Electron Beam Deposition Into the KrF Laser Gas F. Hegelera), M.C. Myers, M. Friedman, J.D. Sethian, S.B. Swanekampb), D.V. Rosec), D.R. Welchc), and M. Wolfordd) Naval Research Laboratory Plasma Physics Division Washington, DC 20375 a) Commonwealth Technology, Inc., Alexandria, VA 22315 b) TITAN/JAYCOR, McLean, VA 22102 c) Mission Research Corporation, Albuquerque, NM 87110 d) SAIC, McLean, VA 22102 Work supported by the U.S. Department of Energy High Average Power Laser Program Workshop, Naval Research Laboratory, December 5, 2002

Summary Should be able to meet ultimate goal of 80% e-beam energy deposition efficiency during the flat-top portion of the pulse at 750 kV. Experimental measurements, plus 1-D and 3-D simulations that have been benchmarked with experiments, point the way: No anode foil is needed (Demonstrated) Pattern and counter-rotate e-beam to “miss” ribs (Demonstrated) Reduce pressure foil from 2 mil to 1 mil (Demonstrated) Shallower hibachi ribs (To be evaluated with new hibachi in early Spring 2003) 750 keV beam (as in ultimate application) Eliminated electron beam halo with floating electric field shapers.

The Electra Laser Facility First Generation system can run at 5 Hz for 5 hours Excellent test bed for developing laser components The Electra Laser Facility Two beams: 30 cm x 100 cm, 500 keV, 90 kA, 100 nsec @ 5 Hz

The key components of a Krypton Fluoride (KrF) Laser Input Laser Gas Recirculator Pulsed Power System Cathode Amplifier Window Electron Beam Foil Support (Hibachi) Laser Cell (Kr + F2) ENERGY + (Kr + F2)  (KrF)* + F  (Kr + F2) + h (248 nm)

Opposite electron beams pump the laser cell Top view 120 cm 30 cm 1.8 m Cathode Hibachi Side view Hibachi

Diode Energy Distribution of a 6.1 kJ Pulse risetime 0.7 kJ fall time 1.2 kJ 100 ns flat-top 4.2 kJ Voltage and current waveforms from a single diode.

Goal: Maximize e-beam energy deposition in the laser cell (> 80% at 750 kV during the flat-top) Two innovations have allowed high hibachi transmission: 1. Eliminate the anode foil 2. Pattern the electron beam to “miss” the hibachi ribs Rib Laser Gas Kr + Ar Vacuum Pressure Foil .001”Ti Emitter e-beam Anode foil Issues High Current “edge” from each strip Non-uniform electric field at anode causes beam spreading Beam rotates and skews between cathode and anode

3-D LSP simulation show that an anode foil can be eliminated E-beam beam spreading in the non-uniform electric field is controlled by the cathode strip width. e-beam hits hibachi ribs Non-uniform foil heat loading. 100 90 80 70 60 50 40 30 20 10 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 cathode strip width % beam energy deposited in gas cathode field shaper ribs foil Simulations by D. Rose & D. Welch MRC Albuquerque

Floating “Field Shapers” on perimeter of cathode strips eliminate enhanced edge emission Emitter Floating Field Shaper A/cm2 WITHOUT WITH FIELD SHAPER Line out of Radiachromic film image of beam at anode F. Hegeler, et al., Physics of Plasmas, vol. 9, October 2002, pp. 4309-4315.

Position of the hibachi ribs The emitter strips are counter-rotated so that the beam goes straight through the hibachi ribs Cathode strips rotated 6 degrees 27 cm 96 cm Position of the hibachi ribs Radiachromic Film: Time integrated current profile at the pressure foil

Measured Electron Beam Deposition Profile

Strip cathode increases energy deposition by 26% over monolithic cathode; performance close to “rib-less” hibachi Electron beam energy deposition efficiency at 500 kV, 1.2 atm. of Kr, and a 50 mm (2 mil) thick Ti pressure foil. Emitter Pressure Foil e-beam Solid cathode, no ribs Entire pulse Flat-top portion Simulation 67% 72% (1-D Tiger) Ribs Solid cathode, with ribs 93% of max Experiment 47% 53% >26% Increase Strip cathode, with ribs Experiment 62% 67%

Flat-top energy deposition efficiency of up to 75% is achieved for 500 keV electrons, with a 1 mil Ti pressure foil Flat-top e-beam energy deposition efficiency with a strip cathode at 400-500 kV and 25-50 mm thick Ti pressure foils. Diode voltage Flat-top efficiency with 50 mm thick Ti pressure foil Flat-top efficiency with 25 mm thick Ti pressure foil 400 kV 57%(1) 71%(2) 500 kV 67%(3) 75%(4) Required laser cell gas pressure to “stop” the e-beam: (1) 1 atm. at 40% Kr and 60% Ar (2) 1 atm. at 60% Kr and 40% Ar (3) 1.2 atm. at 100% Kr (4) 1.3 atm. at 100% Kr

3-D LSP simulations and experimental measurements agree well for the counter-rotated strip cathode E-beam deposition efficiency for the flat-top portion of the e-beam (500 kV) 80% with 2 mil Ti pressure foil 6 degree counter rotation 1.2 atm Kr in laser cell 2 mil Ti foil Simulations: 66% efficiency Experiments: 67% efficiency 70% 60% 50% deposited energy 40% 30% 20% 10% 0% laser gas pressure foil hibachi ribs Deposition efficiency = Energy deposited in laser gas/energy in diode (for flat top portion of beam) Simulations by D. Rose & D. Welch, MRC Albuquerque

3-D simulation of a single cathode strip and ribs confirms the 1-D results Static fields simulation with periodic boundaries in X; 2-cm wide cathode, 4-cm rib spacing, 30-cm of 1.2 atm Kr, 500 kV, 1 mil Ti foil. Foil Rib Kr Gas Periodic Boundary Number Density Energy deposition fractions: (for a 2-cm wide cathode) Gas: 76.6% Ribs: 12.6% Foil: 10.3% Other: <1%

New Hibachi with shallower ribs will increase the e-beam deposition efficiency E-beam spreading is minimized Electric field in A-K gap is more uniform compared to deep rib Hibachi 1 mil Ti pressure foil Hibachi ribs Preliminary simulation using new hibachi gives 74% energy deposition into gas at 430 kV.

1-D Tiger simulations at 750 kV 1-D codes predict a maximum energy deposition efficiency of 81% for a 750keV electron beam.