ion source and injector experiments at the hif/vnl

The Heavy Ion Fusion Virtual National Laboratory

Ion Source and Injector Experiments at the HIF/VNL

J. W. Kwan, D. Baca, E. Henestroza, J. Kapica, F. M. Bieniosek, W.L. Waldron, J.-L. Vay, S. Yu, LBNL

G.A. Westenskow, D. P. Grote, E. Halaxa, LLNLI. Haber, Univ. of Maryland

L. Grisham, PPPL

HIF Symposium, Princeton, NJJune 7, 2004


This talk is dedicated to the amazing Cicada

In the hope that the HIF symposium 2021, Princeton, NJ

will tell the story of heavy ion beams achieving fusion


A summary of main experiments

Experiment Purpose Facility

Large diameter ion diode

Study large beam optics and benchmark simulation

STS-500

RF plasma source

Prepare source for Merging Beamlets

STS-100

Merging Beamlets

High average current density (J) injector

STS-500

Negative ions Check if Cl- is applicable for HIF

STS-100

Accel-decel injection

High line charge density () beam for solenoid focusing

NDCX

Al-Si source development

Long and short pulse length for special applications

STS-50


Experiments on STS-500 to study beam optics

500 kV, 17 s pulse, 1.0 s rise time


Experimental Apparatus

10-cm diameter K+ Al-Si source with Pierce electrode

For Beam Imaging, use:

Kapton

100 m Alumina scintillatorFaraday cup with electron suppressor using a honeycomb bottom

Slit scanners: 2 mils, 17.8 cm apart


Warp simulations

Good agreement between experimental results and simulation predictions

Experimental results

150 kV48A heater

Emittance taken here

10 cm source, 21 cm diode gap,Space charge limited mode

2/3

2/351044.0

AVP


The emission-limited under-dense beam did not show much aberration


5.0 cm aperture

Aperturing the large beam7.5 cm aperture

aperture

Aperture Beam fraction

Norm. emittance

Brightness ratio

None 100% 0.60 17.5 cm 55% 0.152 8.65.0 cm 25% 0.048 39

Brightness comparison


The apertured beam showed no aberrations

Optical image from the alumina scintillator taken with a gated camera

-100

-50

0

50

100

0 0.5 1

Normalized intensity

posi

tion

[mm

]

Integrated current density profile (compares to a slit cup measurement)

7.5 cm aperture


Red--expt. data;black--simulation

Time-dependent adaptive-mesh simulation shows how to achieve a fast rise time

Current at Faraday cup

• The current pulse rises faster than the applied voltage pulse.

• Capacitive coupling softens the signal rise time.

• One dimensional theoretical model:

• Example: 50ns/350ns

Applied Diode Voltage

3/ 242 2 2( ) 4

9 27p p

qe qeJ JV t dt t

A m A m


Merging high density beamlets is an innovative approach to build compact multi-beam HIF injector

x-z

y-z

• Achieve high current, and high average current density

• Minimize the injector and matching section size for a compact multi-beam HIF driver system

WARP-3D simulation to study emittance growth

39.9 m

0.5 m past column 1.9 m

4.1 m

91 beamlets (each semi-Gaussian, 0.006 A, 0.003 π-mm-mrad, 160k particles), 1.2-1.6 MeV, 1024x1024, 1 cm/step

After the beamlets are merged, the emittances settle down at about 1.0 pi-mm-mrad.

Emittance is optimized if the number of beamlets is large and the beamlets are slight converging, but only weakly dependent on the emittance of each beamlet.

4.1 m1.9 m

Configuration Phase


Testing Plasma Source on STS-100

RF-driven 26 cm diam. multi-cusp source inside ceramic insulator

500s, 20kW, ~ 10 MHz Compact RF oscillator


Characterization of the RF plasma source

18 kW of 13 MHz RF,multicusp Argon plasma source at optimum pressure of 2 mTorr

Multi-aperture extracts61 beamlets at 100 mA/cm2 using high gradient insulator

Einzel lens to focus beamlets and examine charge exchange loss


RF plasma source beamlets results

020924

0

0.2

0.4

0.6

0.8

1

8 13 18RF power (kW)

Ar 3+

Ar 2+

Ar 1+

030117

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.00 5.00 10.00 15.00 20.00 25.00

RF drive (kW)

80kV70kV60kV50kV40kV

Achieve 100 mA/cm2

0.000

0.005

0.010

0.015

0.020

47 47.5 48 48.5 49 49.5 50 50.5Beam Energy (kV)

20ms

16ms

12ms

8ms

4ms

90% Ar+

< 0.5% low energy componentElectrostatic energy analyser


Full Gradient test on STS-500 will begin this month

This experiment will confirm full current density, its uniformity, and voltage gradient across vacuum gap.


Merging Beamlets test will begin in September

Apparatus is full scale in dimension, but1/4 scale in voltage,so 1/8 in current.

The experiment will study emittance growth physics, beam matching parameters, and beam halos.

Success in this experiment will establish the basis for building a (future) driver-scale injector.


Negative ion beams is an innovative idea in response to the gas and electrons problem

Avoid the problem of electrons being trapped in positive ion beams

No charge exchange problem to cause energy dispersion

Low ion temperature for both negative and positive halogen ions

Can be efficiently converted to atomic neutrals by laser photo-detachment, if this can be of advantage to the final focusing at the fusion chamber.


Negative ion sources for HIF Drivers

0.94

0.96

0.98

1

1.02

1.04

1.06

10 15 20 25 30 35

Source pressure (mTorr)

Ion

curr

ent (

mA

)

0

10

20

30

40

50

60

70

80

Elec

tron

curr

ent (

mA)

Cl-

electron

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 3 6 9 12 15 18

Extraction voltage (kV)

Ion

curr

ent (

mA)

012345678910

Elec

tron

curr

ent (

mA)

Cl-Cl+electron

We have already demonstrated 45 mA/cm2 of pure Cl- ions with relatively low co-extracted electrons (7:1) from a single aperture.

Current density scaled almost linearly with RF power (12.56 MHz).

Current density of Cl+ ~ 1.3 x Cl-.

A new experiment will run on STS-100 this summer to examine the negative ion production from a large source, measure emittance, and form an array of beamlets.


At 3.3 mC/m, the HEDP is > 10x the present HCX experiment.

Longitudinal emittance can coupling to transverse emittance

Possible compression limit when the bunch’s forward kinetic energy becomes comparable to the beam potential.

30kV -350kV 0V

Solenoid

Ion Source

The accel-decel injector is an innovation to meet our HEDP challenge: build a low energy high current driver to hit target

• In an accel-decel injector, a long pulse is compressed when decelerates into a solenoid, the Super-High (line charge density) bunch is then accelerated without expansion.

= I/vThe situation is similar to loading passengers into a roller coaster train.

10A x 100ns= 0.3m x 3.3 C/m


0

0.01

0.02

0 0.2 0.4 0.6 0.8 1 1.2

0

0.01

0.02

00.050.10.150.20.250.30.350.4

60 cm solenoid located 5 cm from ground plate(winding:7.7cm ID, 9.2 cm OD,1 Mega Amp-Turn)

Bz/100(Tesla)

30 kV 0 kV-220 kV -35 kV -55 kV

A proof-of-principle Super-High experiment

K+ Gun (using Al-Si source)

E.H.20.MAY.04

NDCX-1


Conclusion

Several ion source/injector experiments at the HIF/VNL are aimed at:-- supporting on-going HIF needs, -- developing future HIF driver, -- innovative concepts (high J, high , fast rise,

negative ions) In response to funding difficulty, the injector test

facility at LLNL is scheduled to terminate in March 2004.

We hope STS-100 can be moved to LBNL to continue ion source development.


What is unchanged is the constantly changing direction.

What is certain is the permanently uncertain state.

After Thought

ion source and injector experiments at the hif/vnl

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