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	191: Critical electric field for runaway electron growth and decay under quiescent conditions
Name:	Carlos Paz-Soldan paz-soldan@fusion.gat.com	Affiliation:	Columbia University
Research Area:	Disruption Mitigation	Presentation time:	Requested
Co-Author(s):	N. Eidietis, R. Granetz, E. Hollmann, A. Tronchin-James, J. Wesley	ITPA Joint Experiment :	Yes
Description:	This experiment aims to study the critical toroidal electric field (Ecrit) required for runaway electron (RE) growth under quiescent plasma conditions. A plasma target developed for extremely low-density operation (125010, n_e < 5E12 cm^-3) will be used. Quiescent RE plasmas are of fundamental interest because they are free from the complicating transient and impurity effects associated with disruption-generated RE beams. To distinguish the observed RE dynamics from the Dreicer mechanism, the plasma current or density will be systematically increased and decreased during the discharge to assess hysteresis in the RE growth and decay (see background). These measurements will ultimately assess and clarify the anomalous RE loss mechanisms needed to match the data. Furthermore, the development of a robust and quiescent plasma target with a significant RE population could lead to significant advances in the science of RE control and measurement. Piggyback studies will use this experiment to improve equilibrium reconstructions of the RE beam. Depending on the robustness and magnitude of the RE beam, experiments suggested in other RoF proposals could also be applied to this target discharge in the future.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	This experiment will begin by reproducing 125010, a discharge developed for error field control (EFC) measurements that was recently discovered to contain a measurable RE population that was quiescent for over 3 seconds without locked modes. Excellent EFC is critical to the success of this experiment, as the error field ultimately limits low-density operation via locked modes. When low-density RE growth is observed, the plasma current (proportional to the toroidal electric field) will be slowly increased and decreased, in order to measure the levels of RE beam growth and decay. The density will then be decreased and increased slowly and the same measurements made.
Background:	The required Ecrit for RE growth is the dominant parameter for gauging whether collisional suppression of the RE beam is likely during MGI. However, observations on C-mod and DIII-D suggest that Ecrit is anomalously large when compared to existing collisional theories [R. Granetz, ITPA 2012], both in quiescent and post-disruption plasmas. Furthermore, the Ecrit required for the suppression of an already-avalanching RE beam can be measured by decreasing the toroidal electric field (Ephi) until RE decay is observed. It is expected that the Ecrit for RE decay will be smaller than for RE formation. Equivalently, the required density for RE formation is expected to be lower than that for RE decay at constant Ephi.
Resource Requirements:	Mostly Ohmic only operation. Beam blips for MSE, 1 gyrotron for possible RE seed generation. 4 SPAs operational.
Diagnostic Requirements:	ZNS or FPLASTIC in low gain mode. Thomson, MSE, SXR, ECE, UCSD fast camera, CO2 and 288 GHz interferometers
Analysis Requirements:	May enable development of new tools for RE equilibrium measurement
Other Requirements:	--