Experimental verification of nonconstant potential and density on magnetic surfaces of helical nonneutral plasmas

Abstract

For the first time, nonconstant space potential s and electron density n e on magnetic surfaces of helical nonneutral plasmas are observed experimentally. The variation of s grows with increasing electron injection energy, implying that thermal effects are important when considering the force balance along magnetic field lines. These observations confirm the existence of plasma equilibrium having nonconstant s and n e on magnetic surfaces of helical nonneutral plasmas.

Experimental Studies of Helical Nonneutral Plasmas with a Helical Magnetic Axis

“…This reflects the profile of inferred n e , showing that n e in the lower region is certainly larger than in the upper one. These results are similar to observations in past CHS experiments [7].…”

“…The pulse width of V acc applied to the cathode is also variable in the range between ∼ 10 μs and ∼ 100 ms, but it is likewise fixed at 10 ms for this research. The electron temperature T e , ϕ s , and n e are measured with the same probing method employed in past CHS experiments [7]. The probe can be used as not only an emissive probe but also a Langmuir probe.…”

“…To explain this phenomenon, numerical work has been performed, and recently, outward orbits that extend to the inward part of the closed HMS have been found [6]. Another significant feature is that the plasma equipotential surfaces do not coincide with the HMS [7]. Thus, the space potential ϕ s is not constant along magnetic field lines.…”

Probing of Toroidal Electron Plasmas Confined on Helical Magnetic Surfaces

“…Figure 5 shows typical results of the (a) φ s (t) measurement and (b) I p (t) measurement where V p = φ s . These data are measured at r = 114 cm in the 5O horizontal crosssection of CHS [16]. In this shot, the acceleration voltage V acc of electrons is about −1 kV and the measured φ s is about −700 V. Despite such large values of both V acc and φ s , data are clearly measured with the circuit system shown in Fig.…”

Pure electron plasmas confined for 90 ms in a stellarator without electron sources or internal objects

“…The injection of an energetic electron beam near the last closed flux surface of a stellarator allows creation of pure electron plasmas without internal objects but with a Debye length on the order of the system size and a high electron temperature (on the order of 100 eV) [9] . Experiments in the Columbia Non-neutral Torus (CNT) stellarator have previously demonstrated that pure electron plasmas with a small Debye length and relatively low temperature (< 5 eV) can be routinely created -however, this was in the presence of macroscopic internal objects [10,11].…”