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...
