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Dynamic Electron Injection for Improved IEC-POPS Operation

11 th US-Japan IEC Workshop. 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. Outline. Periodically Oscillating Plasma Sphere By R. Nebel and J. Park

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Dynamic Electron Injection for Improved IEC-POPS Operation

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  1. 11th US-Japan IEC Workshop 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

  2. 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

  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. 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

  5. Outer grid Inner grid Electron emitter Emissive probe 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, H2, and Neon Diagram of LANL IEC device

  6. Near Harmonic Potential Observed • Average electron density in the well ~ 3.3×106 cm-3 • Off-peak radial density profile: stable profile from fluid dynamics standpoint

  7. POPS Resonance Measurement • 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. Scaling of POPS Frequency • 3 ion species (H2+, He+ and Ne+) have been used. • Resonance frequency exhibit Vwell1/2 scaling • Resonance frequency exhibit 1/(ion mass)1/2 scaling • POPS frequency calculation with rVC =rgrid (no free parameter) • Excellent agreement with theoretical calculations (in absolute values)

  9. Motivation of Present Work: Virtual Cathode Instability was Observed • Stability limit: (1) (1) (2) (2) • Gradual well depth decrease

  10. Proper Space-charge Neutralization is required to maintain Virtual Cathode Before Compression After Compression • 1D particle code shows that insufficient space-charge neutralization distorts the plasma potential well • Ramping electron injection during compression phase is proposed ni ~ 106/cc ni ~ 108/cc ne ~ 107/cc ne ~ 107/cc ni ne ni > ne

  11. Ramping Electron Injection will neutralize Ion built up Solid-State Marx Modulator architecture Proprietary LANL technology (ISR-6) High efficiency & fault tolerant Modular and scalable design Prototype Pulsed Power System Operate 50 Hz to 1 kHz Reliable & Long lifetime Modulator Specifications • 10 stage solid-state Marx modulator • Fiber-optic trigger control system

  12. Preliminary Power Supply Test Short pulse test Long pulse test High duty ration test Arbitrary voltage controller voltage channels

  13. Improved Virtual Cathode Feedback Control • POPS frequency feedback tuning to adjust applied RF-frequency to match changing potential well depth Frequency tuning to match gradual decay of virtual cathode

  14. Virtual Cathode Dynamics are Studied using a 2D PIC Code 10 [cm] Injection electron current : 1 [A] Injection electron energy : 300 [eV] Transparent anode Φ=300[V] Injection boundary Φ=0[V]

  15. Space-charge limited Virtual Cathode might be more stable 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. 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.

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