Study of the magnetic configuration of an l=3 torsatron by the triode and the luminescent rod methods

Abstract

The magnetic field configuration of the Uragan-3M l=3 torsatron, which has a p=4 multipole vertical magnetic field compensation system, was studied using two methods to map the contours of the magnetic flux surfaces. The first method, the so-called triode method with a constant voltage electron source, measures the current emitted by an open thermoelectron emitter and the portion drawn by a highly transparent grid located in a poloidal cross-section of the torus. The second method involves the use of a conducting luminescent rod which scans the torus cross-section and lights up when struck by electrons emitted by an electron gun. The information on the magnetic surface structure obtained by these two techniques is compared. The characteristics of the two methods are discussed, giving special attention to the triode method because it allows an objective criterion for the quality of the magnetic surface structure to be introduced. It is shown how advantageous both methods are for rapid adjustment and optimization of the magnetic configurations in a stellarator when perturbations are present and what improvements could be achieved on Uragan-3M. Also discussed are experiments on generating electron clouds in Uragan-3M during the ramp-up phase of the magnetic field pulse

Influence of Experimental Conditions on Distribution of Divertor Plasma Flow in the Heliotron-E Fusion Device

“…Such an asymmetry has been found during precise measurements of the magnetic structure in this device [23].…”

Toroidal stellarator diode: discharge features and electron transport

“…It can be seen that the suppression of cathode emission on these becomes even stronger as U a increases. It is also found that the so-called tips at the centre of the I em (x) curves at U a = 30 V and U a = 50 V (figure 2(a)) indicate the position of the lower magnetic axis for the typical double-axis topology with the figure-of-eight inner separatrix (figure 2(b)) (X-point topology [14]). …”

“…In the experiments in U-3M [14] and U-2M [20,21], the thermionic emitter-cathode presented a cylindrical tungsten helix with a conductor 0.01 cm thick (the helix being 0.1 cm in diameter and 0.12 cm in length, with five turns). The hot emitter-cathode works efficiently when its current is I c = 0.8 A, i.e.…”

“…One of the estimates of the CMS quality in the stellarator magnetic configuration is the ratio of emission current measured inside the CMS region to that measured outside the CMS region [6,8,14,21]. Generally, if this ratio is more than three orders of magnitude, the magnetic system is said to work well.…”

“…The degree of suppression of the thermionic cathode emission in the TSD system characterizes the electron confinement on the magnetic surfaces. The confinement of electrons defined by the degree of suppression of electron emission from the emitter was used in [6,8,10,14,15] as an objective quality criterion for the magnetic surface structure of stellarator-type traps. This method of magnetic surface quality definition may be applied to more accurate alignment of the magnetic configurations.…”