The international community is in agreement that a lepton collider in the TeV centre of mass energy range is required to leverage discoveries made at the Large Hadron Collider and expand the physics programme. Two future colliders are proposed. The International Linear Collider (ILC) will collide electron and positron bunches at a centre of mass energy of 500 GeV, upgradable to 1 TeV. The Compact Linear Collider (CLIC) is designed to reach 3 TeV. This thesis investigates the wakefields, which degrade the beam quality, and beam dynamics in the main linacs of the ILC, presenting the first direct comparison of beam dynamics for linacs made up of the alternative high gradient superconducting cavity designs - the Reentrant and Ichiro cavities. Higher order modes of the electromagnetic field in the cavities, which will be excited by the passage of the bunches, are calculated using finite difference and finite element techniques. A trapped dipole mode in the Ichiro cavity at 2.4498 GHz is identified. These modes are used as the basis for the beam dynamics studies. These simulations have demonstrated that ILC linacs made up of the new high gradient cavities, with targeted damping, would meet wakefield requirements for delivering high quality beams for particle physics studies. This result is important since any upgrade of the ILC from 500 GeV to 1 TeV centre of mass energy would make use of one of these high gradient cavity designs in the extension to the linacs. Beam dynamics in the CLIC beam delivery system (BDS), are also detailed. Simulations included deflecting mode Crab Cavities required to maximise collision luminosity when there is a crossing angle, and verify analytic results for the required deflecting voltage and tolerances to phase differences. The tolerance to crab cavity roll angle is found to be extremely tight, at 5.9 millidegrees. Additionally, results in this thesis uncover a problem with the BDS magnet layout which must be addressed.

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A prototype system which has integrated a linear accelerator (linac) with a magnetic resonance imager (MRI) has been constructed at the Cross Cancer Institute. The real time operation of a linac-MRI system will require proper radio frequency (RF) shielding such that the MRI images can be acquired without extraneous RF noise from the linac. This thesis reports on the steps taken to successfully RF-shield the linac from the MRI such that the two devices can operate independently of one another. The RF noise from functioning multileaf collimators (MLC) is measured using near field probes and MRI images are acquired with the MLC near the MRI. This included measuring the RF noise as a function of applied magnetic field strength. Several measurement and simulation scenarios are discussed to determine the major sources of RF noise generation from the modulator of a linac. Finally RF power density levels are reported internally and externally to the RF cage which houses the linac and the MRI. The shielding effectiveness of the RF cage has been measured in the frequency range 1 50 MHz and is presented. MRI images of two phantoms are presented during linac operation. The MLC studies illustrate that the small RF noise produced by functioning MLC motors can be effectively shielded to avoid signal-to-noise degradation in the MRI image. A functioning MLC can be incorporated into a linac-MRI unit. The RF noise source investigations of the modulator of a linac illustrate that the major source of RF noise involves the operation of a magnetron. These studies also eliminate the pulse forming network (PFN) coil and the grid voltage spikes on the thyratron as possible major sources of RF noise. The main result is that for linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI. Finally imaging work with the linac operating illustrates that the accelerating structure of a linac and an MRI can be housed within the same RF cage. The 6 MV linac can be operated to produce radiation with no experientially measurable degradation in image quality due to RF effects. / Medical Physics

Hypofractionated conformal stereotactic radiotherapy (HCSRT) has been used for the treatment of AVMs at the Umeå University Hospital since 1986. From this year and onwards an increasing number of patients with single or oligo brain metastases have also been treated using this technique. In paper I we have retrospectively evaluated our treatment results of AVMs in terms of obliteration and complications. The rates of obliteration and complications seem to be comparable with SRS even if the AVM volumes in our series were larger than in most series with SRS. In paper II we have retrospectively evaluated the results in terms of local control, survival and complications in two groups of patients with single or oligo brain metastases. One group was treated with HCSRT alone and the other group was treated with whole brain radiotherapy in combination with a stereotactic boost. Controversy still exists concerning the benefit of additional use of WBRT in combination with stereotactic irradiation. The survival times were equal in the two groups and no significant difference in local control was observed. The omission of WBRT seems to carry a higher risk for development new brain metastases distant from the irradiated area. In paper III we report the treatment results in a subgroup of AVMs treated with a combination of embolisation and HCSRT. We also focus on the reduction of vascular density within the nidus of an AVM and propose a method to digitally compare images and more objectively assess a reduction in vascular density following embolisation. Obliteration rates seem comparable with other series using a combination of SRS and embolisation even if our rate of complications was higher than what is usually reported. Using luminescence as measure of vascular density all AVMs seemed to be less dense after embolisation. Treatment accuracy in terms of reproducibility of the isocenter in consecutive treatment sessions is crucial in fractionated radiotherapy. In paper IV we have radiologically evaluated the reproducibility of the isocenter in successive treatment sessions using the non invasive relocatable Fixster frame. There was a high degree of reproducibility and only small errors that most likely is of no clinical importance. A reliable dose plan is equally important as a tool to predict the dose delivered inside and outside the target volume. In paper V we have evaluated the reliability of treatment plans in HCSRT for targets of different geometry and size. A liquid ion chamber and gel dosimeter was used for assessment of dose distribution and absorbed dose. The doseplanning system proved to be accurate in predicting the absorbed dose and dose distribution for the different targets.

A Varian 2100C 18 MV photon beam has been modeled in this work using the MCNP5 Monte Carlo particle transport user code. The subsequent beam irradiation was also delivered to a water phantom and benchmarked against experimentally measured depth dose data. The model presented in this work establishes the foundation to which further beam characteristics tuning is required in order to realistically model the beam mentioned above. It has been determined in this work that the initial electron beam energy of this beam model is sufficiently close to the electron beam energy from the linear accelerator used to obtain the benchmark depth dose data.

A superconducting microwave cavity has been designed to test advanced materials for use in the accelerating structures contained within linear colliders. The electromagnetic design of this cavity produces surface magnetic fields on the sample wafer exceeding the critical limit of Niobium. The ability of this cavity to push up to 4 times the critical field provides, for the first time, a short sample method to reproducibly test these thin films to their ultimate limit. In order for this Wafer Test cavity to function appropriately, the large sapphire at the heart of the cavity must have specific inherent qualities. A second cavity was constructed to test these parameters: dielectric constant, loss tangent, and heat capacity. Several tests were performed and consistent values were obtained. The consequences of these measurements were then applied to the Wafer Cavity, and its performance was evaluated for different power inputs. The Q_0 of the cavity could be as low as 10^7 because of the sapphire heating, therefore removing the ability to measure nano-resistances. However, with additional measurements in a less complex environment, such as the Wafer Test Cavity, the Q_0 could be higher than 10^9.

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Linear colliders are an option for lepton collision at several TeV. The Compact Linear Collider (CLIC) aims at electron and positron collisions at a centre of mass energy of 3 TeV. In CLIC, the main accelerating structures are designed to operate at an X-band frequency of 12 GHz with an accelerating gradient of 100 MV/m. Two significant issues in linear accelerators that can prevent high gradient being achieved are electrical breakdown and wakefields. The baseline design for the CLIC main linacs relies on a small aperture size to reduce the breakdown probability and a strong damping scheme to suppress the wakefields. The strong damping scheme may have a higher possibility of electrical breakdown. In this thesis an alternative design for the main accelerating structures of CLIC is studied and various aspects of this design are discussed. This design is known as a Damped and Detuned Structure (DDS) which relies on moderate damping and strong detuning of the higher order modes (HOMs). The broad idea of DDS is based upon the Next Linear Collider (NLC) design. The advantages of this design are: well damped wakefields, minimised rf breakdown probability and reduced size of the structure compared to the strong damping design. Procedures necessary to minimise the rf monopole fields and enhance the wakefield suppression are discussed. The rf as well as mechanical designs of a test structure are presented. This unique design forms the basis of this research and allows both the electrical breakdown and beam dynamics constraints to be simultaneously satisfied.