Observation of an Enhanced Magnetic Helicity Injection Mode by a Rotating Plasma Annulus

Wang, ‪Zhehui; Si, Jiahe; Li, Hui

doi:10.1007/s10894-006-9057-y

Cited by 3 publications

(2 citation statements)

References 9 publications

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“…An enhanced helicity injection mode was recently reported in spheromak experiments with large plasma rotation [9]. Althought not analyzed in terms of boundary flows, this observation could support the feasibility of the mechanism proposed and studied in this Letter.…”

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confidence: 73%

Spheromak formation and sustainment by tangential boundary flows

García-Martínez

Farengo

2010

Physics of Plasmas

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The nonlinear, resistive, 3D magnetohydrodynamic equations are solved numerically to demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can by explained in terms of helicity injection and differs from other helicity injection methods employed in the past. Several features which were also observed in previous dc helicity injection experiments are identified and discussed.Spheromak plasmas have been formed using several different techniques [1,2]. The existence of multiple formation methods clearly shows that the spheromak is a preferred (lowest energy) state toward which a magnetohydrodynamic (MHD) system naturally evolves when the appropriate boundary conditions are imposed. The physical process responsible for the formation is magnetic relaxation: on the time scale of MHD instabilities the plasma relaxes to the minimum energy state compatible with its magnetic helicity content (which remains approximately constant) [3]. Once formed, the spheromak will decay in a resistive time scale because resistivity does dissipate magnetic helicity. For this reason, the sustainment of the configuration during times longer than the resistive decay time requires some helicity injection method. Since relaxation operates on a shorter time scale, the spheromak configuration is maintained regardless of the details of the specific helicity injection mechanism. Some particular examples are the coaxial helicity injection method (CHI) [4][5][6], the merging of helicitycarrying filaments (MHF) [7] and the helicity injected torus with steady inductive helicity injection (HIT-SI) [8].In this Letter we report the first evidence coming from nonlinear, resistive, 3D MHD numerical simulations that demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can by explained in terms of helicity injection and differs from other helicity injection methods employed in the past (CHI, MHF and HIT-SI).An enhanced helicity injection mode was recently reported in spheromak experiments with large plasma rotation [9]. Althought not analyzed in terms of boundary flows, this observation could support the feasibility of the mechanism proposed and studied in this Letter.We model the plasma using the resistive MHD equations with finite viscosity and zero β. The evolution equations for u and B are:where E = −u × B + ηJ and Π = (∇u + ∇u T ) − (2/3) × (∇·u) (see Ref.[10] for further details). These equations are normalized with a (chamber radius), ψ G (imposed flux) and c A (Alfvèn speed). In addition, B and J are scaled with √ µ 0 . Time is expressed in units of the Alfvèn time τ A = a/c A . The normalized resistivity η and the kinematic viscosity ν are set to 5 × 10 −5 . With these values the resistive time is τ r ∼ 800 (defined as in Ref.[11]). The resulting system is solved with the Versatile Advection Code [12].The domain is a cylinder of elongation h/a = 1, with perfectly conducting wall conditions (B · n = 0 ...

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supporting

confidence: 73%

Spheromak formation and sustainment by tangential boundary flows

García-Martínez

Farengo

2010

Physics of Plasmas

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“…28,29 FMP has a 4.5 m long, 0.75 m radius cylindrical vacuum chamber and the same coaxial plasma gun as the previous Compact Toroid eXperiment ͑CTX͒ at Los Alamos about two decades ago. 30 An electrolytic capacitor bank stores up to 300 kJ of energy and applies a maximum 900 V between the electrodes.…”

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confidence: 99%

Dust trajectories and diagnostic applications beyond strongly coupled dusty plasmas

2007

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Plasma interaction with dust is of growing interest for a number of reasons. On the one hand, dusty plasma research has become one of the most vibrant branches of plasma science. On the other hand, substantially less is known about dust dynamics outside the laboratory strongly coupled dusty-plasma regime, which typically corresponds to 1015m−3 electron density with ions at room temperature. Dust dynamics is also important to magnetic fusion because of concerns about safety and potential dust contamination of the fusion core. Dust trajectories are measured under two plasma conditions, both of which have larger densities and hotter ions than in typical dusty plasmas. Plasma-flow drag force, dominating over other forces in flowing plasmas, can explain the dust motion. In addition, quantitative understanding of dust trajectories is the basis for diagnostic applications using dust. Observation of hypervelocity dust in laboratory enables dust as diagnostic tool (hypervelocity dust injection) in magnetic fusion. In colder plasmas (∼10eV or less), dust with known physical and chemical properties can be used as microparticle tracers to measure both the magnitude and directions of flows in plasmas with good spatial resolution as the microparticle tracer velocimetry.

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Equilibrium and magnetic properties of a rotating plasma annulus

Wang

Liu

et al. 2008

Physics of Plasmas

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Local linear analysis shows that magneto-rotational instability can be excited in laboratory rotating plasmas with a density of 1019m−3, a temperature on the order of 10eV, and a magnetic field on the order of 100G. A laboratory plasma annulus experiment with a dimension of ∼1m, and rotation at ∼0.5 sound speed is described. Correspondingly, magnetic Reynolds number of these plasmas is ∼1000, and magnetic Prandtl number ranges from about one to a few hundred. A radial equilibrium, ρUθ2∕r=d(p+Bz2∕2μ0)∕dr=K0, with K0 being a nonzero constant, is proposed for the experimental data. Plasma rotation is observed to drive a quasisteady diamagnetic electrical current (rotational current drive) in a high-β plasma annulus. The rotational energy depends on the direction and the magnitude of the externally applied magnetic field. Radial current (Jr) is produced through biasing the center rod at a negative electric potential relative to the outer wall. Jr×Bz torque generates and sustains the plasma rotation. Rotational current drive can reverse the direction of vacuum magnetic field, satisfying a necessary condition for self-generated closed magnetic flux surfaces inside plasmas. The Hall term is found to be substantial and therefore needs to be included in the Ohm’s law for the plasmas. Azimuthal magnetic field (Bθ) is found to be comparable with the externally applied vacuum magnetic field Bz, and mainly caused by the electric current flowing in the center cylinder; thus, Bθ∝r−1. Magnetic fluctuations are anisotropic, radial-dependent, and contain many Fourier modes below the ion cyclotron frequency. Further theoretical analysis reflecting these observations is needed to interpret the magnetic fluctuations.

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Observation of an Enhanced Magnetic Helicity Injection Mode by a Rotating Plasma Annulus

Cited by 3 publications

References 9 publications

Spheromak formation and sustainment by tangential boundary flows

Spheromak formation and sustainment by tangential boundary flows

Dust trajectories and diagnostic applications beyond strongly coupled dusty plasmas

Equilibrium and magnetic properties of a rotating plasma annulus

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