The U.S. fusion community continues to focus effort on studying the physics of disruptions in tokamaks, and on finding ways to mitigate their deleterious consequences. The emphasis falls into four categories:

·

Disruption characterization (i.e. experimental measurements)

·

Modeling of current quench, halo currents, and forces

·

Disruption mitigation, killer pellet injection, and modeling

·

Runaway electron generation and modeling

I.

Disruption characterization

I.1)

Install and test AXUV diode array diagnostic on DIII-D for fast disruption radiometry.

Status: Phase I test AXUV array being fabricated with installation scheduled for spring of 98. Engineering design completed and vacuum components being fabricated and tested. Preliminary design reviews completed.

I.2)

Document post-thermal quench characteristics on Alcator C-Mod, paying particular attention to core and SOL temperatures, for use in future modeling efforts.

Status: Now planned for 1998 run campaign. Was planned for 1997, but delayed due to technical difficulties with edge/x-point Thomson scattering system.

I.3)

Investigate non-axisymmetry of plasma column during current quench.

Status: Proposed for future work ( > 1998) using a fast-framing camera. Goal is to test hypothesis of Pomphrey concerning 1/1 and/or 2/1 distortion of plasma surface as the cause of halo current asymmetry.

II.

Modeling of current quench, halo currents, and forces

II.1)

Apply models of disruption-driven halo currents to Alcator C-Mod and other larger tokamaks to understand the difference between halo currents on different machines.

Status: In progress; report on application of model to various machines expected by December 1998.

II.2)

Update the ITER conducting structure and poloidal field coil models in TSC, and cross-check against the latest SPARK model.

Status: Completed.

II.3)

Analyze six ITER disruption and VDE scenarios with TSC.

Status: Completed; memo written.

II.4)

Perform scans of halo temperature and width for several ITER disruption and VDE scenarios with TSC

Status: This has been completed and a report has been written.

II.5)

Perform scans of halo temperature and width for several ITER disruption and VDE scenarios using the DINA code.

Status: Completed; results reported in August, 1997 (GA-C22692). Disruption scoping studies were performed for both VDE's and major disruptions using the DINA code. The results predicted halo currents on ITER that are significantly lower than predicted by the multi-machine database and within the ITER allowables.

II.6)

Analyze DIII-D major disruptions and VDE's with DINA (combined with preceding task)

Status: Completed; results reported in August, 1997 (GA-C22692). Comparison of DIII-D triggered VDE experiments with corresponding DINA simulations showed that DINA disruption models were able to reproduce experimental halo current and plasma geometry reasonably well. The best simulation of halo current evolution (using equal plasma and halo temperatures to minimize the number of free parameters) was achieved with approximately constant halo+core plasma area.

II.7)

Develop analytic model of axisymmetric disruption halo currents.

Status: Completed; Analytic model development complete and comparison with DIII-D experiments performed. The peak axisymmetric halo current is found to be determined primarily by the core current decay rate and the vertical instability growth rate. Results presented at 1997 APS meeting and a publication is now in the review process.

II.8)

Perform comparisons between analytic axisymmetric halo model and the DINA code results (combined with preceding tasks)

Status: Completed; results of comparison between model and code reported in August, 1997 (GA-C22692). Comparisons of VDE scoping study using DINA code with predictions of a simple circuit model of halo current evolution show good agreement.

II.9)

Analyze common ITER cases with DINA and TSC to verify consistency of their respective models.

Status: Completed; results reported in August, 1997 (GA-C22691). Results showed relatively poor quantitative agreement between the two simulations (factor of three), however, it was confirmed that both codes are driving the halo currents consistent with the present physics understanding of halo sources. The differences are attributed to the details of the plasma evolution and point to the importance of performing scoping studies to determine the proper range of halo currents rather than relying on a limited number of simulations.

II.10)

Determine poloidally- and toroidally-dependent halo current ``footprints'' for several TSC cases using a halo current asymmetry model.

Status: A Nuclear Fusion paper will appear shortly with the results of this work.

III.

Disruption mitigation, killer pellet injection, and modeling

III.1)

Explore the use of a massive gas injection for disruption mitigation.

Status: Injection of massive helium gas puff into a VDE was performed on DIII-D in Feb. 1998. Reduction of halo currents was observed. Analysis in progress. Analysis of experimental results is ongoing, and further experiments are planned. Reporting of results expected by Dec. 1998.

III.2)

Physics design of a fast plasma shutdown scenario for ITER.

Status: This work is continuing. The goal is to find a pellet injection sequence for shutting down an ITER discharge safely.

III.3)

Analyze DIII-D impurity-injection scenarios with DIII-D pellet ablation/radiation code. Model role of radiation-induced pressure-driven instabilities and rapid cooling of bulk electron temperature on the production of runaway electrons.

Status: Initial report completed in August, 1997. Two publications in review on pellet/ablation/radiation code and on anomalous pellet penetration during experiments on DIII-D. Proposed work in 1998: New experimental runaway generation results obtained with argon killer pellets will be compared with predictions from the DIII-D pellet ablation/radiation code (KPRAD). The experiments suggest that a more advanced ablation light model is required in order to connect the measured ablation signal with the argon ablation physics. Our goal is to implement a more advanced ablation light model in KPRAD and compare it with the experimental argon killer pellet results. Expect to complete KPRAD ablation light model and benchmark by October 1998. Analysis of experimental argon killer pellet results expected by December 1998.

III.4)

Liquid jet injection for disruption mitigation.

Status: Considerable work has been done on this task and the results have been reported in a recent Nuclear Fusion article and in August, 1997 (GA-C22682). It is expected that this work will continue in 1998.

III.5)

Inject impurities into SOL/halo region to reduce halo current.

Status: Proposed for 1998; based on modeling results of D. Humphreys, which indicate that increasing the SOL/halo resistivity should reduce halo current magnitude.

III.6)

Analyze TFTR and DIII-D impurity-injection scenarios with TSC.

Status: This is on hold because of the departure of Z. Chang.

III.7)

Massive deuterium killer pellet experiments on Alcator C-Mod.

Status: Completed July, 1997. Near simultaneous injection of 20 D₂ pellets raised density up to 2×10²¹ m^-3, causing disruptions, but no acceleration of quench was observed, nor any mitigation of halo currents. Reported at disruption meeting at DIII-D, July 1997.

IV.

Runaway electron generation and modeling

IV.1)

Perform experiment to characterize disruption-generated runaway electrons and investigate their suppression by magnetic perturbations.

Status: Experiment was performed in Feb. 1998. Runaway electrons successfully generated in pre-emptive disruptions using argon ``killer'' pellets. Mitigation using externally applied magnetic perturbations was explored. A test of using lithium pellets in the current quench to mitigate the runaways was also tried. Analysis of results of experiment is ongoing, with completion expected by December 1998.

IV.2)

Test diagnostic for detection of relativistic runaway electrons by means of their synchrotron emission using a ``Two Color'' fast IR diode array.

Status: Diagnostic installed in Sept. 1997 and experiment was performed in Feb. 1998. Runaway electrons successfully generated in pre-emptive disruptions using argon ``killer'' pellets. IR diode signals were detected during disruption thermal and current quench phases. Hard x-ray pickup and other sources of noise were successfully eliminated. IR signals being analyzed and interpreted by UCSD group.

IV.3)

Develop model of runaway-electron generation in fast-shutdown scenarios.

Status: Using the bounce-averaged Fokker-Planck code, CQL3D, it was shown that even when the electric field is small for production of Dreicer runaways, knock-on collisions of energetic electrons with the bulk electrons can produce significant runaway electrons. It was also shown that a burst of Dreicer runaways may occur if the cooling rate due to the radiation from impurity pellets is rapid enough. The results were reported in GA-A22678 and are now in the journal review process. Work on the effect of magnetic perturbations on the confinement of these runaways was presented at the 1997 APS meeting. It is expected that this work will continue in 1998.

IV.4)

Develop runaway electron model for TSC.

Status: This is delayed because of the departure of Chang.

IV.5)

Review and summarize experimental data on runaway electrons.

Status: Completed; report submitted June, 1997. Recent data from TEXTOR, JET, JT-60U, and DIII-D was examined. Some modeling results were also reported.

IV.6)

Investigate core turbulence levels during post-thermal quench phase on Alcator C-Mod.

Status: Proposed for future work ( > 1998) using new fluctuation diagnostics being installed now on C-Mod. The goal is to answer questions concerning the re-healing of flux surfaces after the thermal quench, which bears on the issue of runaway avalanching.