Experimental investigation of electron velocity distribution functions in the UT Helimak
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.