Evidence for Separatrix Formation and Sustainment with Steady Inductive Helicity Injection

Victor, B.S.; Jarboe, T. R.; Hossack, Aaron; Ennis, D.A.; Nelson, B.A.; Smith, Roger; Akçay, C.; Hansen, Chris; Marklin, G.J.; Hicks, N.; Wrobel, J.S.

doi:10.1103/physrevlett.107.165005

Cited by 16 publications

(15 citation statements)

References 18 publications

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“…where I tor is the symmetric current and I inj is the quadrature injector current I inj = (I 2 x,inj + I 2 y,inj ) 1/2 averaged over an injector cycle. The non-symmetric edge magnetic fields on HIT-SI agree with that of the injector component of the Taylor state which is proportional to the measured injector current [13,14,21]. Therefore, I inj is proportional to the imposed fluctuation amplitude and its square is the correct quantity to use as the source term.…”

Section: Idcd Modelsupporting

confidence: 52%

“…With IDCD the fluctuations required for dynamo current drive are produced by the helicity injectors instead of by plasma instability. The discovery is based on data obtained in a recent breakthrough in parameters achieved in the helicity injected torus with steady inductive helicity injection (HIT-SI) experiment [13]. The current drive mechanism is analogous to resistive wall mode locking where asymmetric perturbations in a conducting plasma rotating next to a resistive wall cause a magnetic drag force on the plasma.…”

Section: Introductionmentioning

confidence: 99%

“…It then relaxes to the n = 0 spheromak state because the n = 0 is the minimum energy state [19]. The injector field then aligns with and ramps up the spheromak forming a separatrix [13].…”

Section: Introductionmentioning

confidence: 99%

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Imposed-dynamo current drive

Jarboe¹,

Victor²,

Nelson³

et al. 2012

Nucl. Fusion

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A mechanism for steady inductive helicity injection (SIHI) current drive has been discovered where the current driving fluctuations are not generated by the plasma but rather are imposed by the injectors. Sheared flow of the electron fluid distorts the imposed fluctuations to drive current. The model accurately predicts the time dependent toroidal current, the injector impedance scaling, and the profile produced in the HIT-SI experiment. These results show that a stable equilibrium can be efficiently sustained with imposed fluctuations and the current profile can, in principle, be controlled. Both are large steps for controlled fusion. Some of the effects of the fluctuations on the confinement of tokamak and spheromak reactors are assessed and the degradation may be tolerable. The mechanism is also of interest to plasma self-organization, fast reconnection and plasma physics in general.

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Section: Idcd Modelsupporting

confidence: 52%

Section: Introductionmentioning

confidence: 99%

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Imposed-dynamo current drive

Jarboe¹,

Victor²,

Nelson³

et al. 2012

Nucl. Fusion

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“…One then relies on non-axisymmetric MHD activity to drive current on closed field lines through the action of a dynamo current drive mechanism. This approach, known as driven CHI, is being currently investigated in small machines, 16,17 and also being studied through MHD simulations. 18 This paper is organized as follows.…”

Section: Introductionmentioning

confidence: 99%

Physics of forced magnetic reconnection in coaxial helicity injection experiments in National Spherical Torus Experiment

et al. 2014

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We numerically examine the physics of fast flux closure in transient coaxial helicity injection (CHI) experiments in National Spherical Torus Experiment (NSTX). By performing resistive Magnetohydrodynamics (MHD) simulations with poloidal injector coil currents held constant in time, we find that closed flux surfaces are formed through forced magnetic reconnection. Through a local Sweet-Parker type reconnection with an elongated current sheet in the injector region, closed flux surfaces expand in the NSTX global domain. Simulations demonstrate outflows approaching poloidally Alfv enic flows and reconnection times consistent with the Sweet-Parker model. Critical requirements for magnetic reconnection and flux closure are studied in detail. These primary effects, which are magnetic diffusivity, injector flux, injector flux footprint width, and rate of injector voltage reduction, are simulated for transient CHI experiments. The relevant time scales for effective reconnection arewhere s V is the time for the injector voltage reduction, s A is the poloidal Alfv en transit time, s R is the global resistive diffusion time, and Pm and S are Prandtl and Lundquist numbers. V C 2014 AIP Publishing LLC. [http://dx.

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“…Optimal plasma performance is achieved by conditioning the hydroscopic walls with many helium discharges, followed by 2-4 high-performing deuterium discharges [24]. The wall pumping enables lower spheromak plasma density and more optimal ratio of plasma current to density, j/n.…”

Section: Resultsmentioning

confidence: 99%

Plasma response to sustainment with imposed-dynamo current drive in HIT-SI and HIT-SI3

Hossack¹,

Jarboe²,

Chandra³

et al. 2017

Nucl. Fusion

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The helicity injected torus-steady inductive (HIT-SI) program studies efficient, steady-state current drive for magnetic confinement plasmas using a novel experimental method. Stable, high-beta spheromaks have been sustained using steady, inductive current drive. Externally induced loop voltage and magnetic flux are oscillated together so that helicity and power injection are always positive, sustaining the edge plasma current indefinitely. Imposeddynamo current drive (IDCD) theory further shows that the entire plasma current is sustained. The method is ideal for low aspect ratio, toroidal geometries with closed flux surfaces. Experimental studies of spheromak plasmas sustained with IDCD have shown stable magnetic profiles with evidence of pressure confinement. New measurements show coherent motion of a stable spheromak in response to the imposed perturbations. On the original device two helicity injectors were mounted on either side of the spheromak and the injected mode spectrum was predominantly n = 1. Coherent, rigid motion indicates that the spheromak is stable and a lack of plasma-generated n = 1 energy indicates that the maximum q is maintained below 1 during sustainment. Results from the HIT-SI3 device are also presented. Three inductive helicity injectors are mounted on one side of the spheromak flux conserver. Varying the relative injector phasing changes the injected mode spectrum which includes n = 2, 3, and higher modes.

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Evidence for Separatrix Formation and Sustainment with Steady Inductive Helicity Injection

Cited by 16 publications

References 18 publications

Imposed-dynamo current drive

Imposed-dynamo current drive

Physics of forced magnetic reconnection in coaxial helicity injection experiments in National Spherical Torus Experiment

Plasma response to sustainment with imposed-dynamo current drive in HIT-SI and HIT-SI3

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