H-mode behavior induced by cross-field currents in a tokamak

Abstract

A sharp transport barrier, accompanied by a bifurcated poloidal rotation and a radial electric field, is formed at the plasma edge by driving a radial current across the outer magnetic surfaces of a tokamak. A decrease in particle transport is observed for negative radial E fields. When the radial current is turned off, the E field and the rotation damp on a time scale comparable with the ion-ion collision time.PACS numbers: 52.25.Fi, 52.55.Pi, 52.70.Ds An intensive effort is in progress, worldwide, to correlate the large and growing data base of experimental observations on plasma transport in magnetic fusion containment devices with theoretical models. In particular, the so-called "//^-mode" regimes in tokamaks have attracted strong interest due to their enhanced particle and energy confinement properties. ^""^ The transition of a plasma into the H mode is marked by a sudden decrease in the hydrogenic light emission from the plasma edge, followed by a prolonged increase in the plasma density. The reduction of hydrogen light (H^ or H^) indicates that the incoming neutral particle flux is reduced, presumably because of a decrease of the outgoing plasma flux, leading to a reduction in "recycling." The improvement in the energy confinement is generally less than the increase in particle confinement, ^-mode measurements also reveal the formation of sharp density and temperature gradients inside the last closed magnetic surfaces, which represents a transport barrier. Despite the magnitude of the effort aimed at modeling the H mode, no clear mechanism has been identified, although radial electric fields are thought to play a role."*'^ In this Letter, experimental observations confirming the importance of the radial E field and the associated plasma rotation for 7/-mode confinement are presented.In 1979, electron injection was used to modify the edge potentials in order to reduce ion sputtering in the Macrotor tokamak.^ Subsequently, improved particle confinement and a concomitant impurity accumulation were observed,^ apparently giving rise to an H mode. These effects were attributed to the creation of edge radial electric fields and associated negative plasma potentials much larger in magnitude than Tgia), where a is the plasma radius. Recently we have extended this earlier work using the new, titanium-gettered. Continuous Current Tokamak (CCT) at the University of California, Los Angeles. The recent experiments clearly show the //-mode signatures found in other tokamaks in various limiter, divertor, and auxiliary-heating configurations. The previously seen impurity limitations^ have also been eased by new electrode designs. ^ For the //-mode-regime studies, CCT was operated in the pulsed neo-Alcator regime, with central parameters R^l.5 m, a =0,4 m, Bt=3 kG, /p=50 kA, ne=5 xlO*Vcm^ Kioop<1.5 V, Te>l50 eV, and T/> 100 e = 180 INSULATOR ELECTRODE = 0" C/2 Q 1.5 1.8 MAJOR RADIUS (M) FIG. 1. (a) Cross section of tokamak, a ^40 cm, showing the location of the exciting electrode, re ^25 cm, and the "rake" probe arrays used...

Drift waves and transport

“…The achievement of the edge H mode has been repeated with many different techniques and is now assumed to be essential for the successful operation of a nuclear fusion reactor based on low beta toroidal confinement. The H-mode transition from biased limiters was found by Taylor et al (1989) and Weynants et al (1991).…”

Neoclassical physics in full distribution function gyrokinetics

“…2,[28][29][30] 4. An increasing number of experimental observations [31][32][33][34][35] insists on the central role of plasma rotation (toroidal and poloidal) for the onset and control of enhanced confinement regimes and often report strong departures of poloidal rotation from its oft-expected neoclassical predictionpossibly due to the turbulence 2,36,37 -either before, at, or enduring past the transition to an enhanced confinement state.…”

Electrode and Langmuir probe tools used for flow damping studies in the Helically Symmetric Experiment

“…HSX has not observed a biased H-mode in traditional sense, 7 but a reduction in the amplitude of low frequency ͑Ͻ100 kHz͒ floating potential fluctuations has been observed during the bias pulse. Further studies of the effect of the electrode bias on plasma turbulence will be performed in the future.…”

“…This "return current" flowing through the plasma applies a torque to the plasma, leading to plasma rotation. This biasing technique was pioneered on the B-3 stellarator 6 and has been used to great advantage at, for instance, CCT, 7 TEXTOR, 8 Phaedrus-T, 9 TUMAN-3, 10 T-10, 11 ISSTOK, 12 RFX, 13 MST, 14 CAT, 15 and IMS. 16 Of the incomplete sampling of biased electrode experiments given above, it appears that only the systems at IMS and MST were designed to allow the transient phase of the plasma spin-up or spin-down to be studied.…”