Flux amplification in helicity injected spherical tori

Journal Article · Fri Apr 15 04:00:00 EDT 2005 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.1884127· OSTI ID:20736495

Tang, X Z; Boozer, A H ^[1]

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1. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

An important measure of the effective current drive by helicity injection into spheromaks and spherical tori is provided by the flux amplification factor, defined as the ratio between the closed poloidal flux in the relaxed mean field and the initial injector vacuum poloidal flux. Flux amplification in magnetic helicity injection is governed by a resonant behavior for Taylor-relaxed plasmas satisfying j=kB. Under the finite net toroidal flux constraint in a spherical torus (ST), the constrained linear resonance k{sub 1}{sup c} is upshifted substantially from the primary Jensen-Chu resonance k{sub 1} that was known to be responsible for flux amplification in spheromak formation. Standard coaxial helicity injection into a ST operates at large M, with M the characteristic dimensionless parameter defined as the ratio between the toroidal flux in the discharge chamber and the injector poloidal flux. Meaningful flux amplification for ST plasmas is limited by a critical k{sub r} at which edge toroidal field reverses its direction. The k{sub r} is downshifted from k{sub 1} by a small amount inversely proportional to M. The maximum flux amplification factor A{sub r}{identical_to}A(k=k{sub r}) scales linearly with M. At the other end of k, substantial flux amplification A(k=k{sub o}){approx}1 becomes available for k{sub o} that scales inversely proportional to M, a significant departure from that in spheromak formation. These important parameters follow the inequality k{sub o}<k{sub r}<k{sub 1}<k{sub 1}{sup c}. Even though A{sub r} is greater than M in a typical ST, detailed q-profile considerations further constrain the maximum useful flux amplification factor in a ST to be smaller than M. The scaling laws are given analytically in the asymptotic limit of M>>1, but numerical solutions indicate that they are useful even for M{approx}1.

• OSTI ID: 20736495
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 4 Vol. 12; ISSN PHPAEN; ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
AMPLIFICATION
HELICITY
MAGNETIC FLUX
PLASMA
PLASMA CONFINEMENT
SPHEROMAK DEVICES
TORI