Current Start-Up Using the New CHI System

Abstract

Coaxial Helicity Injection (CHI) has now been implemented in QUEST. The goals for the first transient CHI experiments were to establish reliable gas breakdown conditions, and to measure CHI-produced toroidal current generation. Both these objectives were successfully met. Toroidal currents up to 29 kA were measured. Interestingly, these first plasmas on QUEST also suggest the formation of small amounts of closed magnetic flux surfaces. Establishment of methods for solenoid-free plasma current start-up, with robust and stable non-inductive current drive on tokamaks increases the prospects for a compact and low aspect ratio fusion reactor, which has the advantages of lower construction cost and higher plasma beta. QUEST is a mid-size spherical tokamak (ST) device [1], in which plasmas are produced mainly by electron cyclotron heating (ECH), and with minimal use of the central solenoid. Non-inductive toroidal plasma currents up to 70 kA have been achieved on QUEST using 28 GHz microwave injection [2]. We have now implemented coaxial helicity injection (CHI) capability on QUEST. We aim to generate more than 100 kA of initial toroidal plasma current with transient CHI and ramp this seed current noninductively using the high-power 28 GHz microwave system. In this paper we report results from the first CHI experiments on QUEST.CHI [3], a useful method for non-inductive current drive, has been studied in the HIT-II and NSTX STs, in which the compatibility of CHI startup with conventional Ohmic heating and current drive has been verified [4,5]. It is projected that the capability of CHI for current startup would be significantly enhanced if ECH is used to heat the CHI plasma, and this is planned to be tested in NSTX-U for a full non-inductive current start-up and ramp-up scenario [6]. author's e-mail: kuroda@triam.kyushu-u.ac.jpQUEST is equipped with important capabilities that will extend CHI studies to new parameter regimes. The internal vessel walls on QUEST are all metallic; this should be beneficial to CHI as it would reduce the influx of low-Z impurities in to the plasma, which are the prime source of energy loss in low temperature plasmas. The CHI electrode configuration on QUEST is also simpler, and different than the ones on HIT-II and NSTX, and it may be easier to implement this configuration in a fusion reactor [7]. The primary difference is that on NSTX and HIT-II, the insulator is also the vacuum boundary, whereas on QUEST, the insulator is sandwiched between the electrode plate and the divertor plate. However, CHI start-up in this new electrode configuration needs to be demonstrated. This is an important part of the CHI program on QUEST. Figure 1 shows the side view of the QUEST internal structure. The nominal toroidal field at the machine axis (at R 0 = 0.64 m) is 0.25 T at its maximum limit. There are four poloidal field coils above and below the vessel mid-plane; these are normally operated in a series configuration, with two coils connected to a single power supply. As shown in Fig. 1, we installed an e...

Fully non-inductive second harmonic electron cyclotron plasma ramp-up in the QUEST spherical tokamak

“…The QUEST CHI system began to be commissioned for the non-inductive plasma startup. The 29 kA plasma current was successfully started up in the first experimental campaign with the QUEST CHI system [30]. The large plasma current ramped up to a 100 kA level is expected in the high density plasmas, providing a source of high edge density for EBWHCD studies.…”

Application of transient CHI plasma startup to future ST and AT devices

“…Fortunately, new and upcoming experiments will investigate a variety of ring electrode concepts in detail. The QUEST device has recently implemented a single ring electrode in the lower divertor area which is biased relative to the vessel wall [30,24]. Further transient CHI experiments are expected to begin on the URANIA device (formerly PEGASUS) at the University of Wisconsin.…”

Initial results from solenoid-free plasma start-up using Transient CHI on QUEST

“…For that objective, we plan to implement further improvements such as the use of refractory electrodes and introduce plasma wall conditioning techniques, specifically aimed at improving electrode surfaces to minimize the level of low-Z oxidized impurities, and to reduce electrode sputtering, both of which are needed to reduce radiative losses and to allow the plasma to heat up to high temperatures so that the resulting L/R current decay time could be longer. This paper expands on the work of the first transient CHI experiments on QUEST, which were reported in the short Rapid Communication paper [25]. In this paper, we describe the CHI system in much more detail, including the conditions needed for initial gas breakdown, the bubble burst current requirements, and the current multiplication factor on QUEST.…”