 5. E+19.Diagnostic includes Langmuir probe and visible spectrometers. In addition, the DivCam imaging system originally designed and built to obtain 2D images of the MAST spherical Tokamak Scrape Off Layer, was used. The DivCam imaging system has enabled to obtain high resolution images of the plasma emission when interacting with the neutral gas. It appears evident that the Electron-Ion Recombination is strongly dependent upon radial transport of plasma particles: light emission attributed to EIR is only observed at a large cross field distance from the plasma source. Moreover, fast imaging of the plasma has also shown the presence of a plasma filament forming and propagating crossfield at the same region of the plasma where the EIR light is observed.To interpret the experimental observations obtained with DivCam, the OSM 1D fluid plasma solver and the EIRENE neutral Monte Carlo solver have been implemented in the linear geometry of the ULS linear system. Both the OSM and the EIRENE solvers were originally intended for tokamak and large magnetic confinement devices. Modelling of the EIR emissivity in the ULS device has demonstrated the importance of the inclusion of turbulent and blob transport in the model to obtain reasonable agreement between the observations and the theoretical predictions. The central density of the plasma filament has been estimated to be approximately .7 E+19 m-3 using EIRENE results.The emission attributed to hydrogenic ions (negative atomic H- and positive molecular ions H2+) and related to Molecular Assisted Recombinations can be estimated within EIRENE using the AMJUEL database. The database provides ion population estimations for three different collisional regimes: in the first regime a large population of vibrational excited hydrogen molecules are assumed to exist within the plasma volume; the second assumes strong Charge Exchange reactions and not vibrational excited molecule; the third assumes electron impact collisions with ground states molecule to be the only ion source. A reasonable agreement between the observations and the EIRENE prediction is only found when using the third estimation suggesting that molecular excitation and charge exchange processes are relatively unimportant under the experimental conditions considered.

Synchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation. These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields. This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments. Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters. Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated. In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation. Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken. The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy. The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses. In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations. A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields.

Durch die Einführung von Synchrozyklotronen und Laser-Teilchenbeschleunigern, entwickelt mit dem Ziel günstigere und kompaktere Protonentherapieanlagen bereitzustellen, werden stark gepulste Strahlenfelder möglicherweise Anwendung in der Teletherapie finden. Darüber hinaus bergen stark gepulste Strahlenfelder das Potential klinischer Vorteile durch eine bessere Schonung gesunden Gewebes oder die verbesserte Behandlung bewegter Tumore. Allerdings ergeben sich neue Herausforderungen im Bereich der Dosimetrie, der Grundlage für eine präzise therapeutische Anwendung ionisierender Strahlung. Diese Herausforderungen betreffen sowohl den Bereich der klinischen Dosimetrie für die unmittelbare Strahlenanwendung als auch die Strahlenschutzdosimetrie zum Schutz von Umwelt und Personal. Luftgefüllte Ionisationskammern, die primären Messinstrumente der klinischen Dosimetrie, sind von einem zunehmenden Signalverlust aufgrund von Volumenrekombination betroffen, da stark gepulste Strahlenfelder eine hohe Ionisationsdichte innerhalb eines sehr kurzen Zeitraums erzeugen. Beschreibungen für diese Effekte sind zwar gut etabliert für die moderat gepulsten Felder im gegenwärtigen klinischen Einsatz (Boags Theorie), allerdings sind die dafür nötigen Näherung höchst wahrscheinlich unzureichend für die stark gepulsten Strahlenfelder zukünftiger Beschleuniger. Ferner sind Dosisleistungsmessgeräte, welche im Strahlenschutz als fest installierte oder mobile Überwachungsdosimeter eingesetzt werden, nur für kontinuierliche Strahlenfelder geprüft und bauartzugelassen, was Zweifel an ihrer Eignung für die Messung gepulster Felder eröffnet. In dieser Arbeit wurden beide Bereiche der Dosimetrie, sowohl Strahlenschutz als auch klinische Dosimetrie, untersucht, um die medizinische Anwendung stark gepulster Strahlung zu ermöglichen. Für ein möglichst umfassendes Verständnis wurden dabei experimentelle Untersuchungen mit theoretischen Überlegungen und Entwicklungen verzahnt. Mit dem ELBE-Forschungsbeschleuniger wurde ein gepulster 20 MeV Elektronenstrahl und somit ein gepulstes Strahlungsfeld erzeugt, welches eine systematische Untersuchung in einem großen Bereich in Bezug auf Pulsdosis und Pulsdauer erlaubte. Ionisationskammern für den klinischen Einsatz wurden mit diesem Elektronenstrahl direkt bestrahlt und ein Faraday-Becher diente als unabhängige Referenzmessung. Dosisleistungsmessgeräte hingegen wurden im, durch den Elektronenstrahl im Faraday-Becher erzeugten, Bremsstrahlungsfeld bestrahlt. Dabei fungierte die Ionisationskammer vor dem Faraday-Becher als Strahlmonitor und diente zur Bestimmung der Referenzdosis des Bremsstrahlungsfeldes über eine Querkalibrierung mit Thermolumineszenzdosimetern. Es wurden drei Dosisleistungsmessgeräte basierend auf unterschiedlichen Messprinzipien untersucht, die damit einen großen Teil der im Strahlenschutz eingesetzten Messprinzipien abdecken: Die Ionisationskammer RamION, das Proportionalzählrohr LB1236-H10 und der Szintillationsdetektor AD-b. Für die klinische Dosimetrie wurden zwei verbreitete Ionisationskammergeometrien untersucht: die Advanced Markus Kammer als Flachkammer und die PinPoint Kammer als Kompaktkammer. Zusätzlich zu der üblichen Luftfüllung wurde außerdem eine Füllung mit reinem Stickstoff und zwei Flüssigionisationskammern mit Isooctan und Tetramethylsilan untersucht. Ferner wurde eine numerische Berechnung der Volumenrekombination in Ionisationskammern durch die Beschreibung der Prozesse von Ladungsfreisetzung, Ladungstransport und Reaktion entwickelt, um eine Beschreibung zu erhalten, die ohne die für Boags Theorie notwendigen Näherungen auskommt. Insbesondere berücksichtigt diese Berechnung den Einfluss der freigesetzten Ladungen auf das elektrische Feld, der in Boags Theorie vernachlässigt wird. Von den drei untersuchten Dosisleistungsmessgeräten zeigte nur das RamION Messungen innerhalb der gegebenen Toleranzen in den untersuchten Strahlungsfeldern. Die unerwartet schlechte Präzision des AD-b Szintillationsdetektors, der keinen prinzipiellen Beschränkungen in gepulsten Feldern unterliegen sollte, wurde auf die Signalverarbeitung im Messgerät zurückgeführt, welche das prinzipielle Problem einer unbekannten Signalverarbeitung in kommerziellen Geräten hervorhebt. Das LB 1236-H10 Proportionalzählrohr andererseits maß den Erwartungen entsprechend. Dies unterstützt zwar die in DIN IEC/TS 62743 dargelegten Erwartungen für zählende Dosimeter, zeigt allerdings zugleich die allgemeine Unzulänglichkeit solcher Instrumente für die Messung stark gepulster Felder und demonstriert die Notwendigkeit für weitere normative Bestrebungen, um einheitliche Bedingungen für die Untersuchung nicht-zählender Dosimeter (wie das RamION) zu schaffen. Durch die Aufnahme dieser Ergebnisse in die Literatur der Strahlenschutzkommission wurde hier der Grundstein für eine solche Entwicklung gelegt. Die Untersuchung der Ionisationskammern für klinische Dosimetrie zeigte z.T. starke Abweichungen zwischen Boags Theorie und experimentellen Beobachtungen. Boags Theorie beschreibt Volumenrekombination hinreichend genau lediglich für die zwei Flüssigionisationskammern. Im Falle sämtlicher gasgefüllter Kammern waren effektive Parameter notwendig, deren Wert kaum einen Zusammenhang mit der ursprünglichen Definition besaß. Doch auch dieser Ansatz versagt jedoch für die Advanced Markus-Kammer bei Sammelspannungen ≥ 300 V und Pulsdosen ab ca. 100 mGy. Das entwickelte numerische Berechnungsverfahren lieferte eine deutlich passendere Berechnung der Volumenrekombination und ermöglichte es, die Ursache für die Unterschiede zu Boags Theorie in dem Einfluss der freigesetzten Ladungen auf das elektrische Feld zu identifizieren. Eine aufgrund der erhöhten Pulsdosis erhöhte positive Raumladung verlangsamt die Sammlung der normalerweise schnellen freien Elektronen, welche von Volumenrekombination zunächst unbeeinträchtigt sind. Aufgrund der längeren Verweildauer im Kammervolumen, lagert sich jedoch ein höherer Anteil der Elektronen an und bildet negative Ionen. Der daraus resultierende höhere Anteil an Ladungen die Volumenrekombination ausgesetzt sind, zusätzlich zu der erhöhten Ladungsmenge, bedingt eine Erhöhung der Volumenrekombination mit der Pulsdosis, die sich nicht durch Boags Theorie beschreiben lässt. Insbesondere von Bedeutung ist dieser Effekt bei hohen elektrischen Feldstärken und kleinen Elektrodenabständen, die in einem hohen Anteil freier Elektronen resultieren. Des Weiteren erlaubt das numerische Verfahren die Berechnung für beliebige Pulsdauern, wohingegen Boags Theorie auf verschwindend geringe Pulsdauern beschränkt ist. Im Allgemeinen ergab das numerische Berechnungsverfahren Ergebnisse in guter Übereinstimmung mit den experimentellen Beobachtungen für die sehr verschiedenartigen Füllungen von Luft, Stickstoff und Flüssigkeiten. Auch die geometrisch komplexere Kompaktkammer konnte prinzipiell damit beschrieben werden, wobei sich jedoch für die untersuchte PinPoint-Kammer einige Diskrepanzen zu den experimentellen Beobachtungen ergaben. Eine vielversprechende Weiterentwicklung der Berechnung wäre die verbesserte Beschreibung der Sammelspannungsabhängigkeit der Volumenrekombination. In ihrer derzeitigen Form erfordert die Berechnung eine Charakterisierung jeder Kammer und Spannung, was durch eine Weiterentwicklung der Berechnung möglicherweise eliminiert werden könnte. Nichtsdestotrotz stellt die entwickelte numerische Berechnung eine deutliche Verbesserung gegenüber Boag's Theorie durch die korrekte Beschreibung der Pulsdosis- und Pulsdauerabhängigkeit der Volumenrekombination in stark gepulsten Felder dar, was prinzipiell eine absolute Dosimetrie dieser Felder ermöglichen sollte. / Synchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation. These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields. This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments. Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters. Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated. In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation. Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken. The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy. The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses. In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations. A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields.

Durch die Einführung von Synchrozyklotronen und Laser-Teilchenbeschleunigern, entwickelt mit dem Ziel günstigere und kompaktere Protonentherapieanlagen bereitzustellen, werden stark gepulste Strahlenfelder möglicherweise Anwendung in der Teletherapie finden. Darüber hinaus bergen stark gepulste Strahlenfelder das Potential klinischer Vorteile durch eine bessere Schonung gesunden Gewebes oder die verbesserte Behandlung bewegter Tumore. Allerdings ergeben sich neue Herausforderungen im Bereich der Dosimetrie, der Grundlage für eine präzise therapeutische Anwendung ionisierender Strahlung. Diese Herausforderungen betreffen sowohl den Bereich der klinischen Dosimetrie für die unmittelbare Strahlenanwendung als auch die Strahlenschutzdosimetrie zum Schutz von Umwelt und Personal. Luftgefüllte Ionisationskammern, die primären Messinstrumente der klinischen Dosimetrie, sind von einem zunehmenden Signalverlust aufgrund von Volumenrekombination betroffen, da stark gepulste Strahlenfelder eine hohe Ionisationsdichte innerhalb eines sehr kurzen Zeitraums erzeugen. Beschreibungen für diese Effekte sind zwar gut etabliert für die moderat gepulsten Felder im gegenwärtigen klinischen Einsatz (Boags Theorie), allerdings sind die dafür nötigen Näherung höchst wahrscheinlich unzureichend für die stark gepulsten Strahlenfelder zukünftiger Beschleuniger. Ferner sind Dosisleistungsmessgeräte, welche im Strahlenschutz als fest installierte oder mobile Überwachungsdosimeter eingesetzt werden, nur für kontinuierliche Strahlenfelder geprüft und bauartzugelassen, was Zweifel an ihrer Eignung für die Messung gepulster Felder eröffnet. In dieser Arbeit wurden beide Bereiche der Dosimetrie, sowohl Strahlenschutz als auch klinische Dosimetrie, untersucht, um die medizinische Anwendung stark gepulster Strahlung zu ermöglichen. Für ein möglichst umfassendes Verständnis wurden dabei experimentelle Untersuchungen mit theoretischen Überlegungen und Entwicklungen verzahnt. Mit dem ELBE-Forschungsbeschleuniger wurde ein gepulster 20 MeV Elektronenstrahl und somit ein gepulstes Strahlungsfeld erzeugt, welches eine systematische Untersuchung in einem großen Bereich in Bezug auf Pulsdosis und Pulsdauer erlaubte. Ionisationskammern für den klinischen Einsatz wurden mit diesem Elektronenstrahl direkt bestrahlt und ein Faraday-Becher diente als unabhängige Referenzmessung. Dosisleistungsmessgeräte hingegen wurden im, durch den Elektronenstrahl im Faraday-Becher erzeugten, Bremsstrahlungsfeld bestrahlt. Dabei fungierte die Ionisationskammer vor dem Faraday-Becher als Strahlmonitor und diente zur Bestimmung der Referenzdosis des Bremsstrahlungsfeldes über eine Querkalibrierung mit Thermolumineszenzdosimetern. Es wurden drei Dosisleistungsmessgeräte basierend auf unterschiedlichen Messprinzipien untersucht, die damit einen großen Teil der im Strahlenschutz eingesetzten Messprinzipien abdecken: Die Ionisationskammer RamION, das Proportionalzählrohr LB1236-H10 und der Szintillationsdetektor AD-b. Für die klinische Dosimetrie wurden zwei verbreitete Ionisationskammergeometrien untersucht: die Advanced Markus Kammer als Flachkammer und die PinPoint Kammer als Kompaktkammer. Zusätzlich zu der üblichen Luftfüllung wurde außerdem eine Füllung mit reinem Stickstoff und zwei Flüssigionisationskammern mit Isooctan und Tetramethylsilan untersucht. Ferner wurde eine numerische Berechnung der Volumenrekombination in Ionisationskammern durch die Beschreibung der Prozesse von Ladungsfreisetzung, Ladungstransport und Reaktion entwickelt, um eine Beschreibung zu erhalten, die ohne die für Boags Theorie notwendigen Näherungen auskommt. Insbesondere berücksichtigt diese Berechnung den Einfluss der freigesetzten Ladungen auf das elektrische Feld, der in Boags Theorie vernachlässigt wird. Von den drei untersuchten Dosisleistungsmessgeräten zeigte nur das RamION Messungen innerhalb der gegebenen Toleranzen in den untersuchten Strahlungsfeldern. Die unerwartet schlechte Präzision des AD-b Szintillationsdetektors, der keinen prinzipiellen Beschränkungen in gepulsten Feldern unterliegen sollte, wurde auf die Signalverarbeitung im Messgerät zurückgeführt, welche das prinzipielle Problem einer unbekannten Signalverarbeitung in kommerziellen Geräten hervorhebt. Das LB 1236-H10 Proportionalzählrohr andererseits maß den Erwartungen entsprechend. Dies unterstützt zwar die in DIN IEC/TS 62743 dargelegten Erwartungen für zählende Dosimeter, zeigt allerdings zugleich die allgemeine Unzulänglichkeit solcher Instrumente für die Messung stark gepulster Felder und demonstriert die Notwendigkeit für weitere normative Bestrebungen, um einheitliche Bedingungen für die Untersuchung nicht-zählender Dosimeter (wie das RamION) zu schaffen. Durch die Aufnahme dieser Ergebnisse in die Literatur der Strahlenschutzkommission wurde hier der Grundstein für eine solche Entwicklung gelegt. Die Untersuchung der Ionisationskammern für klinische Dosimetrie zeigte z.T. starke Abweichungen zwischen Boags Theorie und experimentellen Beobachtungen. Boags Theorie beschreibt Volumenrekombination hinreichend genau lediglich für die zwei Flüssigionisationskammern. Im Falle sämtlicher gasgefüllter Kammern waren effektive Parameter notwendig, deren Wert kaum einen Zusammenhang mit der ursprünglichen Definition besaß. Doch auch dieser Ansatz versagt jedoch für die Advanced Markus-Kammer bei Sammelspannungen ≥ 300 V und Pulsdosen ab ca. 100 mGy. Das entwickelte numerische Berechnungsverfahren lieferte eine deutlich passendere Berechnung der Volumenrekombination und ermöglichte es, die Ursache für die Unterschiede zu Boags Theorie in dem Einfluss der freigesetzten Ladungen auf das elektrische Feld zu identifizieren. Eine aufgrund der erhöhten Pulsdosis erhöhte positive Raumladung verlangsamt die Sammlung der normalerweise schnellen freien Elektronen, welche von Volumenrekombination zunächst unbeeinträchtigt sind. Aufgrund der längeren Verweildauer im Kammervolumen, lagert sich jedoch ein höherer Anteil der Elektronen an und bildet negative Ionen. Der daraus resultierende höhere Anteil an Ladungen die Volumenrekombination ausgesetzt sind, zusätzlich zu der erhöhten Ladungsmenge, bedingt eine Erhöhung der Volumenrekombination mit der Pulsdosis, die sich nicht durch Boags Theorie beschreiben lässt. Insbesondere von Bedeutung ist dieser Effekt bei hohen elektrischen Feldstärken und kleinen Elektrodenabständen, die in einem hohen Anteil freier Elektronen resultieren. Des Weiteren erlaubt das numerische Verfahren die Berechnung für beliebige Pulsdauern, wohingegen Boags Theorie auf verschwindend geringe Pulsdauern beschränkt ist. Im Allgemeinen ergab das numerische Berechnungsverfahren Ergebnisse in guter Übereinstimmung mit den experimentellen Beobachtungen für die sehr verschiedenartigen Füllungen von Luft, Stickstoff und Flüssigkeiten. Auch die geometrisch komplexere Kompaktkammer konnte prinzipiell damit beschrieben werden, wobei sich jedoch für die untersuchte PinPoint-Kammer einige Diskrepanzen zu den experimentellen Beobachtungen ergaben. Eine vielversprechende Weiterentwicklung der Berechnung wäre die verbesserte Beschreibung der Sammelspannungsabhängigkeit der Volumenrekombination. In ihrer derzeitigen Form erfordert die Berechnung eine Charakterisierung jeder Kammer und Spannung, was durch eine Weiterentwicklung der Berechnung möglicherweise eliminiert werden könnte. Nichtsdestotrotz stellt die entwickelte numerische Berechnung eine deutliche Verbesserung gegenüber Boag's Theorie durch die korrekte Beschreibung der Pulsdosis- und Pulsdauerabhängigkeit der Volumenrekombination in stark gepulsten Felder dar, was prinzipiell eine absolute Dosimetrie dieser Felder ermöglichen sollte.:1 Introduction 2 Scientific Background 2.1 General Aspects of Dosimetry 2.1.1 The Radiation Dose 2.1.2 Limitations of Absorbed Dose 2.1.3 Radiation Therapy vs. Radiation Protection 2.2 Pulsed Radiation 2.2.1 Terminology 2.2.2 Sources 2.3 Ionization Chambers for Radiation Therapy Dosimetry 2.3.1 Principle of Operation 2.3.2 Calibration and Correction Factors 2.3.3 Saturation Correction and Volume Recombination 2.4 Numerical Solution of Advection-Diffusion-Reaction Equations 2.5 Dose Rate Meters for Radiation Protection Dosimetry 2.5.1 Counting Tubes 2.5.2 Scintillation Detectors 2.5.3 Current Regulatory Developments 3 Material and Methods 3.1 Common Experimental Setup 3.1.1 Radiation Source ELBE 3.1.2 Beam Monitoring Equipment 3.2 Dose Rate Meter Measurements 3.2.1 Measurement Series and Procedure 3.2.2 Reference Measurements 3.3 Ionization Chamber Measurements 3.3.1 Measurement Series and Procedure 3.3.2 Experimental Determination of Volume Recombination 3.4 Numerical Calculation of Volume Recombination 3.4.1 Plane-parallel Chamber Geometry 3.4.2 Adaption to Thimble Chamber Geometry 3.4.3 Input Parameters 4 Dose Rate Meter Investigation 4.1 Results 4.2 Discussion and Conclusion 5 Ionization Chamber Investigation 5.1 Field Homogeneity and Stability 5.2 Uncertainty Considerations 5.3 Advanced Markus Chamber in Air 5.3.1 Experimental and Calculation Results 5.3.2 Comparison to Literature 5.3.3 Validity of the Numerical Model 5.3.4 Discussion of the Recombination Rate 5.3.5 Relevance of the Free Electron Fraction 5.4 Advanced Markus Chamber in N 2 5.4.1 Experimental and Calculation Results 5.4.2 Discussion of the Electron-Ion Recombination 5.5 PinPoint Chamber 5.5.1 Results and Discussion 5.6 Liquid Ionization Chamber 5.6.1 Experimental and Calculation Results 5.6.2 Discussion 5.7 Conclusion and Outlook 6 Summary 7 Zusammenfassung Bibliography Appendix A Evaluation of the Faraday Cup Data B Description of the Implemented Numerical Solver Danksagung / Synchrocyclotrons and laser based particle accelerators, developed with the goal to enable more compact particle therapy facilities, may bring highly pulsed radiation field to external beam radiation therapy. In addition, such highly pulsed fields may be desirable due to their potential clinical benefits regarding better healthy tissue sparing or improved gating for moving tumors. However, they pose new challenges for dosimetry, the corner stone of any application of ionizing radiation. These challenges affect both clinical and radiation protection dosimetry. Air-filled ionization chambers, which dominate clinical dosimetry, face the problem of increased signal loss due to volume recombination when a highly pulsed field liberates a large amount of charge in a short time in the chamber. While well established descriptions exist for this volume recombination for the moderately pulsed fields in current use (Boag's formulas), the assumptions on which those descriptions are based will most likely not hold in the prospective, highly pulsed fields of future accelerators. Furthermore, ambient dose rate meters used in radiation protection dosimetry as survey meters or fixed installations are generally only tested for continuous fields, casting doubt on their suitability to measure pulsed fields. This thesis investigated both these aspects of dosimetry - clinical as well as radiation protection - to enable the medical application of highly pulsed radiation fields. For a comprehensive understanding, experimental investigations were coupled with theoretical considerations and developments. Pulsed fields, varying in both dose-per-pulse and pulse duration over a wide range, were generated with the ELBE research accelerator, providing a 20 MeV pulsed electron beam. Ionization chambers for clinical dosimetry were investigated using this electron beam directly, with an aluminium Faraday cup providing the reference measurement. Whereas the dose rate meters were irradiated in the photon field generated from stopping the electron beam in the Faraday cup. In those measurements, the reference was calculated from the ionization chamber, then serving a an electron beam monitor, cross-calibrated to the photon field with thermoluminescent dosimeters. Three dose rate meters based on different operating principles were investigated, covering a large portion of the operating principles used in radiation protection: the ionization chamber based RamION, the proportional counter LB 1236-H10 and the scintillation detector AD-b. Regarding clinical dosimetry, measurements of two prominent ionization chamber geometries, plane-parallel (Advanced Markus chamber) and thimble type (PinPoint chamber), were performed. In addition to common air-filled chambers, chambers filled with pure nitrogen and two non-polar liquids, tetramethylsilane and isooctane, were investigated. In conjunction with the experiments, a numerical solution of the charge liberation, transport, and recombination processes in the ionization chamber was developed to calculate the volume recombination independent of the assumptions necessary to derive Boag's formulas. Most importantly, the influence of the liberated charges in the ionization chamber on the electric field, which is neglected in Boag's formulas, is included in the developed calculation. Out of the three investigated dose rate meters only the RamION could be identified as an instrument truly capable of measuring a pulsed field. The AD-b performed below expectations (principally, a scintillator is not limited in detecting pulsed radiation), which was attributed to the signal processing, emphasizing the problem of a typical black-box signal processing in commercial instruments. The LB 1236-H10, on the other hand, performed as expected of a counting detector. While this supports the recent effort to formalize these expectations and standardize testing for counting dosimeters in DIN IEC/TS 62743, it also highlights the insufficiency of counting detectors for highly pulsed fields in general and shows the need for additional normative work to establish requirements for dose rate meters not based on a counting signal (such as the RamION), for which no framework currently exists. With these results recognized by the German radiation protection commission (SSK) the first steps towards such a framework are taken. The investigation of the ionization chambers used in radiation therapy showed severe discrepancies between Boag's formulas and the experimentally observed volume recombination. Boag's formulas describe volume recombination truly correctly only in the two liquid-filled chambers. All the gas-filled chambers required the use of effective parameters, resulting in values for those parameters with little to no relation to their original meaning. Even this approach, however, failed in the case of the Advanced Markus chamber for collection voltages ≥ 300 V and beyond a dose-per-pulse of about 100 mGy. The developed numerical model enabled a much better calculation of volume recombination and allowed the identification of the root of the differences to Boag's formulas as the influence of the liberated charges on the electric field. Increased positive space charge due to increased dose-per-pulse slows the collection and reduces the fraction of fast, free electrons, which are unaffected by volume recombination. The resultant increase in the fraction of charge undergoing volume recombination, in addition to the increase in the total amount of charge, results in an increase in volume recombination with dose-per-pulse that is impossible to describe with Boag's formulas. It is particularly relevant in the case of high electric fields and small electrode distances, where the free electron fraction is large. In addition, the numerical calculation allows for arbitrary pulse durations, while Boag's formulas apply only to very short pulses. In general, the numerical calculation worked well for plane-parallel chambers, including those filled with the very diverse media of liquids, nitrogen and air. Despite its increased complexity, the thimble geometry could be implemented as well, although, in the case of the PinPoint chamber, some discrepancies to the experimental data remained, probably due to the required geometrical approximations. A possible future development of the numerical calculation would be an improved description of the voltage dependence of the volume recombination. At the moment it requires characterizing a chamber at each desired collection voltage, which could be eliminated by an improved modeling of the volume recombination's dependence on collection voltage. Nevertheless, the developed numerical calculation presents a marked improvement over Boag's formulas to describe the dose-per-pulse dependence and pulse duration dependence of volume recombination in ionization chambers, in principle enabling the application of ionization chambers in the absolute dosimetry of highly pulsed fields.:1 Introduction 2 Scientific Background 2.1 General Aspects of Dosimetry 2.1.1 The Radiation Dose 2.1.2 Limitations of Absorbed Dose 2.1.3 Radiation Therapy vs. Radiation Protection 2.2 Pulsed Radiation 2.2.1 Terminology 2.2.2 Sources 2.3 Ionization Chambers for Radiation Therapy Dosimetry 2.3.1 Principle of Operation 2.3.2 Calibration and Correction Factors 2.3.3 Saturation Correction and Volume Recombination 2.4 Numerical Solution of Advection-Diffusion-Reaction Equations 2.5 Dose Rate Meters for Radiation Protection Dosimetry 2.5.1 Counting Tubes 2.5.2 Scintillation Detectors 2.5.3 Current Regulatory Developments 3 Material and Methods 3.1 Common Experimental Setup 3.1.1 Radiation Source ELBE 3.1.2 Beam Monitoring Equipment 3.2 Dose Rate Meter Measurements 3.2.1 Measurement Series and Procedure 3.2.2 Reference Measurements 3.3 Ionization Chamber Measurements 3.3.1 Measurement Series and Procedure 3.3.2 Experimental Determination of Volume Recombination 3.4 Numerical Calculation of Volume Recombination 3.4.1 Plane-parallel Chamber Geometry 3.4.2 Adaption to Thimble Chamber Geometry 3.4.3 Input Parameters 4 Dose Rate Meter Investigation 4.1 Results 4.2 Discussion and Conclusion 5 Ionization Chamber Investigation 5.1 Field Homogeneity and Stability 5.2 Uncertainty Considerations 5.3 Advanced Markus Chamber in Air 5.3.1 Experimental and Calculation Results 5.3.2 Comparison to Literature 5.3.3 Validity of the Numerical Model 5.3.4 Discussion of the Recombination Rate 5.3.5 Relevance of the Free Electron Fraction 5.4 Advanced Markus Chamber in N 2 5.4.1 Experimental and Calculation Results 5.4.2 Discussion of the Electron-Ion Recombination 5.5 PinPoint Chamber 5.5.1 Results and Discussion 5.6 Liquid Ionization Chamber 5.6.1 Experimental and Calculation Results 5.6.2 Discussion 5.7 Conclusion and Outlook 6 Summary 7 Zusammenfassung Bibliography Appendix A Evaluation of the Faraday Cup Data B Description of the Implemented Numerical Solver Danksagung

Les colonnes de plasma entretenues par un champ électrique (continu ou alternatif) à haute pression (p > 10 Torr) sont affectées par les phénomènes de contraction (réduction de la section radiale de la décharge) et de filamentation (fragmentation de la section de plasma en plusieurs filaments). La compréhension de ces phénomènes ainsi que le développement d’une méthode pouvant les supprimer demeurent une étape essentielle pour l’optimisation de certains procédés plasma. Dans cette optique, un premier objectif de notre travail était de déterminer les mécanismes à l’origine de la contraction et de la filamentation dans les décharges créées dans des gaz rares. Ainsi, nous avons montré que dans les plasmas micro-ondes contractés la cinétique de la décharge est contrôlée par les ions moléculaires et que la contraction est liée à l’influence du gradient de la température du gaz sur la concentration de ces ions. De plus, nous avons mis en évidence que la filamentation apparaît lorsque l’inhomogénéité radiale du champ électrique devient importante. Dans un second temps, nous avons développé une méthode de décontraction et de défilamentation de la décharge, qui consiste à ajouter à une décharge initiale de gaz rare des traces d’un autre gaz rare de plus faible potentiel d’ionisation. Dans le cas des plasmas décontractés, nous avons démontré que la cinétique de la décharge n’est plus contrôlée par les ions moléculaires, ce qui confirme bien l’importance de ces ions dans la description de la contraction. Pour terminer, nous avons étendu à la pression atmosphérique la technique d’absorption optique de mesure de densité des états métastables et résonnants à l’aide d’une lampe spectrale, ce qui n’avait été réalisé jusqu’ici que pour des pressions inférieures à 10 Torr. Ces états jouent un rôle essentiel dans l’ionisation des décharges contractées alors que dans les décharges décontractées leur désexcitation par les atomes du gaz adjuvant est l’étape fondamentale du processus de changement de cinétique menant à la décontraction. / Plasma columns sustained at high pressures (p > 10 Torr) by an electric field (of constant or varying intensity) are affected by the contraction phenomena (reduction of the radial section of the discharge) and filamentation (breaking of the plasma column into several filaments). The understanding of these phenomena and the development of methods to suppress them are essential steps in the optimization of some plasma processes. In this context, the initial objective of our work was to determine the mechanisms at the origin of plasma contraction and filamentation in rare gas discharges. Along that line, we have shown that the discharge kinetics of micro-wave contracted plasmas is controlled by the presence of molecular ions and that contraction relates to the influence of the radial gradient of gas temperature on the concentration of these ions. In addition, we have evidenced that filamentation shows up whenever the radial inhomogeneity of the electric field intensity becomes important enough. In a second step, we have developed a method for eliminating plasma contraction and filamentation. It consists in adding to a contracted rare-gas discharge, a small amount of another rare gas having a lower ionization potential. In the case of the expanded plasmas obtained in this way, the discharge kinetics has been shown to be no longer controlled by molecular ions, thereby confirming their essential role in the contraction mechanism. Finally, we have extended to atmospheric pressure the technique of optical absorption that uses a spectral lamp to measure metastable-atom and resonant-atom densities. Until now, this technique has been used only at gas pressures lower than 10 Torr. Our interest in measuring metastable-state atom density is related to their participation in the step-wise ionization of contracted plasmas while, in expanded discharges, the fact that are desexcited by collisions with the added gas atoms is the essential step in modifying the kinetics of the discharge and preventing it to contract.

Les colonnes de plasma entretenues par un champ électrique (continu ou alternatif) à haute pression (p > 10 Torr) sont affectées par les phénomènes de contraction (réduction de la section radiale de la décharge) et de filamentation (fragmentation de la section de plasma en plusieurs filaments). La compréhension de ces phénomènes ainsi que le développement d’une méthode pouvant les supprimer demeurent une étape essentielle pour l’optimisation de certains procédés plasma. Dans cette optique, un premier objectif de notre travail était de déterminer les mécanismes à l’origine de la contraction et de la filamentation dans les décharges créées dans des gaz rares. Ainsi, nous avons montré que dans les plasmas micro-ondes contractés la cinétique de la décharge est contrôlée par les ions moléculaires et que la contraction est liée à l’influence du gradient de la température du gaz sur la concentration de ces ions. De plus, nous avons mis en évidence que la filamentation apparaît lorsque l’inhomogénéité radiale du champ électrique devient importante. Dans un second temps, nous avons développé une méthode de décontraction et de défilamentation de la décharge, qui consiste à ajouter à une décharge initiale de gaz rare des traces d’un autre gaz rare de plus faible potentiel d’ionisation. Dans le cas des plasmas décontractés, nous avons démontré que la cinétique de la décharge n’est plus contrôlée par les ions moléculaires, ce qui confirme bien l’importance de ces ions dans la description de la contraction. Pour terminer, nous avons étendu à la pression atmosphérique la technique d’absorption optique de mesure de densité des états métastables et résonnants à l’aide d’une lampe spectrale, ce qui n’avait été réalisé jusqu’ici que pour des pressions inférieures à 10 Torr. Ces états jouent un rôle essentiel dans l’ionisation des décharges contractées alors que dans les décharges décontractées leur désexcitation par les atomes du gaz adjuvant est l’étape fondamentale du processus de changement de cinétique menant à la décontraction. / Plasma columns sustained at high pressures (p > 10 Torr) by an electric field (of constant or varying intensity) are affected by the contraction phenomena (reduction of the radial section of the discharge) and filamentation (breaking of the plasma column into several filaments). The understanding of these phenomena and the development of methods to suppress them are essential steps in the optimization of some plasma processes. In this context, the initial objective of our work was to determine the mechanisms at the origin of plasma contraction and filamentation in rare gas discharges. Along that line, we have shown that the discharge kinetics of micro-wave contracted plasmas is controlled by the presence of molecular ions and that contraction relates to the influence of the radial gradient of gas temperature on the concentration of these ions. In addition, we have evidenced that filamentation shows up whenever the radial inhomogeneity of the electric field intensity becomes important enough. In a second step, we have developed a method for eliminating plasma contraction and filamentation. It consists in adding to a contracted rare-gas discharge, a small amount of another rare gas having a lower ionization potential. In the case of the expanded plasmas obtained in this way, the discharge kinetics has been shown to be no longer controlled by molecular ions, thereby confirming their essential role in the contraction mechanism. Finally, we have extended to atmospheric pressure the technique of optical absorption that uses a spectral lamp to measure metastable-atom and resonant-atom densities. Until now, this technique has been used only at gas pressures lower than 10 Torr. Our interest in measuring metastable-state atom density is related to their participation in the step-wise ionization of contracted plasmas while, in expanded discharges, the fact that are desexcited by collisions with the added gas atoms is the essential step in modifying the kinetics of the discharge and preventing it to contract.