Transcript 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,

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
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 1
Outline

Periodically Oscillating Plasma Sphere
•

Research Motivation and Goal
•

Space charge neutralization by dynamic electron injection
Experimental Approaches
•
•

By R. Nebel and J. Park
Ramping emitter bias
POPS frequency feedback
Summary
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 2
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
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 3
Periodically Oscillating Plasma Sphere (POPS, by D.
Barnes and R. Nebel)

Electron Density
Plasma Potential
Harmonic potential with uniform
density
1.2
•
•
0.8
•
External electron injection
Constant density electron
background in a sphere
Spherical harmonic potential well
for ions
1
0.6
0.4
0.2
0

Phase lock with external
modulation
•
•
•
-0.2
-1.5
Ions created by ionization and
oscillate radially in the well
Same frequency, regardless of
amplitude (harmonic oscillator)
POPS frequency for ions
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
-1
f POPS
-0.5
0
Radius
0.5
1
1.5
2Vwell / M ion

rvirtual cathode
Slide 4
Experimental Setup for POPS

6 Electron Emitters
•
•

•
•
Outer grid
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
Electron
emitter
Emissive probe
•

Inner grid
Spherical Grids
•

Dispenser cathode type
Square-pulse bias voltage (~ 10 ms)
floating potential and its time variation
Low operating pressure (1×10-6 torr)
•
Emissive probe
Diagram of LANL IEC device
Fill gas: He, H2, and Neon
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 5
60
50
Gas Pressure:1x10-6 torr
Middle, Outer, Extractor
Grid @ 100V
40
30
-0.5
Middle Grid
0
0.5
1
1.5
Radius (inch)
2
Calculated electron density profile
8
-3
Measured plasma potential and polynomial fitting
80
selected measured voltage
2nd order voltage fit
70
4th order voltage fit
6th order voltage fit
6
Electron density (10 cm )
Plasma Potential (V)
Near Harmonic Potential Observed
ne_2nd order
ne_4t h order
ne_6t h order
avg. ne (4t h and 6t h)
6
4
2
Middle Grid
2.5
0
-0.5
0
0.5
1
1.5
Radius (inch)
2
2.5

Average electron density in the well ~ 3.3×106 cm-3

Off-peak radial density profile: stable profile from fluid dynamics
standpoint
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 6
POPS Resonance Measurement
Potential Well Depth (V)
200
Variation in virtual cathode decay time
with rf oscillation of the inner grid bias.
•
No rf
300 kHz
350 kHz
380 kHz
150
Improvement in
virtual cathode lifetime
100
Inner Grid = 250V
Outer Grid = 300V
Emitter Grid = 134V
50
POPS Resonance (@350 kHz) and 1/2
harmonic observed (expected from
Mathieu equation).
•
Resonance frequency independent of
outer grid and extractor grid bias.
•
0
0
1
2
3
4
Time (ms)
Well depth ~ 148 V
Grid radius ~ 6.25 cm
t (ms)
2
1
Resonance frequency
(~ 350 kHz)
0 1/2 harmonic
(~ 175 kHz)
100
300
500U N C L A S 700
900
SIFIED
rf driving
frequency
(kHz,
Operated by Los Alamos National Security, LLC
for the U.S. Department
of Energy’s
NNSA 8V amplitude)
1100
Slide 7
Scaling of POPS Frequency
3 ion species (H2+, He+ and
Ne+) have been used.
•
Comparison between meausred
and calculated POPS frequency
POPS Resonance Frequency (kHz)
700
Resonance frequency exhibit
Vwell1/2 scaling
•
600
+
H ions
500
2
Resonance frequency exhibit
1/(ion mass)1/2 scaling
•
400
300
f POPS
+
He ions
200
2Vwell / M ion

rvirtual cathode
POPS frequency calculation
with rVC =rgrid (no free parameter)
•
100
0
+
Ne ions
0
50
Excellent agreement with
theoretical calculations (in
absolute values)
UNCLASSIFIED
100 150 200 250
Potential well depth (V)
300
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
•
Slide 8
Motivation of Present Work:
Virtual Cathode Instability was Observed
(1)
(1)
(2)
(2)

Gradual well depth
decrease

UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Stability limit:
Te  ni
 1
Ti  ne
Slide 9
Proper Space-charge Neutralization is required to
maintain Virtual Cathode
Before
Compression
After
Compression
ni ~ 106/cc
ni ~ 108/cc
ne ~ 107/cc
ne ~ 107/cc
ni  ne
ni > n e

1D particle code shows that insufficient space-charge neutralization
distorts the plasma potential
well
UNCLASSIFIED
Slide 10
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA

Ramping electron injection during compression phase is proposed
Ramping Electron Injection will neutralize Ion built up
Solid-State Marx Modulator architecture
Prototype Pulsed Power System
Proprietary LANL technology (ISR-6)
Operate 50 Hz to 1 kHz
High efficiency & fault tolerant
Reliable & Long lifetime
Modular and scalable design
Modulator Specifications
Phase I test module
10
stage solid-state
Marx modulator
Fiber-optic
trigger
control system
Architecture
10 stage Marx with 1.3
kV/stage
Output voltage
1.3kV- 13 kV
Rep. Rate
50 -1 kHz
Pulse Duration
50 s - 1 ms
Output current
13 A (max)
Pulse droop
0.1% - 5%
Peak power
169 kW
Power
8.45 kW
Lifetime
> 109 pulses
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 11
Preliminary Power Supply Test
Short pulse test
Long pulse test
High duty ration test
voltage
Arbitrary voltage controller
Slide 12
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
channels
Improved Virtual Cathode Feedback Control

POPS frequency feedback
tuning to adjust applied RFfrequency to match changing
potential well depth
f POPS
2Vwell / M ion

rvirtual cathode
200
Potential Well Depth (V)
tuning
to match gradual
decay of virtual
cathode
No rf
300 kHz
350 kHz
380 kHz
Frequency
150
Improvement in
virtual cathode lifetime
100
Inner Grid = 250V
Outer Grid = 300V
Emitter Grid = 134V
50
0
0
1
2
3
4
Time (ms)
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 13
Virtual Cathode Dynamics are Studied using a 2D PIC
Code
10 [cm]
Injection electron current : 1 [A]
Injection electron energy : 300 [eV]
U N C L A anode
SSIFIED
Transparent
Injection boundary
Φ=300[V]
Φ=0[V]
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s
NNSA
Slide 14
Space-charge limited Virtual Cathode might be more
stable
•
•
Injection electron current : 0.1 [A]
Injection electron current : 1 [A]
Injection electron energy : 150 [eV]
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
UNCLASSIFIED
Slide 15
stability of virtual cathode.
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
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.
UNCLASSIFIED
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
Slide 16