1. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Slide 1 Dynamic Electron Injection for Improved IEC-POPS Operation Yongho Kim, Aaron McEvoy, and Hans Herrmann Los Alamos National Laboratory, Los Alamos, NM October 12, 2009 11 th US-Japan IEC Workshop

2. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Outline Periodically Oscillating Plasma Sphere By R. Nebel and J. Park Research Motivation and Goal Space charge neutralization by dynamic electron injection Experimental Approaches Ramping emitter bias POPS frequency feedback Summary Slide 2

3. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Slide 3 Negative Electrostatic Potential Well (= Virtual Cathode Mode) Symmetric injection of electrons into a transparent spherical anode Previous work 1954 Wells 1956 Farnsworth 1959 Elmore 1968 Hirsh 1973 Swanson Advantage of VC mode Perfect ion confinement High density & high kinetic energy at the center 1959 Elmore, etc

4. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Periodically Oscillating Plasma Sphere (POPS, by D. Barnes and R. Nebel) Harmonic potential with uniform density External electron injection Constant density electron background in a sphere Spherical harmonic potential well for ions Phase lock with external modulation Ions created by ionization and oscillate radially in the well Same frequency, regardless of amplitude (harmonic oscillator) POPS frequency for ions Slide 4

5. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Experimental Setup for POPS 6 Electron Emitters Dispenser cathode type Square-pulse bias voltage (~ 10 ms) Spherical Grids Outer grid: control electron density profile Inner grid: confinement, 1 cm spacing (vs. Debye length ~ 1.8 cm) RF modulation to inner grid to excite POPS oscillation and phase-lock Emissive probe floating potential and its time variation Low operating pressure (1×10 -6 torr) Fill gas: He, H 2, and Neon Slide 5 Diagram of LANL IEC device Electron emitter Emissive probe Outer grid Inner grid

6. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Near Harmonic Potential Observed Average electron density in the well ~ 3.3×10 6 cm -3 Off-peak radial density profile: stable profile from fluid dynamics standpoint Slide 6

7. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D POPS Resonance Measurement Slide 7 Variation in virtual cathode decay time with rf oscillation of the inner grid bias. POPS Resonance (@350 kHz) and 1/2 harmonic observed (expected from Mathieu equation). Resonance frequency independent of outer grid and extractor grid bias.

8. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Scaling of POPS Frequency Slide 8 3 ion species (H 2 +, He + and Ne + ) have been used. Resonance frequency exhibit V well 1/2 scaling Resonance frequency exhibit 1/(ion mass) 1/2 scaling POPS frequency calculation with r VC =r grid (no free parameter) Excellent agreement with theoretical calculations (in absolute values)

9. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Motivation of Present Work: Virtual Cathode Instability was Observed Slide 9 (1) (2) (1) Stability limit: Gradual well depth decrease

10. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Proper Space-charge Neutralization is required to maintain Virtual Cathode Slide 10 Before Compression After Compression n i ~ 10 6 /cc n e ~ 10 7 /cc n i  n e n i ~ 10 8 /cc n e ~ 10 7 /cc n i > n e 1D particle code shows that insufficient space-charge neutralization distorts the plasma potential well Ramping electron injection during compression phase is proposed

11. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Ramping Electron Injection will neutralize Ion built up Slide 11 Solid-State Marx Modulator architecture Proprietary LANL technology (ISR-6) High efficiency & fault tolerant Modular and scalable design  Fiber-optic trigger control system Prototype Pulsed Power System Operate 50 Hz to 1 kHz Reliable & Long lifetime Phase I test module Architecture10 stage Marx with 1.3 kV/stage Output voltage1.3kV- 13 kV Rep. Rate50 -1 kHz Pulse Duration 50  s - 1 ms Output current13 A (max) Pulse droop0.1% - 5% Peak power169 kW Power8.45 kW Lifetime> 10 9 pulses Modulator Specifications  10 stage solid-state Marx modulator

12. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Preliminary Power Supply Test Slide 12 Short pulse testLong pulse test High duty ration test Arbitrary voltage controller channels voltage

13. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Improved Virtual Cathode Feedback Control POPS frequency feedback tuning to adjust applied RF- frequency to match changing potential well depth Slide 13  Frequency tuning to match gradual decay of virtual cathode

14. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Virtual Cathode Dynamics are Studied using a 2D PIC Code Slide 14 Injection boundary Φ=0[V] Transparent anode Φ=300[V] 10 [cm] Injection electron current : 1 [A] Injection electron energy : 300 [eV]

15. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Space-charge limited Virtual Cathode might be more stable Slide 15 Injection electron current : 0.1 [A] Injection electron energy : 150 [eV] Injection electron current : 1 [A] Injection electron energy : 150 [eV] At high electron injection current (1 A), space-charge limited virtual cathode was calculated. If the plasma has a deep potential well then the electron energy might not be greater than the ion temperature, which is favorable to the stability of virtual cathode.

16. Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA U N C L A S S I F I E D Summary Objective of present work is to enhance virtual cathode stability Dynamic electron injection was proposed to compensate ion accumulation at the center of potential well (  quasi-neutral limit). Ramping emitter bias voltage will maintain ne > ni and avoid instability. Feedback POPS frequency control will phase-lock POPS and extend virtual cathode lifetime. CELESTE (2D PIC) code is used to study virtual cathode stability. Slide 16