Workshop on Transients

Chair: Charles Greenfield (General Atomics)
Co-Chair: Raffi Nazikian (Princeton Plasma Physics Laboratory)
DOE FES Liaison: Mark Foster

CONTENTS

Panels

The Transients workshop is organized in two panels, each with three sub-panels (Fig. 1). Each sub-panel will consider a complete research program in their area, which in most cases will include elements of experiment, theory, and modeling. Since there are obvious overlaps with the Workshop on Integrated Simulations, where appropriate one member of each panel or sub-panel can be designated to serve jointly on an Integrated Modeling panel.

Our task is largely that of revisiting Thrust 2 in the 2009 ReNeW report, taking into account the ensuing six years of progress – and discovery of new issues. The research plan we will develop may go into more detail than ReNeW, but we are once again being asked to develop a spanning set of research activities rather than prioritizing them.

All sub-panels should consider two time scales for research. The most rapid progress is needed to ad-dress areas that will impact safe operation of ITER. In some cases, additional progress will be needed beyond ITER in order to be able to safely address transients in more demanding future devices such as an FNSF or DEMO.

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Details about the panels


PANEL 1: Preventing device damage from disruptions

Lead: Charles Greenfield (General Atomics)
Co-lead: Dylan Brennan (Princeton University) joint with Integrated Modeling Workshop

Sudden terminations of a fusion grade plasma can be triggered by either MHD instabilities or hardware failure. Developing an approach to preventing damage related to the sudden release of the plasma’s thermal and magnetic energy content will require progress in three broad areas. When a disruption is imminent and unavoidable, a mitigation system will be deployed to safely shut down the discharge. This should be a last resort, with preference to efforts to completely avoid the disruption via plasma control. The decision making - steps to take for avoidance, when to engage the mitigation system, etc. – will require advances in predictive capabilities.


Sub-panel 1. DISRUPTION PREDICTION

Leads: Steve Sabbagh (Columbia) and Chris Hegna (Wisconsin)

Members: P. deVries (ITER), N. Ferraro (GA), J. Ferron (GA), R. Granetz (MIT), S. Kruger (TechX), R. La Haye (GA), D. Maurer (Auburn), B. Tobias (PPPL), K. Tritz (JHU)

There has been significant progress in disruption prediction in individual devices. Examples include the analysis performed on NSTX experiments that can identify most disruptions from the existing database with few false positives. Another system has been deployed on JET that is routinely used in real-time to trigger its mitigation system. Advances are needed to improve the accuracy and reduce the incidence of false positives in such systems through additional measurements and physics models (some capable of computation in real-time) determining proximity to disruption triggering conditions. Improved systems would be aimed to be deployed on present and future tokamak devices such as ITER, or DEMO with an acceptable minimum training set.


Sub-panel 2. DISRUPTION AVOIDANCE

Leads: Ted Strait (GA) and David Gates (PPPL)

Members: J. Hanson (Columbia), S. Gerhardt (PPPL), D. Humphreys (GA), E. Kolemen (Princeton), R. La Haye (GA), M. Lanctot (GA), S. Sabbagh (Columbia), J. Snipes (ITER)

Most disruptions occur when the plasma approaches known stability limits. A challenge for plasma control is to be able to operate near these limits without crossing them. In some cases, active suppression of instabilities can be done under plasma control (e.g. ECCD used to “search and suppress” NTMs in DIII-D).


Sub-panel 3. DISRUPTION MITIGATION

Leads: Val Izzo (UCSD) and Bob Granetz (MIT)

Members: N. Eidietis (GA), M. Lehnen (ITER), R. Raman (Washington), D. Rasmussen (ORNL)

This area is led by the U.S. Burning Plasma Organization Task Group on Disruptions, in turn led by Val Izzo (UCSD) and Bob Granetz (MIT).

As a last resort, the plasma must be safely terminated. Mitigation systems deployed on present-day tokamaks rely on injection of massive quantities of materials to radiate away the plasma’s energy content via gas injection or shattered pellet injection. Other methods have been proposed but studied in much less detail. Several issues remain under study, such as radiation asymmetries and the generation of a “runaway” electron population that can cause severe, local, damage to in-vessel components. This area is time critical as the ITER Disruption Mitigation System (DMS) is scheduled to undergo a final design review in 2017.

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PANEL 2: Avoiding deleterious effects of ELMs in high performance plasmas

Lead: Raffi Nazikian (PPPL)
Co-lead: John Canik (ORNL) joint with the PMI workshop

Edge Localized Modes, or ELMs, are repetitive instabilities that are effective in flushing impurities from the core of H-mode plasmas but are also capable of depositing large amounts of heat and particles in concentrated target areas in the divertor. Several techniques are currently under study for use in ITER and subsequent devices for the suppression or mitigation of these instabilities, and for the development of alternate operational scenarios that are free of ELMs and meet impurity control requirements. Since these areas are also related to the physics of the H-mode pedestal and boundary physics, there may be connections (and joint members) with panels within the Workshop on Plasma-Materials Interactions.


Sub-panel 4. ELM SUPPRESSION OR MITIGATION WITH RESONANT MAGNETIC PERTURBATIONS

Leads: Max Fenstermacher (LLNL) and Oliver Schmitz (Wisconsin)

Members: J.-W. Ahn (ORNL), C.S. Chang (PPPL), T. Evans (GA), N. Ferraro (GA), A. Loarte (ITER), R. Moyer (UCSD), J.-K. Park (PPPL), R. Nazikian (PPPL), C. Paz-Soldan (GA), F. Waelbroeck (Texas)

ELM suppression and/or mitigation with magnetic perturbations generated by externally powered coils has been demonstrated in DIII-D, and subsequently explored on several devices in the US and elsewhere. The promise of this approach led to it’s becoming the lead-ing method for ELM control in ITER, with the planned addition of an internal coil set to the ITER baseline design. While the current progress in experiments is encouraging, the under-standing of the suppression or mitigation mechanisms in a range of existing devices is still insufficient for robust extrapolation to ITER, FNSF or beyond. This panel will address the outstanding issues and research needs in experiment, modeling and theory in this area.


Sub-panel 5. NATURALLY ELM-FREE OPERATING SCENARIOS

Leads: Jerry Hughes (MIT) and Wayne Solomon (PPPL)

Members: K. Burrell (GA), A. Garofalo (GA), G. Huijsmans (ITER), D. Mansfield (PPPL, ret), J. Rice (MIT)

Alternate operational scenarios, such as the QH-mode and I-mode, have been identified which combine the favorable confinement properties of ELMing H-mode plasmas with an edge region that is naturally free from ELMs. This panel will address research needs and identify paths for applying these or similar scenarios to ITER and subsequent devices.


Sub-panel 6. ELM PACING

Leads: Larry Baylor (ORNL) and Gary Jackson (General Atomics)

Members: A. Bortolon (PPPL), N. Commaux (ORNL), S. Diem (ORNL), G. Huijsmans (ITER), T. Jernigan (ORNL, ret), A. Loarte (ITER), D. Mansfield (PPPL, ret), T. Osborne (GA), D. Shiraki (Columbia)

Rapid, repetitive injection of pellets of deuterium or lithium have been shown to trigger ELMs, thereby increasing the ELM frequency and decreasing the impulsive fluxes produced by each individual ELM to a potentially tolerable level. Further research will explore the parameter space where this technique is capable of ameliorating the negative consequences of ELMs in high performance scenarios and to continue to build a scientific basis for extrapolation to future devices.

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