1. Runaway Electron Mitigation Collaboration on J-TEXT David Q. Hwang UC Davis Sixth US-PRC Magnetic Fusion Collaboration Workshop Collaborating Institutions:

2. Application of accelerated CT for Runaway Electron (RE) mitigation RE Simulation** ** Smith, H.M. et al. Plasma Phys & Control Fusion 51(2009) 124008

3. Comparison of stopping power of RE in various noble gas media by collisional (dashed lines) and bremsstrahlung cooling (solid lines) * Bakhtiari, M., G.J. Kramer, M. Takechi, H. Tamai, Y. Miura, Y. Kusama, and Y. Kamada, Role of Bremsstrahlung Radiation in Limiting the Energy of Runaway Electrons in Tokamaks. Physical Review Letters, 2005. 94(21): p. 215003. Theoretical Comparison of Bremsstrahlung vs. Collision*

4. Experimental setup for Present CT Injector

5. The CTIX injector is unique: (1) the injector operates repetitively, (2) breakdown and formation are initiated by fast gas injection, and (3) the acceleration bank is delayed and switched with saturable core inductors. Formation Circuit Saturable core inductor. Acceleration Circuit Injector 0.15 micro F 50 kV 50 micro F 40 kV 720 kA 50 micro F 40 kV 720 kA 0.15 micro F 50 kV 100 ohm 500 ohm Power supply 20 kV, 8 kW Power supply 20 kV, 8 kW

6. CT Penetration of Vacuum Magnetic Field * Hwang et.al, Nucl. Fusion 40,#5 (2000)

7. Ratio of Magnetic Field to Wave Forces Plasma-beta F wave / F field * * Newcomb MHD model Phys. : Of Fluids B3 (1991)

8. Curves of constant magnetic field corresponding to kinetic energy density equaling magnetic energy density show tokamak fueling/impurity injection requirements in this plot of CT velocity vs. mass density.

9. Additional Magnetic Perturbation Effects by CTs Compressibility of CT leads to increase of internal magnetic field at stopping location Condition of stopping is equilibrium of the internal and external magnetic pressure CT resistivity leads to reconnection of the CT field and remaining tokamak field. The reconnection will spoil the tokamak field and limit the RE energy (similar to edge magnetic resonance RE mitigation)

10. Accelerator He Injection No Gas Injection He Injection

11. Rutherford Backscattering (RBS) spectrum after 20-shot run with Kr puff Si-28 Cl-35 Cr-52 Fe-56 Cu-63 Kr-84 Au-197

12. New Collaborative Investigation in Runway Electron Mitigation (REM) on J-TEXT Relativistic theory shows RE stopping force by bremsstrahlung cooling can be more effective than collisional stopping of RE. The effect is more efficient at higher RE energy found in larger tokamaks such as EAST, ITER Mitigation most effective by delivery of high-Z ions on the magnetic axis Compact Toroid can deliver noble gas ions to magnetic axis in  sec time scale. Collaborative project of high-Z CT injection of J- TEXT disruption studies HEEM Test facility for diagnostics calibration and simulation code benchmark

13. JCT injector on J-TEXT Characteristics of JCT injector Conical Electrodes Initial inner diameter 0.4 m Initial outer diameter 0.5 m Final inner diameter 0.2 m Final outer diameter 0.4 m Straight length 0.5 m Taper length 2 m Stored capacitor energy 250 kJ (formation) 500 kJ (acceleration) Peak current 500 kA (formation) 2 MA (acceleration) CT composition H 2 + (Ne, Ar, Kr, Xe) JCT pulsed-power (PFN, switches) JCT pulsed-power (PFN, switches) JCT injector

14. High Energy Electron Magnetized (HEEM) test facility for X-ray diagnostics calibration and simulation code benchmarking Specifications: 1. Transverse magnetic field: 0.5 T to 1 T 2. Energetic pulsed electron beam: ~1 MeV at a current of 1 A 3. HEEM e-beam pulse length: ~10 μs 4. High-Z CT electron density: ~10 15 cm -3 5. High-Z CT velocity: ~100 km/s 6. High-Z CT noble gas species: He, Ne, Ar, Kr, Xe Purpose: 1. High-Z CT penetration of a transverse magnetic field 2. High-Z CT interaction with an energetic electron beam 3. Testing and calibration of JCT injector, X-ray diagnostics and data acquisition system 4. Simulation code benchmarking

15. J-TEXT Collaboration in REM RE dominated on magnetic axis profile CT penetration to tokamak center CT penetration model determined Internal CT field can spoil the RE acceleration path via reconnection CT deliver high-Z noble ions in msec time scale New REM method via Bremsstrahlung cooling International collaborative experiment on J-TEXT using US JCT injector at RE up to 5 MeV HEEM test stand for injector, diagnostic calibration and Code benchmark at 1 MeV