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Internal Magnetic Structure and Electric Fields in the Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI)

Author: Aaron J. Redd
Requested Type: Consider for Invited
Submitted: 2006-12-18 13:23:02

Co-authors: T.R.Jarboe, R.Z.AboulHosn, C.Akcay, W.T.Hamp, G.Marklin, B.A.Nelson, R.G.O'Neill, R.Raman, P.E.Sieck, R.J.Smith, B.T.Stewart, and J.S.Wrobel

Contact Info:
University of Washington
Univ of Washington, Box 352250
Seattle, WA   98195-2
USA

Abstract Text:
The Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI)
spheromak experiment [Sieck, Nucl. Fusion v.46, p.254 (2006)] addresses
critical issues for spheromaks, including current drive, high-beta operation,
confinement quality and efficient steady-state operation. HIT-SI has a
"bow-tie" shaped axisymmetric confinement region (major radius R=0.33 m,
axial extent of 0.57 m) and two half-torus helicity injectors, one mounted on
each end of the flux conserver. HIT-SI has produced spheromaks with up
to 30 kA of toroidal current, using up to 6 MW of applied power, demonstrating
that Steady Inductive Helicity Injection can generate and sustain discharges
with modest power requirements.

A recently-published model [Jarboe, Phys.Rev.Lett. v.97, p.115003 (2006)]
provides a mechanism for calculating the fully-relaxed (Taylor) state field
throughout the HIT-SI volume. This model only requires as inputs the magnitude
of the even-symmetry (spheromak) component and the magnitudes of the two
odd-symmetry injector components. The injector magnitudes are straightforward
to measure from the injector currents and fluxes. The even-symmetry magnitude
is determined using external measurements of the n=0 averaged poloidal fields.
Data will be shown that demonstrate remarkable agreement between the model
prediction and independent internal and external measurements of the magnetic
field. This agreement suggests the HIT-SI plasma is very close to the
fully-relaxed state, and so the large toroidal plasma currents observed in
HIT-SI (relative to the injector currents) are indicative of formation of a
spheromak separatrix. External measurements will be presented which also
indicate the presence of a spheromak equilibrium.

Also, internal electric field measurements indicate that a significant fraction
of the applied injector loop voltage is dissipated in the helicity injectors.
However, this dissipation voltage appears to be independent of either the
applied loop voltage or the injector flux. Thus, increases to the driving
loop voltage will produce significant increases in the helicity injection rate
and injector-applied power.

Characterization: A3,E3

Comments:
If appearing as a poster, please place next to other HIT-SI posters.