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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

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

outline
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
negative electrostatic potential well virtual cathode mode
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

periodically oscillating plasma sphere pops by d barnes and r nebel
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
experimental setup for pops

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

near harmonic potential observed
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
pops resonance measurement
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.
scaling of pops frequency
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)
motivation of present work virtual cathode instability was observed
Motivation of Present Work: Virtual Cathode Instability was Observed
  • Stability limit:

(1)

(1)

(2)

(2)

  • Gradual well depth decrease
proper space charge neutralization is required to maintain virtual cathode
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

ramping electron injection will neutralize ion built up
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
preliminary power supply test
Preliminary Power Supply Test

Short pulse test

Long pulse test

High duty ration test

Arbitrary voltage controller

voltage

channels

improved virtual cathode feedback control
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

virtual cathode dynamics are studied using a 2d pic code
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]

space charge limited virtual cathode might be more stable
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.
summary
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.