Musings on Intense Beam Generation, Propagation and Application + - B.V. Oliver Sandia National Laboratories, Albuquerque, NM 87185

Transcription

1 Musings on Intense Beam Generation, Propagation and Application B.V. Oliver Sandia National Laboratories, Albuquerque, NM Laboratory of Plasma Studies Cornell University Oct. 6, SAND PE Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy s National Nuclear Security Administration under contract DE-NA

2 The Russian Invasion and Electron Hall MHD A host of plasma and pulsed power physicists from Russia descended upon LPS in the late 80s and early 90s, L.I. Rudakov, G. Mesyats, D.D. Ryutov, V. P. Smirnov, A particularly fruitful theoretical interaction on Electron Hall MHD began in the Summer of L. I. Rudakov introduced us to the convective skin effect and KMC (Kingsep, Mokhov and Chukbar) shock wave field penetration. A collisionless fast field penetration mechanism driven by field advection with the electron fluid u = 1 λ 1 n Ω e n x which could be much faster than the diffusive velocities (η/t) 1/. We also considered the role of electron inertia.which is important near boundaries and in developing high k spatial structures (vorticies) on order the collisionless skin depth λ = c and again could result in field penetration at velocities significantly faster than diffusion This initial work carried me into the realm of current neutralization of beams and rings in plasma. Everything I needed to know, Rudakov summarized in ω pe 1. L.I. Rudakov, Macroscopic instabilities of a high-current beam in a gas in a guiding magnetic field, Sov. J. Plasma Phys., 14, 1988

3 Beam neutralization in plasma when plasma ions are nearly immobile (m i ) e - H + e - e H + Ion-beam attracts plasma electrons Ohm s law Creates a plasma channel so beam + plasma is quasi-neutral: n i - n e + n b 0 v e j e = (E+ B); σ = conductivity c j b 4π B= j +j ; j 0 j e c e b i 1 B B j E=- ; c t t t dv m i = q(e + v x B) dt Hybrid fluid/pic codes were used to study beam and plasma dynamics Plasma return current driven by binductive electric field and determined by generalized Ohm s law 3

4 Beam injection into plasma filled magnetic lenses 1 Field line advection, whistler wave generation and current diffusion all fun stuff B c 4π c 4π = - B - jb B + B - jb t 4πen e c 4πσ c Hall current advection Resistive diffusion 1. B.V. Oliver and R.N. Sudan, Phys. Plasmas 3,

5 South to the Naval Research Labs: Ion Beams and Rod-Pinches Arrival on stirring day! Self-pinched beam equilibria in vacuum.which followed the laminar equilibria of Creedon 1 from magnetic insulation theory. 1. John Creedon. JAP 46, 946 (1975) e v v m 4e (n Zn b ) B 4 c j b en v v c B Momentum Poisson Ampere - 1; 1/(1 - v /c ) Cons. of energy, b=mc /4e n b 1 d r 1 b r dr 1 I net (I b ) 1/..And, Gerry Cooperstein s favorite diode, the Rod-Pinch. 5

6 The Rod-Pinch diode: an example of selfmagnetically insulated flow with ions Diode current well modeled by critical current formulation 1 :. I αi,.0 α.6 crit γ 1 Icrit 8.5 ka, γ 1 ev/mc ln r / r c a Operation and is described by self-insulated flow theory with the inclusion of ions cathode anode r s rod-pinch diode e - flow B ion flow Region I Region II Region I Region II n e - n i, - n i. B = neve rc + ve B=0, niv i j c r Ions are absolutely necessary for operation! 3/ 4 (γa γs) Ji. Child Langmuir like 9 (r r ) I 17 r s s J i a [γ a 1] 1. G. Cooperstein et al. Phys. Plasmas, 8, 4618 (001). B.V. Oliver et al. Phys. Plasmas, 11, (004); 1/4 ka. R (mm) Normal Rod pinch diode Anode Cathode V=4.1 MV I=78 ka Virtual Cathode ( E ~0 ) Surface 58 ka X-ray Z (mm) Fig. courtesy of S. Swanekamp, NRL 6

7 Electron backscatter can be significant in cylindrical diodes, results in decreased but stable impedance! cathode No static solution. Current oscillates CL ions primary e- scattered e- anode j j cl E scat 1 9π α scat j βe ev m incident incident V d r c r a =0 =1 As the fraction of reflected beam current goes up, so does the total current. However, there is a maximum and the current is stable. 1. N.R. Pereira, JAP 54, 6307 (1986) D. Mosher, G. Cooperstein et al, Proc. 11 th Intl. Beams Conf. (1996) V. Engelko, V. Kusnetsov et al. JAP 88, 3879 (000) B.V. Oliver, T.C. Genoni et al., JAP 90, 4951 (001) 7

8 Magnetized e-beams for x-ray radiography applications The Immersed B z diode 1 : the electron beam is created in the accelerating gap of a high current diode and guided in vacuum to an anode/target via an applied Solenoidal magnetic field Anode Cathode MITL Needle cathode A-K gap Anode/ target Bremmstrahlung x-rays are created when the e-beam is stopped in a high atomic number converter. cathode e - x-ray Energy E b = -10 MeV, Current I b = ka, Pulse length t b = ns 1. M.G. Mazarakis et al. Appl. Phys. Lett 70, 83 (1997) anode target 8

9 Beam target interactions led to development of 3-D simulations (LSP) and nonlinear modeling of ionhose instability. Immersed-B 1 : Diode used for creating high intensity bremsstrahlung radiation. Beam spot on target determined by ion-hose saturation amplitude c I rsat I e b A Cathode needle Anodetarget A-K gap <r> <y> <x> E-beam Target ions e-beam offset and rms radius cathode anode 3-D PIC simulations of immersed-b diode electron and ion dynamics 1. M.G. Mazarakis et al. Appl. Phys. Lett 70, 83 (1997) D.R. Welch et al. Laser and Particle Beams 16, 85, (1998) 9

10 Paraxial diode: a classic beam propagation problem in overdense plasma n b /n e << 1. Gas-cell acts as a ¼ betatron focusing lens 1. Gas breakdown sufficient for complete charge neutralization but incomplete current neutralization. d r 1 I ε - +, I I I dz r I r b net 3 b A b net b plasma Beam density contours from Lsp simulations For << R I net /I A R I I A F, I net net I = γβ 17 (ka), A ε = 4 <r ><r >-<rr > R F Contours of I net Net current (beam + plasma) I net = crb / Focal sweeping due to time dependent net current is the primary contributor to larger than desired time integrated spots. I b =3 ka I net = 13 ka 1. B.V. Oliver, D. Short, G. Cooper and J. McLean, IEEE Trans. on Plasma Science 33, 704 (005). 10

11 Plasma cells have advantages provided if one controls kinetic effects and anomalous resistivity. Theory/simulation 1 of cross field plasma currents show susceptibility to unstable Bernstein modes (Resistance as high as 60x classical) ( ) ( ) 1, e i pe n e In e pi n e In i e n1 kv ( ) i n 1 ( n i ) d ne Resistivity nearly classical for: < 0.5 ka at cm -3 plasma density < 10 ka net current at cm -3 density Ion phase space for kg, cm -3 plasma density Peak Collision Freq. / ie 1. Welch, Genoni, Oliver et al., Phys. Plasmas 13, (006) R v d e /v th ie 11

12 Coming full circle:.plasma cells are nice but electron advection from the boundaries still causes issues. Bryan, the future of plasma physics is that you ll do the same problem on a bigger computer!, Rod Mason, 1994 Net current grows near target region. This is due to electron inertial effects at wall 1 and advection with the plasma return current electrons electron configuration space Contours of I net A-K gap A-K gap R Plasma cell Plasma cell I net ~0 ka Z Simulations of 0 torr H, 1.3% ionized (10 16 cm -3 ) 1 B. V. Oliver, L. I. Rudakov, R. J. Mason, and P. L. Auer, Phys. Fluids B 4, 94 (199). A.S. Kingsep, L.I. Rudakov and Chuckbar, Sov. Phys. Dokl 7, 140 (198) 1

13 Now...a partial lobotomy and oversite of pulsed power facilities Nothing good can come from a theorist having oversight of pulsed power facilities in the 1-0 TW range. RITS-6 Accelerator, 10 MeV, 180 ka, 70ns e-beam driver RITS-6 PFL Marx tank Induction Induction cells cells X-ray Diode source region Saturn Accelerator, 1.6 MeV, 10 MA, 40ns beam driver How to control vacuum on expts. at SuperSwarf, AWE.but how it makes you appreciate pen, paper and the computer and drive you to consider new ideas. 13

14 New diodes and power-flow on Saturn Massively parallel rod-pinch array 1 Clam Shell MITL Combine the power from Saturn s 36 modules into a single radial disk feed without magnetic-null losses, invert the voltage polarity, and drive large-area ion diode. 1. B.V. Oliver et. al., Proc. Euro-Asian Pulsed Power Conf. (016).. P. Vandevender et. al., Phys. Rev. Accel. Beams, 18, (015). 14

15 New applications: Ion beams to replicate neutron damage in electronics Ion beam irradiation of transistors can emulate the effects of neutron damage N t / (cm -1 ) SPR Neutrons 4.5 MeV Si Ions V =/- VP + V -/0 + V* -/ T (K) Si BJT Neutron Ion DLTS Equiv.opj Deep Level Transient Spectroscopy 1 can interrogate the damage to transistors. - We can study materials like III-V GaAs under neutron irradiation - Neutron damage exhibits deep broad DLTS features, suggestive of field-dependent emission (clustering effects more pronounced in III-V materials) 1. D. V. Lang, JAP, 45, 303 (1974) 15

16 Thanks for listening. T (k) 16