Runaway Electron Mitigation Collaboration on

J-TEXT

David Q. HwangUC Davis

Sixth US-PRC Magnetic Fusion Collaboration Workshop

Collaborating Institutions:

Application of accelerated CT for Runaway Electron (RE) mitigation

RE Simulation**

** Smith, H.M. et al. Plasma Phys & Control Fusion 51(2009) 124008

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*

Experimental setup for Present CT Injector

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 coreinductor.

Acceleration Circuit

Injector

0.15 micro F50 kV

50 micro F40 kV720 kA

50 micro F40 kV720 kA

0.15 micro F50 kV

100 ohm

100 ohm

500 ohm

500 ohm

Power supply20 kV, 8 kW

Power supply20 kV, 8 kW

CT Penetration of Vacuum Magnetic Field

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1

Vf /

Vo

1

target field in Teslas

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1 1

rigid body theory

Internal compressional CT Energy

* Hwang et.al , Nucl. Fusion 40,#5 (2000)

Ratio of Magnetic Field to Wave Forces Fwave

Ffield 4

pct

Plasma-beta

Fwave/ Ffield

*

* Newcomb MHD model Phys. : Of Fluids B3 (1991)

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.

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)

Accelerator He Injection

No Gas Injection

He Injection

1.2x1015

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

elec

trons

/cm

^3

35302520151050µsec

Interferometer at z=142 cm

60304

1.2x1015

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

elec

trons

/cm

^3

35302520151050µsec

Interferometer at z=142 cm

60323

Rutherford Backscattering (RBS)spectrum after 20-shot run with Kr puff

Si-28

Cl-35Cr-52

Fe-56

Cu-63

Kr-84

Au-197

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 msec time scale.

• Collaborative project of high-Z CT injection of J-TEXT disruption studies

• HEEM Test facility for diagnostics calibration and simulation code benchmark

JCT injector on J-TEXTCharacteristics of JCT injectorConical 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 H2 + (Ne, Ar, Kr, Xe)

JCT pulsed-power(PFN, switches)

JCT injector

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 T2. Energetic pulsed electron beam: ~1

MeV at a current of 1 A3. HEEM e-beam pulse length: ~10 μs4. High-Z CT electron density: ~1015 cm-3

5. High-Z CT velocity: ~100 km/s6. High-Z CT noble gas species: He, Ne,

Ar, Kr, Xe

• Purpose:1. High-Z CT penetration of a

transverse magnetic field2. High-Z CT interaction with an

energetic electron beam3. Testing and calibration of JCT

injector, X-ray diagnostics and data acquisition system

4. Simulation code benchmarking

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