Second stability in the ATF torsatron

Abstract

Access to the MHD second stability regime has been achieved in the ATF torsatron. Experimental p values (j3 0 < 3%, with fast ions contributing « j of the pressure at high p) are well above the theoretical transition value (p c « 1.3% for ideal modes) required to reach this regime. The relatively low p c results from operation with peaked pressure profiles. The measured magnetic fluctuations decrease with increasing /?, and the pressure profile broadens. This behavior is consistent with theoretical predictions for p self-stabilization of resistive interchange modes. PACS numbers: 52.55.Hc, 52.30.Jb, 52.35.PyThe ideal magnetohydrodynamic (MHD) second stability regime has attracted increasing attention in toroidal confinement fusion research 1 ' 2 because it offers the hope of operation at high p=2fiop/B 2 (the ratio of the plasma kinetic pressure to the magnetic pressure) with favorable confinement, which would improve the prospects for a viable D-T fusion reactor. This theory predicts that the changes in the internal magnetic surfaces (plasma axis shift and shape) caused by an increase in plasma pressure act to stabilize instabilities driven by unfavorable field-line curvature. l As a result, the plasma can become more stable as p increases ("/? self-stabilization"), with an accompanying reduction in the anomalous transport induced by the curvature-driven instabilities. In this Letter we describe experiments in which operation in the second stability regime has been achieved at relatively low p in a torsatron (a type of stellarator).Because of their external control of magnetic configuration and the absence of plasma current, stellarators are well suited for exploring the second stability regime and studying p self-stabilization. In stellarators with significant shear (dx/dr > 0, where % is the rotational transform and r is the plasma minor radius) the dominant instabilities are interchange modes. 3,4 These modes can be stabilized by the magnetic well produced by the outward magnetic axis shift at finite p (Shafranov shift), and this p self-stabilization effect can open a stable path to the second stability regime. 5 ' 6 The ATF torsatron design was optimized to explore this possibili- ty. 7ATF is an / -2, twelve-field-period torsatron with major radius /?o™2.10 m, average minor radius a *=0.27 m, magnetic field on axis 5o<2T, central rotational transform £o«0.3, and edge transform ^f l «l. For the profile assumed in the ATF design studies 6 the p selfstabilization effect should dominate at /?o«5%. Electron-beam field mapping in early 1988 revealed substantial magnetic islands (6 cm wide at the x ™ T surface and smaller at other rational surfaces). 8 These islands, now corrected, acted as a magnetic limiter and effectively reduced the plasma radius to r p *=*0.6d (increasing the effective aspect ratio A from 8 to 10) and the effective edge transform Xa » 0.5. This led to a large increase in the Shafranov shift id/accp^A/xl) that enhanced the self-stabilization effect and afforded the results reported here, which ...

Significance of MHD Effects in Stellarator Confinement

“…Before start of LHD operation, beta values in the 1.5 -2 % range have been reported by several stellarator experiments including Heliotron-E [14], the Advanced Torsatron Facility ATF [15], the Compact Helical System CHS [16] and W7-AS. Here, we are focusing on more recent results from W7-AS and LHD.…”

Stellarators

“…Experiments focused on plasma performance optimization, the dependence of the measured bootstrap current on plasma pressure, field strength and poloidal field shaping; characterization of fluctuations near the plasma edge; and impurity studies. Evidence for beta self-stabilization (second stability) has been obtained in ATF [6]. On-axis beta values (/3(0) < 3%) are well above the theoretical value (i8(0) =• 1.3% for ideal modes) for transition to the second stability regime for the peaked pressure profiles obtained in ATF.…”

Review of recent stellarator results in the USA, the USSR and Japan

“…Beta self-stabilization occurs when the depth and radial extent of the magnetic well increase rapidly enough with increasing beta to offset the destabilizing effect of the increasing plasma pressure, Evidence for this behavior has been obtained in ATF (Harris et al, 1989). Figure ll(a) shows that coherent n = 1 poloidal magnetic fluctuations (/}e) are observed to first increase with beta and then decrease at higher • values, even though the instability should be moving closer to the array of magnetic loops as beta increases.…”

“…In ATF, good agreement was obtained between islands measured with an electron beam and those calculated from field errors due to the current feeds to the helical and vertical • field coils (Colchin et al, 1989). This field perturbation was used to advantage to access the second stability region at lower beta than otherwise possible (Harris et . al., 1989); the field errors were then corrected (Colchin et al, 1989).…”