Plasmoids Formation During Simulations of Coaxial Helicity Injection in the National Spherical Torus Experiment

Ebrahimi, F.; Raman, R.

doi:10.1103/physrevlett.114.205003

Cited by 51 publications

(62 citation statements)

References 29 publications

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“…However, reconnecting current sheets exceeding a certain aspect ratio cannot form because they become unstable to the formation of plasmoids, which break the reconnection layer into shorter elements, consequently leading to a significant increase in the reconnection rate (Daughton et al 2006(Daughton et al , 2009. Hence, the predictions of the classical Sweet-Parker reconnection model (Sweet 1958;Parker 1957) break down for sufficiently large systems such as those typically encountered in astrophysical environments -in these cases, it was shown that the reconnection rate becomes nearly independent of the magnetic diffusivity (Bhattacharjee et al 2009;Huang & Bhattacharjee 2010;Uzdensky et al 2010;Loureiro et al 2012;Takamoto 2013;Comisso et al 2015a;Ebrahimi & Raman 2015;Comisso & Grasso 2016).…”

Section: Introductionmentioning

confidence: 99%

Plasmoid Instability in Forming Current Sheets

Comisso

Lingam

Huang

et al. 2017

ApJ

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The plasmoid instability has revolutionized our understanding of magnetic reconnection in astrophysical environments. By preventing the formation of highly elongated reconnection layers, it is crucial in enabling the rapid energy conversion rates that are characteristic of many astrophysical phenomena. Most of the previous studies have focused on Sweet-Parker current sheets, which, however, are unattainable in typical astrophysical systems. Here, we derive a general set of scaling laws for the plasmoid instability in resistive and visco-resistive current sheets that evolve over time. Our method relies on a principle of least time that enables us to determine the properties of the reconnecting current sheet (aspect ratio and elapsed time) and the plasmoid instability (growth rate, wavenumber, inner layer width) at the end of the linear phase. After this phase the reconnecting current sheet is disrupted and fast reconnection can occur. The scaling laws of the plasmoid instability are not simple power laws, and depend on the Lundquist number (S), the magnetic Prandtl number (P m ), the noise of the system (ψ 0 ), the characteristic rate of current sheet evolution (1/τ ), as well as the thinning process. We also demonstrate that previous scalings are inapplicable to the vast majority of the astrophysical systems. We explore the implications of the new scaling relations in astrophysical systems such as the solar corona and the interstellar medium. In both these systems, we show that our scaling laws yield values for the growth rate, wavenumber, and aspect ratio that are much smaller than the Sweet-Parker based scalings.

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Section: Introductionmentioning

confidence: 99%

Plasmoid Instability in Forming Current Sheets

Comisso

Lingam

Huang

et al. 2017

ApJ

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“…This observation, combined with other observations from a slow mid-plane camera (not shown here), which shows a bright region moving from the lower end-plate to the upper end-plate, indicates that the CHI plasma must be disconnecting from the injector flux in the lower end-plate and drifting up. Such plasmas must carry closed field line currents, as they are no longer connected to the injector flux [8]. In these first experiments, because the PF3-2 and PF5-2 coils are driven in pairs, the PF3-2 coil above the upper end-plate is in a polarity so as to attract the CHI plasma.…”

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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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“…Thus, the role of the asymmetric instability reported here is distinct from that of plasmoid formation as part of the global fluxsurface closure after injection, which is predicted in axisymmetric simulations of NSTX. 18,19 To our knowledge, at the present time there are no experimental studies that can be used to determine if this mode is present in laboratory, helicity-injected plasmas. To the extent that the linear mode is current-driven, it is purely growing with no real part of the frequency and thus would not show up as a coherent oscillation, although Doppler-shift effects could generate oscillations in the laboratory.…”

Section: Discussionmentioning

confidence: 99%

“…The underlying resistive instability is the same basic current-sheet instability that is receiving renewed interest in the context of magnetic reconnection. [15][16][17][18][19]24 That the resulting nonlinear dynamics have greater impact as the modeled impurity concentration is reduced and temperature increases likely reflects a smaller current-sheet width at lower resistivity, which has a role in plasmoid formation. 16,17 However, during the phase of modeled active injection, the footpoints of the current channel are distant, and the instability is not part of the reconnection process that forms closed flux on the global scale.…”

Section: Discussionmentioning

confidence: 99%

“…The structure is reminiscent of the current-sheet instability that underlies plasmoid instability. [15][16][17][18][19] However, unlike basic-physics studies of magnetic reconnection and simulations of axisymmetric flux closure in NSTX, 18,19 the current sheet surrounding the early evolution of the flux bubble is not associated with largescale reconnection.…”

Section: Introductionmentioning

confidence: 99%

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A current-driven resistive instability and its nonlinear effects in simulations of coaxial helicity injection in a tokamak

Hooper

Sovinec

2016

Physics of Plasmas

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An instability observed in whole-device, resistive magnetohydrodynamic simulations of the driven phase of coaxial helicity injection in the National Spherical Torus eXperiment is identified as a current-driven resistive mode in an unusual geometry that transiently generates a current sheet. The mode consists of plasma flow velocity and magnetic field eddies in a tube aligned with the magnetic field at the surface of the injected magnetic flux. At low plasma temperatures (∼10–20 eV), the mode is benign, but at high temperatures (∼100 eV) its amplitude undergoes relaxation oscillations, broadening the layer of injected current and flow at the surface of the injected toroidal flux and background plasma. The poloidal-field structure is affected and the magnetic surface closure is generally prevented while the mode undergoes relaxation oscillations during injection. This study describes the mode and uses linearized numerical computations and an analytic slab model to identify the unstable mode.

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Plasmoids Formation During Simulations of Coaxial Helicity Injection in the National Spherical Torus Experiment

Cited by 51 publications

References 29 publications

Plasmoid Instability in Forming Current Sheets

Plasmoid Instability in Forming Current Sheets

Current Start-Up Using the New CHI System

A current-driven resistive instability and its nonlinear effects in simulations of coaxial helicity injection in a tokamak

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