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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

11th US-Japan IEC Workshop

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

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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

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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

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-1.5 -1 -0.5 0 0.5 1 1.5

Electron DensityPlasma Potential

Radius

cathodevirtual

ionwellPOPS r

MVf

/2

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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, H2, and Neon

Slide 5

Diagram of LANL IEC device

Electronemitter

Emissive probe

Outer gridInner grid

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U N C L A S S I F I E D

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

Slide 6

30

40

50

60

70

80

-0.5 0 0.5 1 1.5 2 2.5

Measured plasma potential and polynomial fitting

selected measured voltage2nd order voltage fit4th order voltage fit6th order voltage fit

Pla

sma

Pot

enti

al (

V)

Radius (inch)

Gas Pressure:1x10-6 torrMiddle, Outer, ExtractorGrid @ 100V

Middle Grid

0

2

4

6

8

-0.5 0 0.5 1 1.5 2 2.5

Calculated electron density profile

ne_2nd orderne_4th orderne_6th orderavg. ne (4th and 6th)

Ele

ctro

n de

nsit

y (1

06 c

m-3

)

Radius (inch)

Middle Grid

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U N C L A S S I F I E D

POPS Resonance Measurement

Slide 7

0

50

100

150

200

0 1 2 3 4

No rf300 kHz350 kHz380 kHz

Po

ten

tial W

ell D

epth

(V

)

Time (ms)

Improvement invirtual cathode lifetime

Inner Grid = 250VOuter Grid = 300VEmitter Grid = 134V

• 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.

0

1

2

100 300 500 700 900 1100

t (

ms)

rf driving frequency (kHz, 8V amplitude)

Resonance frequency(~ 350 kHz)

1/2 harmonic (~ 175 kHz)

Well depth ~ 148 VGrid radius ~ 6.25 cm

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U N C L A S S I F I E D

Scaling of POPS Frequency

Slide 8

cathodevirtual

ionwellPOPS r

MVf

/2

• 3 ion species (H2+, He+ and

Ne+) have been used.

• Resonance frequency exhibit Vwell

1/2 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)

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300

Comparison between meausredand calculated POPS frequency

PO

PS

Res

onan

ce F

requ

ency

(kH

z)

Potential well depth (V)

Ne+ ions

H2

+ ions

He+ ions

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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)(2)

(1)

Stability limit: 1

ei

ie

nT

nT Gradual well depth decrease

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U N C L A S S I F I E D

Proper Space-charge Neutralization is required to maintain Virtual Cathode

Slide 10

BeforeCompression

AfterCompression

ni ~ 106/cc

ne ~ 107/cc

ni ne

ni ~ 108/cc

ne ~ 107/cc

ni > ne

1D particle code shows that insufficient space-charge neutralization distorts the plasma potential well

Ramping electron injection during compression phase is proposed

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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

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

Modulator Specifications

10 stage solid-state Marx modulator

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U N C L A S S I F I E D

Preliminary Power Supply Test

Slide 12

Short pulse test Long pulse test

High duty ration testArbitrary voltage controller

channels

volta

ge

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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

0

50

100

150

200

0 1 2 3 4

No rf300 kHz350 kHz380 kHz

Po

ten

tial W

ell D

epth

(V

)

Time (ms)

Improvement invirtual cathode lifetime

Inner Grid = 250VOuter Grid = 300VEmitter Grid = 134V

Frequency tuning to match gradual decay of virtual cathode

cathodevirtual

ionwellPOPS r

MVf

/2

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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 boundaryΦ=0[V]

Transparent anodeΦ=300[V]

10 [cm]Injection electron current : 1 [A]

Injection electron energy : 300 [eV]

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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.

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