The focus of this work is the experimental investigation of electron velocity distribution functions in the plasma of the Texas Helimak experiment. Texas Helimak has a cylindrical geometry and relatively moderate plasma parameter, which allow the use of a retarding field analyzer that is located approximately in the middle of the vacuum vessel. Electron velocity distribution functions were measured for a variety of different operating conditions and for two gas species, namely argon and helium. Parameter scans, whereby all parameter except the scan parameter were kept constant, were done in order to investigate the influence of the scan parameter on the velocity distribution. It was found, that most electron velocity distribution functions are not alone Maxwellian. One of the reasons therefore is electron cyclotron resonance heating, which was used for ionizing and heating the plasma and what produces a suprathermal tail. It is possible, however, to obtain effective electron temperatures. These effective electron temperatures range from about 10 eV , what is similar to the electron bulk temperature, to more than 150 eV. / text

Profiles for electron density and temperature were determined in a self-consistent way using line emission spectroscopy and collisional radiative models for neutral and singly ionized Argon (Ar I and Ar II) in the Texas Helimak. Neutral Argon density profiles were calculated using a kinetic gas model. Electron-impact excitation and Ionization rates were corrected to account for the electron velocity distribution deviating slightly from a true Maxwellian distribution due to inelastic electron-neutral collisions. Results show an electron temperature which roughly agrees with probe diagnostics. This method gives an electron density that is about twice as high, which may be possible from a power balance perspective. / text

Controlling the scrape-off layer (SOL) properties in order to limit divertor erosion and extend component lifetime will be crucial to successful operation of ITER and devices that follow, where intermittent thermal loads on the order of GW/m² are expected. Steady state transport in the edge region is generally turbulent with large, order unity, fluctuations and is convection dominated. Owing to the success of the past fifty years of progress in magnetically confining hot plasmas, in this work we examine convective transport phenomena in the SOL that occur in the relatively "slow", drift-ordered fluid limit, most applicable to plasmas near MHD equilibrium. Diamagnetic charge separation in an inhomogeneous magnetic field is the principal energy transfer mechanism powering turbulence and convective transport examined in this work. Two possibilities are explored for controlling SOL conditions. In chapter 2 we review basic physics underlying the equations used to model interchange turbulence in the SOL and use a subset of equations that includes electron temperature and externally applied potential bias to examine the possibility of suppressing interchange driven turbulence in the Texas Helimak. Simulated scans in E₀×B₀ flow shear, driven by changes in the potential bias on the endplates appears to alter turbulence levels as measured by the mean amplitude of fluctuations. In broad agreement with experiment negative biasing generally decreases the fluctuation amplitude. Interaction between flow shear and interchange instability appears to be important, with the interchange rate forming a natural pivot point for observed shear rates. In chapter 3 we examine the possibility of resonant magnetic perturbations (RMPs) or more generally magnetic field-line chaos to decrease the maximum particle flux incident on the divertor. Naturally occurring error fields as well as RMPs applied for stability control are known to cause magnetic field-line chaos in the SOL region of tokamaks. In chapter 3 2D simulations are used to investigate the effect of the field-line chaos on the SOL and in particular on its width and peak particle flux. The chaos enters the SOL dynamics through the connection length, which is evaluated using a Poincaré map. The variation of experimentally relevant quantities, such as the SOL gradient length scale and the intermittency of the particle flux in the SOL, is described as a function of the strength of the magnetic perturbation. It is found that the effect of the chaos is to broaden the profile of the sheath-loss coefficient, which is proportional to the inverse connection length. That is, the SOL transport in a chaotic field is equivalent to that in a model where the sheathloss coefficient is replaced by its average over the unperturbed flux surfaces. Both fully chaotic and the flux-surface averaged approximation of RMP application significantly lower maximum parallel particle flux incident on the divertor. / text

A Neutral Beam Probe (NBP) was developed for studying the Texas Helimak plasma experiment. The probe consisted of a beam of neutral sodium atoms that were injected into the magnetized plasma of the Helimak. After some fraction of the atoms underwent electron impact ionization, the resulting ion beam followed a path to an energy analyzer where the change of energy was detected along with the total ion current. The measurement of the change of energy implies a change of potential energy at the point of ionization since all the neutral beam particles enter the plasma with a well determined energy. The total current detected at the energy analyzer also implies a rate of electron impact ionization, which in turn implies an electron density and temperature. The NBP was developed based on the Elmo Bumpy Torus (EBT) Heavy Ion Beam Probe (HIBP), which was operated at Oak Ridge National Labs. In fact, the majority of the equipment that was used in this experiment was taken from that HIBP, and some of it was rebuilt. We generated an estimate of the radial electric field in the Helimak along with an estimate of density changes as a result of biasing experiments. Interestingly, when a bias voltage was applied inside the Helimak, the radial electric field did not change significantly at the sample region, but the electron density did vary. The probe data taken by the Helimak team agree with the density changes. The electric field derived from Langmuir probes is not trivial (especially in plasmas with flows) and was not computed for this thesis. / text