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Title: Observation of a pressure limit in a gun-driven spheromak

Abstract

Density in a spheromak is lowered by action of a m/n=2/4 island, after which the electrons heat rapidly to give profiles of nT{sub before}=nT{sub after}, indicating that the spheromak heats up to a pressure limit. The spheromak core remains at T{sub e}{approx}150 eV for {approx}1/2 ms during a period exhibiting low amplitude incoherent fluctuations consistent with a pressure-driven mode, giving the volume-averaged electron beta <{beta}{sub e}>{approx}3% and peak {beta}{sub e} of {approx}6%. Calculations show that measured pressure profiles are close to the Mercier limit.

Authors:
; ; ; ; ; ; ; ; ;  [1]
  1. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550 (United States)
Publication Date:
OSTI Identifier:
20782771
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 4; Other Information: DOI: 10.1063/1.2188086; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLITUDES; ELECTRONS; EV RANGE; FLUCTUATIONS; MAGNETIC ISLANDS; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; PLASMA INSTABILITY; PLASMA PRESSURE; RADIATION TRANSPORT; SPHEROMAK DEVICES

Citation Formats

Woodruff, S., Hooper, E.B., Pearlstein, L.D., Bulmer, R., Hill, D.N., Holcomb, C.T., McLean, H.S., Moller, J., Stallard, B.W., and Wood, R.D. Observation of a pressure limit in a gun-driven spheromak. United States: N. p., 2006. Web. doi:10.1063/1.2188086.
Woodruff, S., Hooper, E.B., Pearlstein, L.D., Bulmer, R., Hill, D.N., Holcomb, C.T., McLean, H.S., Moller, J., Stallard, B.W., & Wood, R.D. Observation of a pressure limit in a gun-driven spheromak. United States. doi:10.1063/1.2188086.
Woodruff, S., Hooper, E.B., Pearlstein, L.D., Bulmer, R., Hill, D.N., Holcomb, C.T., McLean, H.S., Moller, J., Stallard, B.W., and Wood, R.D. Sat . "Observation of a pressure limit in a gun-driven spheromak". United States. doi:10.1063/1.2188086.
@article{osti_20782771,
title = {Observation of a pressure limit in a gun-driven spheromak},
author = {Woodruff, S. and Hooper, E.B. and Pearlstein, L.D. and Bulmer, R. and Hill, D.N. and Holcomb, C.T. and McLean, H.S. and Moller, J. and Stallard, B.W. and Wood, R.D.},
abstractNote = {Density in a spheromak is lowered by action of a m/n=2/4 island, after which the electrons heat rapidly to give profiles of nT{sub before}=nT{sub after}, indicating that the spheromak heats up to a pressure limit. The spheromak core remains at T{sub e}{approx}150 eV for {approx}1/2 ms during a period exhibiting low amplitude incoherent fluctuations consistent with a pressure-driven mode, giving the volume-averaged electron beta <{beta}{sub e}>{approx}3% and peak {beta}{sub e} of {approx}6%. Calculations show that measured pressure profiles are close to the Mercier limit.},
doi = {10.1063/1.2188086},
journal = {Physics of Plasmas},
number = 4,
volume = 13,
place = {United States},
year = {Sat Apr 15 00:00:00 EDT 2006},
month = {Sat Apr 15 00:00:00 EDT 2006}
}
  • In the Sustained Spheromak Physics Experiment, SSPX, progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1-m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations ({delta}B/B{approx}1% on the midplane edge) yields T{sub e} profiles peaked at > 200eV. Trends indicate a limiting beta ({beta}{sub e} {approx} 4-6%), and so we have been motivated to increase T{sub e} by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: (A) Operation with constant current and spontaneous gun voltagemore » fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with {delta}B/B {approx}2% and large voltage fluctuations ({delta}V {approx} 1kV), giving a 50% increase in current amplification, I{sub tor}/I{sub gun}. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX ({approx}0.7T along the geometric axis). By increasing the time between pulses, a quasi-steady sustainment is produced (with periodic good confinement), comparing well with resistive MHD simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.« less
  • In the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, D. Pearlstein, and D. D. Ryutov, Nucl. Fusion 39, 863 (1999)], progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations ({delta}B/B{approx}1% on the midplane edge) yields T{sub e} profiles peaked at >200 eV. Trends indicate a limiting beta ({beta}{sub e}{approx}4%-6%), and so we have been motivated to increase T{sub e} by operating with stronger magnetic field. Two new operating modes are observed tomore » increase the magnetic field: (A) Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with {delta}B/B{approx}2% and large voltage fluctuations ({delta}V{approx}1 kV), giving a 50% increase in current amplification, I{sub tor}/I{sub gun}. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX ({approx}0.7 T along the geometric axis). By increasing the time between pulses, a quasisteady sustainment is produced (with periodic good confinement), comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.« less
  • Recent improvements in the operation of the CTX spheromak device have produced discharges containing evidence for a pressure-driven instability. The instability leads to a distinct event in the discharge, which can be studied in detail. Data are presented which reasonably discount Taylor relaxation of the current profile as the cause of the event. The critical value of the normalized pressure gradient has been measured and is compared to the Mercier limit.