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icc2011_us_japan_ct_workshop_nagata.pdf2011-09-27 21:22:33Masayoshi Nagata
icc_ctworkshop2011_nagata.pdf2011-09-27 21:22:15Masayoshi Nagata

Flow and dynamo measurements in the HIST double pulsing CHI experiment

Author: Masayoshi Nagata
Requested Type: Consider for Invited
Submitted: 2011-06-11 02:51:16

Co-authors: T. Higashi, M. Ishihara, Y. Kikuchi, N. Fukumoto, T. Kanki

Contact Info:
University of Hyogo
2167 Shosha
Himeji, Hyogo   671-228

Abstract Text:
Flux amplification and sustainment of the spherical torus (ST) configurations by Multi-pulsing Coaxial Helicity Injection (M-CHI) operation have been demonstrated in the Helicity Injected Spherical Torus (HIST) device. The refluxing scenario of CHI has been proposed to achieve a quasi-steady sustainment of a high-temperature spheromak with partial recovery of plasma current between field-building pulses [1]. Successive multi-pulsing discharges on SSPX at LLNL were demonstrated to maintain the magnetic field of spheromak with a high electron temperature in a quasi-steady state [2]. To explore the usefulness of the M-CHI for the ST configurations, we have tested double pulsing operations on HIST. In the double pulsing discharges, we have observed that the plasma current has been effectively amplified against the resistive decay and then the life time has increased up to ~10 ms which is longer than that in the single CHI case.
In the discharges, we have measured the radial profiles of the poloidal and toroidal flow velocities and the electron density during the sustainment and decay phases. The result from the measurements shows that poloidal shear flow exists between the open flux column and the most outer closed flux surface. The poloidal velocity shear may be caused by the diamagnetic drift of ions, i.e. the gradient-driven velocity, because of a steep density gradient there. This flow shear is enhanced and the parallel flow is reduced by the second CHI pulse. The open flux column in the high-q ST configurations has a diamagnetic toroidal magnetic field structure. During the sustainment phase, the ion flow is driven in the same direction as the current in the column region. This may be consistent with electron locking model [3]. The measured radial electric field is determined by the flow velocities and the ion pressure gradient through the radial momentum balance equation.
We have investigated MHD and Hall dynamo mechanisms for current/flux amplifications by using dynamo and Hall probes. The MHD dynamo electric field arises from the correlation between the fluctuating flow velocity and magnetic field. It has been found that a measured fluctuation-induced electromotive electric field sustains the field-aligned current against resistive decay in the parallel Ohm’s law based on the MHD dynamo model. We will understand further about the turbulent transport from the open flux column (anti-dynamo) to the core region (dynamo) by measuring how or where hall dynamo processes become dominant. We will present recent results from the experiments and 3D-MHD numerical simulations (MHDTM-code).

[1] E.B. Hooper, Plasma Phys. Control. Fusion 53, 085008 (2011).
[2] B. Hudson et al., Phys. Plasmas 15, 056112 (2008).
[3] T.R. Jarboe et al., Nucl. Fusion 51, 063029 (2011).

Characterization: D5

This abstract is assigned for the US-Japan CT workshop.

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011