Experimental demonstration of plasma startup by coaxial helicity injection

Raman, R.; Jarboe, T. R.; Nelson, B. A.; Hamp, W. T.; Izzo, V.A.; O'Neill, R.G.; Redd, A. J.; Sieck, P.E.; Smith, Roger

doi:10.1063/1.1688791

Cited by 24 publications

(26 citation statements)

References 11 publications

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“…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].…”

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confidence: 99%

Current Start-Up Using the New CHI System

Kuroda

Raman

Hanada

et al. 2017

Plasma and Fusion Research

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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...

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confidence: 99%

Current Start-Up Using the New CHI System

Kuroda

Raman

Hanada

et al. 2017

Plasma and Fusion Research

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“…The start-up takes place in a manner similar to electrostatic drive for spheromaks, but the intent is to create an equilibrium from which Ohmic current drive can proceed efficiently. Depending on how the electrostatic injection is applied [26], MHD activity and magnetic relaxation [29,30] may be part of the process.…”

Section: Spherical Torus Current Injection and Start-upmentioning

confidence: 99%

“…This has led to exploration of electrostatic helicity injection for start-up in HIT-II at the University of Washington, in NSTX at Princeton Plasma Physics Laboratory, and in the Pegasus ST at the University of Wisconsinsee Refs. [26][27][28]. The start-up takes place in a manner similar to electrostatic drive for spheromaks, but the intent is to create an equilibrium from which Ohmic current drive can proceed efficiently.…”

Section: Spherical Torus Current Injection and Start-upmentioning

confidence: 99%

Numerical Studies of Magnetohydrodynamic Activity Resulting from Inductive Transients Final Report

Sovinec¹

2005

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“…A new method of non-inductive startup, referred to as transient coaxial helicity injection (Transient CHI) [1,2,3], has been successfully developed on the Helicity Injected Torus, experiment [4], and recently implemented on the National Spherical Torus Experiment [5]. In this method, a plasma current is rapidly produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields, shown schematically in Fig.…”

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Solenoid-free Plasma Start-up in HIT-II and NSTX using Transient CHI

et al. 2006

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Elimination of the central solenoid is a consideration for the design of toroidal confinement devices, which then require alternative methods for initiating the plasma current. A new method of non-inductive startup, referred to as transient coaxial helicity injection (Transient CHI) [1,2,3], has been successfully developed on the Helicity Injected Torus, experiment [4], and recently implemented on the National Spherical Torus Experiment [5]. In this method, a plasma current is rapidly produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields, shown schematically in Fig. 1. An initial very small gas puff is introduced near the injector, which is ionized by the applied voltage.The initial poloidal field configuration is chosen such that the plasma rapidly expands into the chamber. When the injector current is rapidly decreased, magnetic reconnection occurs near the injector electrodes, with the toroidal plasma current forming closed flux surfaces. By selfconsistently increasing both the injector flux and the externally produced toroidal flux, the useful CHI-produced current on closed flux surfaces has been increased to 100 kA on HIT-II, and is retained during a subsequent inductive ramp. On HIT-II, CHI-started plasmas outperform discharges initiated by induction alone and consume less volt-seconds. [3] Transient CHI is successfully used on NSTX for an unambiguous proof-of-principle demonstration of closed-flux current generation, of up to 160 kA, without the use of the central solenoid. Significant improvements over the HIT-II results are (a) demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor, (b) a multiplication factor of 60 between the injected current and the achieved toroidal current, compared to six in previous experiments, and (c) significantly more detailed experimental measurements, including, for the first time, fast time scale visible imaging of the entire process, showing discharge formation, disconnection from the injector, and the reconnection of magnetic field lines to form closed flux. In some discharges the generated current persists for an unprecedented, and unanticipated 400 ms (until external equilibrium coils are shut off).These significant results, which were obtained on a machine designed with mainly conventional components and systems, indicate favourable scaling with machine size.From helicity conservation, for a Taylor minimum energy state, λ inj ≥ λ tok , where λ inj =

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Experimental demonstration of plasma startup by coaxial helicity injection

Cited by 24 publications

References 11 publications

Current Start-Up Using the New CHI System

Current Start-Up Using the New CHI System

Numerical Studies of Magnetohydrodynamic Activity Resulting from Inductive Transients Final Report

Solenoid-free Plasma Start-up in HIT-II and NSTX using Transient CHI

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