Ion Source and Injector Experiments at the HIF/VNL

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• 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,
  LBNLG.A. Westenskow, D. P. Grote, E. Halaxa,
  LLNLI. Haber, Univ. of MarylandL. Grisham, PPPL
• HIF Symposium, Princeton, NJJune 7, 2004

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• 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
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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 (l) beam for solenoid focusing NDCX
Al-Si source development Long and short pulse length for special applications STS-50
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Experiments on STS-500 to study beam optics
500 kV, 17 ms pulse, 1.0 ms rise time
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Experimental Apparatus
Slit scanners 2 mils, 17.8 cm apart
10-cm diameter K Al-Si source with Pierce
electrode

• For Beam Imaging, use
• Kapton
• 100 mm Alumina scintillator

Faraday cup with electron suppressor using a
honeycomb bottom
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Good agreement between experimental results and
simulation predictions
Experimental results
10 cm source, 21 cm diode gap,Space charge
limited mode
Warp simulations
150 kV48A heater
Emittance taken here
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The emission-limited under-dense beam did not
show much aberration
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Aperturing the large beam
7.5 cm aperture
aperture
5.0 cm aperture
Brightness comparison
Aperture Beam fraction Norm. emittance Brightness ratio
None 100 0.60 1
7.5 cm 55 0.152 8.6
5.0 cm 25 0.048 39
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The apertured beam showed no aberrations
7.5 cm aperture
Optical image from the alumina scintillator taken
with a gated camera
Integrated current density profile (compares to a
slit cup measurement)
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Time-dependent adaptive-mesh simulation shows how
to achieve a fast rise time
Current at Faraday cup
Applied Diode Voltage
Red--expt. datablack--simulation

• The current pulse rises faster than the
  applied voltage pulse.
• Capacitive coupling softens the signal rise
  time.
• One dimensional theoretical model
• Example 50ns/350ns

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

• Achieve high current, and high average current
  density
• Minimize the injector and matching section
  size for a compact multi-beam HIF driver
  system

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WARP-3D simulation to study emittance growth
Configuration Phase
91 beamlets (each semi-Gaussian, 0.006 A, 0.003
p-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.
1.9 m
0.5 m past column
39.9 m
4.1 m
4.1 m
1.9 m
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Testing Plasma Source on STS-100
RF-driven 26 cm diam. multi-cusp source inside
ceramic insulator
500ms, 20kW, 10 MHz Compact RF oscillator
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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
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RF plasma source beamlets results
Achieve 100 mA/cm2
90 Ar

lt 0.5 low energy component
Electrostatic energy analyser


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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.



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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.

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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.

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Negative ion sources for HIF Drivers

• We have already demonstrated 45 mA/cm2 of pure
  Cl- ions with relatively low co-extracted
  electrons (71) 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.

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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 l (line charge density) bunch is then
  accelerated without expansion.

The situation is similar to loading passengers
into a roller coaster train.
l I/v
10A x 100ns 0.3m x 3.3 mC/m

• At 3.3 mC/m, the HEDP l is gt 10x the present
  HCX experiment.
• Longitudinal emittance can coupling to transverse
  emittance
• Possible compression limit when the bunchs
  forward kinetic energy becomes comparable to the
  beam potential.

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A proof-of-principle Super-High l experiment
60 cm solenoid located 5 cm from ground
plate (winding7.7cm ID, 9.2 cm OD,1 Mega
Amp-Turn)
NDCX-1
Bz/100(Tesla)
30 kV
0 kV
-220 kV
-35 kV
-55 kV
K Gun (using Al-Si source)
E.H.20.MAY.04
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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 l, 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.

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After Thought
What is unchanged is the constantly changing
direction.What is certain is the permanently
uncertain state.