Comparison between Tokamak and Ohmically Heated Stellarator Plasmas

Abstract

Tokamak and Ohmically heated stellarator plasmas have been compared in the CLEO apparatus, using gas current in the range 10-25 kA at a toroidal field of 12.7 kG. It is found that the vacuum poloidal field due to the helical winding has a considerable influence on the containment properties. In particular, for the same current the electron energy per unit length is a factor of 2 greater in the stellarator than in the tokamak.CLEO' is a seven-field period, l =3 toroidal stellaretor with a major radius, Ro of 90 cm and a minor radius, r&, of 13 cm as defined by two circular stainless-steel limiters, fixed inside the 28-cm bore stainless-steel vacuum vessel. The maximum available toroidal field B~is 20 kG, and the helical winding is designed to carry current I, up to 120 kA turns. 25Vo of the torus inner surface is covered by an active film of titanium before, and if necessary during, the day' s run. The base pressure is -10 ' Torr.A tokamak configuration can be set up in this apparatus by not energizing the helical winding, with equilibrium provided by a vertical field. A vertical field is also required for stellarator operation, the typical values at 20-kA externally induced ohmic heating current (I,) being 70 and 30 G for tokamak and stellarator, respectively. By using Ohmically heated hydrogen plasma with I, in the range 5-26 kA, a comparison has been made between stellarator and tokamak configurations, most extensively at 17 kA. For the &~of 12.7 kG and I, of 61 kA turns used in this comparison, the separatrix lies outside the limiter. The calculated value of rotational transform for the stellarator vacuum magnetic surface which touches the limiter is &=0.3. The same value of + at the plasma edge would be produced in the tokamak mode by an equivalent current I, of 21 kA.We have chosen to compare the tokamak with the Ohmically heated stellarator when the plasma edge is determined by the limiter rather than by the separatrix in order to ensure that limiter effects (e.g. , recycling) and wall effects are as similar as possible in the two configurations. The diagnostics used in these experiments were the following: (i) Current and loop voltage mea. -surements. (ii) Position coils. When the vertical field is optimized to produce the longest-lasting tokamak current, these show the current channel to be centered to + 1 cm. (iii) A single-channel 0 Z 10 (a) 10-0-10 'r" Fp'gl-0 (c) E 2 V o l.2 m E 1i 0 10 20 30 Time (ms) A 40 10 20 30 40 Time (ms) B FIG. l. Oscillograms of (a) loop volts, (b) gas current, (c) mean electron density for (A) tokamak and (B) stellarator at 12.7-kG toroidal field. 2-mm microwave interferometer for measurement of n, over a minor diameter. (iv) Photon scattering with a 10-J ruby laser for measurement of n, , T,(&).(v) A neutral particle analyzer, using charge exchange, for measurement of T& (vi) .A detector of the absolute intensity of H emission, to assess recycling and estimatẽ ", the average particle containment time. Figure 1 shows oscillograms of current, loop voltage, and n, for both to...

Energy confinement comparison of Ohmically heated stellarators to tokamaks

“…This scaling is shown in Fig. 2, in which we plot the data in Figure 3 shows Wendelstein VII-A [3] and Cleo [6] stellarator results, plotted in the form of T_/(l+ct) <:T > vs E e <-n e >aR Q /( > a -\ ao )B. The line is an "average" line in Fig where c^ is related to the dimensional constant factor in £(T e ).…”

Culham stellarator programme, 1965–1980

“…The first objective was to establish a comparison between tokamak and ohmically heated stellarator configurations. Preliminary results [23,24] showed that the poloidal field produced by the helical winding did beneficially affect plasma containment. Not only were major disruptions suppressed but also an effective equivalent current Ig could be prescribed from the mean poloidal field, such that the contained energy was proportional to I p (I p + Ig), where I p is the plasma current.…”

Plasma equilibrium in toroidal l = 3 stellarators

“…In most situations of practical interest, the measured profiles of pressure and plasma current do not permit (27) to be solved analytically; however, its numerical solution for given boundary data is very simple. We illustrate the use of the above analysis in calculating specific equilibria by presenting some results obtained for typical equilibria in CLEO stellarator (Atkinson et al 1976). In this device, consisting of a toroidal chamber of major radius 90 cm and minor radius 14 cm, hydrogen plasma with T e ~ 200 eV, and T t ~ 100 eV can be readily produced and sustained with an Ohmic heating current of several kilo-amperes, at an electron density in the range 10 13 -10 14 cm" 3 .…”

“…The boundary value relation between the external vertical field and the plasma surface displacement is calculated in § 5, and in § 6 several interesting special cases are examined. Finally the use of these results is illustrated by application to the case of CLEO stellarator (Atkinson et al 1976).…”