DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	131: Understanding runaway equilibrium to minimize damage hazard
Name:	Alex James jamesan@fusion.gat.com	Affiliation:	Facebook
Research Area:	Disruption Mitigation	Presentation time:	Requested
Co-Author(s):	--	ITPA Joint Experiment :	Yes
Description:	The goal of this proposal is to investigate runaway equilibrium to improve the understanding of radial position control of runaway electrons after rapid-shutdowns. Radial control of runaway electrons is important for ITER and other large tokamaks, where position control and rampdown of runaway current may be an important component of the disruption mitigation system. A theory of the plasma/runaway beam system was previously developed [Z. Yoshida, J. Phys. B, 1989], but has not been experimentally investigated before at parameters close to DIII-D or ITER. The proposed experiments will study the equilibrium by measuring the current profile, and observing the effect of radial position actuators. Data from these experiments will also supplement a study of the critical density and electric field for avalanche suppression proposed by R. Granetz as an ITPA joint experiment.	ITER IO Urgent Research Task :	Yes
Experimental Approach/Plan:	Impurity-free runaway discharges will be established by initializing to just above the critical density for avalanche, then either: a) holding at constant loop voltage while the density is gradually dropped, or b) holding at constant density while the loop voltage is gradually increased until the discharge runs away. By avoiding shutdown gas injection and atomic line emission interfering with MSE, the current profile evolution will be measured throughout the runaway portion of the discharge. Visible synchrotron emission will be compared with projected q-contours to study runaway confinement. If necessary after the runaway discharge is established, thermal plasma current can be further suppressed by injecting deuterium or a small amount of impurity gas such as helium or nitrogen, as long as the MSE system continues to operate normally. After a discharge is established with low thermal current, actuators such as current in positioning coils and loop voltage can be scanned to study their effects on the equilibrium shape, position, and current and density profiles. The observed effects will be compared to results from the ORNL VMEC code to solidify understanding of experimental observations, and to revise theory if necessary.
Background:	Runaway discharge position control has been demonstrated in the vertical direction, but not in the radial direction. Prior efforts at radial position control met difficulties separating the LCFS from the inner wall, which was achieved only by ramping up the current. The plasma-beam equilibrium was investigated at least in the 1989 article by Zensho Yoshida, and at the small TBR-1 tokamak in Brazil.
Resource Requirements:	cryo-pumping
Diagnostic Requirements:	MSE, UCSD fast camera Thompson, density reflectometer
Analysis Requirements:	ORNL VMEC for studying runaway equilibrium response to actuators EFIT modified to include the beam pressure
Other Requirements:	--