Physics and technology of high-voltage pulsed plasmas

1. Physics and technology of high-voltage pulsed plasmas Bob Merlino Plasma Physics Seminar April 5, 2010

2. outline • Controlled thermonuclear fusion • Early work (1950’s) • Stellarators at PPPL • Linear (Z) pinch • Toroidal pinch • Magnetic mirrors • High voltage linear q pinch • Implosion or shock heating • Pulsed power technology • HV toroidal q pinch

3. reaction rate (cm3/s) ion temperature (keV) controlled thermonuclear fusion D + T   (3.5 MeV) + n (14.1 MeV) • heating • confinement • Lawson criterionfor ignition:nt> 1014 cm3 s - n is the plasma density - tis the confinement time

4. ITER – Cadarache, France

5. 2 i 1 minor cross section early work in magnetic fusion • Project Matterhorn begun in 1951 at Princeton University as a classified project • Problem with toroidal devices is that magnetic field is non-uniform (stronger on inner radius than outer radius)  ultimately this leads to plasma moving outward • loss of confinement “pump-out” • First experiments at PPPL studied confinement in Stellarators – toroidal magnetic field with rotational transform i

6. Lyman Spitzer, Jr. CONFIDENTIAL: Such material would cause damageor be prejudicial to national security if publicly available. - US Atomic Energy Commission – AEC - US Energy Research and Development Administration (ERDA) - US Department of Energy (DOE) 1977

7. Model A Figure 8 Stellarator 1954 Spitzer invited Van Allen to come to Princeton in 1953-54 to initiate an experimental program. It was Van Allen’s idea to start with a table-top device to demonstrate the Stellarator concept.

8. Racetrack Stellarator

9. HV capacitor bank JxB Plasma heating in a linear (Z) pinch • Pass a large current through a gas making a linear discharge • JxB force causes pinching • Current heats the plasma • if dI/dt large enough, plasmas with keV temperatures can be formed • how fast? dI/dt ~ 1013 A/s • Since V = L dI/dt, very high voltages and low inductances are required • Problem is that linear pinch is unstable to sausage and kink instabilities

10. ZETA - toroidal pinchAtomic Energy ResearchEstablishment, Harwell, UK Toroidal pinches • Even if the linear pinch were stable, it could never be a viable fusion device because the electrodes introduce impurities that would cause the plasma to radiate away a large fraction of its energy • Must consider closed magnetic configurations with no internal electrodes • Zeta in UK • Perhapsotron at Los Alamos • These are “stabilized” pinches due to presence of toroidal magnetic field

11. Other Plasma heating devices • work at Los Alamos. the Naval Research Lab in DC and the Univ. MD concentrated on methods of producing hot plasmas in linear q pinch devices • groups at Oak Ridge National Lab and Lawrence Livermore National Lab investigated heating and confinement in magnetic mirrors

12.  pinches • simple straight solenoidal magnetic field line devices • magnetohydrodynamically stable • produce very hot plasmas  Ti ~10 keV • very high efficiency for conversion of electrical energy into plasma energy • inherently a pulsed device • confinement limited by plasma squirting out the ends

13. Maryland High Voltage q Pinch one-turn solenoidal coil pre-ionized plasma Pyrex or quartz vacuum vessel Main capacitor bank • solenoid coil was 1m long by 46 cm id • plasma formed in H2 or D2 at P ~ few mtorr • preionization done by discharging capacitor ringing at 150 kHz • initial plasma “fully ionized” at n ~ 1013 cm3 with +/- 400 G bias field • main bank: 250 kV, dI/dt rises in < 10 nsec giving dB/dt ~ 1010 G/s

14.  pinch physics Bo BMain induced current in plasma sheath radially inwardJ x B force • plasmas are diamagnetic, so • currents are induced on the surface • the J x Bforce makes plasma implode, causing it to be heated – shock or implosion heating  snowplow model • to exclude field penetration • plasma resistivity allows magnetic field to penetrate

15. pulsed power technology • how do you produce voltage pulses of hundreds of kA with currents of hundreds of kA in fractions of microseconds? • there are no commercially available devices that can provide this • pulsed power technology developed in US and UK (R. A. Fitch) for Radar in WW II • in US, main development program was at Sandia National Lab

16. 3 mm polyethylene  pinch HV technology oil 2500 g H2O + CuSO4

17. output switch L O A D inversion switch (spark gap) crowbar switch with crowbar I DC with out crowbar t Marx generatorcharge capacitors in parallel then switch to series to get voltage multiplication Marx output voltage = N x # capacitors

18. Magnetic field diagnostics B 4 mm

19. B Main implosion field antiparallel to initial bias field 0 Main implosion field parallel to initial bias field Magnetic field profiles

20. 20 cm 0 -20 cm -70 cm 0 +70 cm Reconnection and tearing in q pinch reversed or antiparallel field configuration t = 440 ns J. H. Irby, J. F. Drake, and H. R. Griem, Phys. Rev. Lett.42, 228 (1979)

21. confinement in  pinch • limited by end loss • e.g., L = 1 m, Te = 1 keV, tconf ~ L/Cs,Cs = 2x105 m/s  tconf ~ 5 ms • suppose density was ~ 1016 cm-3, how long would a  pinch have to be to satisfyn tconf > 1014 cm-3 s ?  tconf = 0.01 s L ~ 2 km • LANL tried plugging the ends with solid plugs of quartz or lithium-deuteride

22. HV generator in transformer oil tank with water + CuSO4 MD High Voltage Toroidal q pinch • goal was to study implosion heating in a toroidal device • determine if Te Ti when fast electron endlosses were eliminated • compliment to SCYLLAC at LANL

23. THOR

24. THOR electrical schematic

25. HV pulsers: swinging LC generators • Charge capacitors in series • use inductors and switches across every other capacitor module to reverse polarity • gives voltage multiplication • use 6 generators connected to toroidal coil with parallel-plate transmission lines • Total energy: 600 kJ* • output voltage: 580 kV • output current: 3 MA in < 1 ms • dI/dt = 6.0 x 1012 A/s • built by Maxwell Labs * 3400 lbs moving at 65 mph

26. Coil current Neutron emission soft x-ray emissionthick Al filter soft x-ray emissionthin Al filter time (ms) Neutron and x-ray emission

27. Magnetic field profiles in toroidal q pinch Antiparallel Parallel

28. toroidal q pinch: conclusions • Electrons attain higher temperatures as compared with linear q pinch, now Te Ti • Implosion, shock or turbulent heating processes remain effective in toroidal geometry • However, attempts to achieve MHD equilibrium through the use of vertical fields and toroidal currents failed • Plasma abruptly drifts to outer wall after implosion phase