Study of plasma parameters in expanders of the Gas Dynamic Trap (GDT)

Recent advances in GDT\footnote{A.A.Ivanov and V.V.Prikhodko, Plasma Phys Control. Fusion 55 (2013) 063001} demonstrate the possibility of electron temperatures (Te) of above 1 KeV in mirror machines.\footnote{P.Bagryansky et al, PRL 114 (2015) 205001} Such high Te can be reached if electron heat losses to end walls are limited. Understanding plasma parameters and potential distribution in expanders, common features in GDT and advanced FRCs at Tri Alpha Energy, is crucial for their efficiency. This poster reviews studies of plasma parameters in GDT expanders at on-axis Te of 300-600 eV. Diagnostics includes Langmuir probes, emissive probes, RPAs, and bolometers. Within few centimeters of the end plates, electron energies of $\sim$ 25eV and plasma potentials of several volts are observed. High potential drops are absent in the sheaths, indicative of the role of cold trapped electrons on the potential profiles. New results agree with prior studies at substantially lower on-axis Te. Scaling of plasma parameters with on-axis Te will be reported. This work was supported by the Ministry of Education and Science of Russia (project RFMEFI61914X0003).