The motivation of Alfven wave current drive for ST

Spherical tokamak (ST) [1] has been shown its attractiveness of because of its high beta (the ratio of plasma pressure to magnetic field pressure) and high good stability properties. However, the difficulty of startup and/or current drive in ST, which is caused by its slim central stack, is the obstacle that has to be leaped over before its application in of fusion plant or component test facility (CTF). Therefore, many non-iinductive startup and/or current drive methods have been applied to STs. However, some promising current drive methods in conventional tokamaks, such as electron cyclotron wave (ECW) and lower hybrid wave (LHW), have encountered difficulties in applying to ST plasmas, which have very high dielectric constants.The electron cyclotron wave (ECW) is widely used in STs to startup or assist startup [2-7], but the high dielectric constants of ST plasmas prevent ECW to penetrate into the core of plasma column. Quite successful ECW startup and current drive experiments are accomplished by the LATE team [7, 8]. In those experiments, nevertheless, only a low density ST plasma is obtained. Similar to ECW, the low hybrid wave (LHW), which is the most efficient method of non-inductive current drive in conventional tokamaks, is unable to reach the center of dense plasma too. The first attempt of LHW in ST, to drive current in the initial ramp-up phase when the plasma density is low, is scheduled in TST-2 [9]. In the frequency range of ion cyclotron wave, high harmonics fast wave (HHFW) has the potential of propagating into the dense ST plasma. From the results of HHFW experiments performed in NSTX [10, 11] and TST-2 [12], however, disappointing us again, the HHFW has a substantial effect of rather heating than current drive. Coaxial helical injection (CHI) is a high efficient non-inductive startup method that has been experimentally approved. Plasma currents of the order of 100 kA have been started up by CHI both in NSTX and HIT-II [13].Besides the methods mentioned above, Alfven wave is another non-inductive current drive method that is supposed to be able to drive current in the interior of bulk plasma without density limit. Thus the difficulties brought by high dielectric constants of ST plasmas are absent for Alfven waves. Indeed, the experimental evidences of Alfven wave current drive have been observed in the Phaedrus-T [14] and the TCABR [15] tokamaks without seeing serious trapped electron effects that was previously thought to reduce or eliminate the effects of current drive [16]. Nevertheless, these two are both conventional tokamaks with relatively higher aspect ratio A=R/a>3, where R and a are the major radius and minor radius respectively. Travelling Alfven wave was firstly proposed to drive current through inverse Landau damping from a longitudinal component of electric field and the transit time magnetic pumping. The driving efficiency of Alfven wave was also roughly calculated, which was inversely proportional to the phase velocity [17][14]. If it were true, the low phase velocity Alfven wave would have great potential in current drive. Nevertheless, this favorable situation was soon in doubt because of the existence of trapped-electrons, which was inversely proportional to the aspect ratio (A=R/a, here R and a are the major radius and minor radius respectively) and seriously diminished the driving efficiency of Alfven wave, in realistic tokamaks with finite aspect ratio [16][15]. In the case of ST, these diminishing effects will become even more serious since the low aspect ratio. Pondermotive force applied on non-resonant electrons provided by helicity injection was thought to be able to overcome the trapped-electron effects [18][16]. Recently, however, this non-resonant drive force was proved to be canceled out when nonlinear effects were included [19, 20][17, 18]. It seems that the only possible way left for driving current by Alfven wave is resonantly driving the electrons near the magnetic axis in a tokamak.

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