Status of Heavy Ion Fusion Research

1
Status of Heavy Ion Fusion Research
Grant Logan Director Heavy Ion Fusion Virtual
National Laboratory (LBNL, LLNL and PPPL HIF
groups) Presented at Fusion Power Associates
Symposium Washington, DC November 19-21, 2003
2
Outline
?Motivation for heavy ion fusion research
?Critical technical issues ?Status of current
research ?Scientific goals for near term future
research
3
Heavy ion fusion research motivation

• World wide experience with high energy
  accelerators support inertial fusion energy
  driver prospects for efficiency, pulse-rate, and
  durability.
• Focusing magnets may survive target radiation and
  debris for many years of operation.
• Expected very good ion-target coupling efficiency
  (classical dE/dx)
• Compatibility with indirect drive and thick
  liquid protected chambers.
• ?These attributes are good for both fusion and
  high energy density physics applications
• Present heavy ion beam research emphasizes
  primary scientific issues intense ion beam
  transport physics, beam-wall interactions,
  focusing, and beam-target plasma interactions.
• Intense ion beam-wall interactions are a common
  area of accelerator science important to
  heavy-ion fusion and high energy and nuclear
  physics.

4
Heavy ions can apply to a variety of targets,
chambers, and focusing schemes, but a key
motivation is the desirability of using thick
liquid-protected fusion chambers with much
reduced materials development
Approaches emphasized in the U.S. program
(primary emphasis)
(secondary)
Accelerator Target Focusing Chamber
Induction Linac Indirect Drive, Distributed Radiator Ballistic, Neutralized Thick-Liquid-Protected Wall
RF Linac Storage Ring Indirect Drive, Hybrid Target Ballistic, Vacuum Thin-Liquid-Protected Wall
Induction Recirculator Indirect Drive, Fast ignition Pinch Modes Solid Dry Wall
High Gradient Line Linacs Direct Drive, Aspherical Granular-Solid Flow Protected Wall
5
Heavy ion beam requirements follow from the
designs of accelerators, chambers and targets
that work together
A self-consistent HIF power plant study was
recently published in Fusion Science and
Technology, 44, p266-273 (Sept. 2003)
Beam brightness Bn t gt 4x106 A.s/(m2rad2) at
target
High gain targets that can be produced at low
cost and injected
Beams at high current and sufficient brightness
to focus
Long lasting, thick-liquid protected chambers for
300 MJ fusion pulses _at_ 5 Hz
6
The science of heavy-ion fusion is unique
To drive inertial fusion energy or high energy
density physics targets, heavy ion beams must be
intense enough that beam space-charge forces
(without plasma neutralization) dominate the ion
particle thermal pressure due to emittance. This
space-charge-dominated regime and the associated
collective phenomena distinguish much of
heavy-ion fusion beam science from that of higher
energy particle accelerators. The primary
scientific challenges are to transport, compress
and focus heavy ion beams onto targets.
A few selected examples of the most important
scientific issues follow.
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Office of Fusion Energy Sciences - Targets and
MeasuresTen Year Measures for Inertial Fusion
Energy and High Energy Density Physics

• Develop the fundamental understanding and
  predictability of high energy density plasmas for
  Inertial Fusion Energy (IFE).
• Minimally Effective Outcome Develop and apply
  physical theories and mathematical techniques to
  model the physical processes in high-energy
  density plasmas and intense beams for inertial
  fusion energy.
• Successful Outcome With the help of
  experimentally validated theoretical and computer
  models, determine the physics limits that
  constrain the use of IFE drivers in future key
  integrated experiments needed to resolve the
  scientific issues for inertial fusion energy and
  high energy density physics.

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An important scientific question fundamental to
future application of heavy-ion beams to both
high energy density physics and inertial fusion
energy

• Can accelerated bunches of heavy ions be
  compressed to sufficient intensity to create the
  high energy density conditions for warm dense
  matter and propagating fusion burn in the
  laboratory?
• Some subsidiary science campaigns needed to
  address this top-level question are
• Determine how well high beam brightness can be
  preserved under transport and focusing of intense
  high current beams.
• Understand how beam-plasma interactions affect
  transverse focusing.
• Explore the shortest pulses achievable with
  longitudinal compression.
• Measure how uniformly warm dense matter can be
  heated with accelerated and tailored ion beams.

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How well can initially compact 6-D beam phase
space density (Ib t /enxenyenz ) be preserved
through acceleration, compression, and focusing
to the target?
Hitting targets allows 10 x lower brightness and
10x higher Dp/p than at injection
Issues that can affect beam emittance e and
brightness Bn Ib/en2
Current experiments
Final focus
Source injector
Accelerator
Drift compression
HCX
PTSX
STS
NTX
Random acceleration and focusing field errors
Beam mismatches
Aberrations, emittance growth, instabilities in
plasma
Beam loss-halos, gas desorption, neutralizing
secondary electrons
Dpz - momentum spread increase with drift
compression
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Example of critical physics issue beam loss in
high intensity accelerators -a current world
research topic (GSI-SIS-18, LANL- PSR, SNS)

• Gas desorption Gas desorbed by ions scraping
  the channel wall can limit average beam current.

• Electron cloud effects Ingress of wall-secondary
  electrons from beam loss and from channel gas
  ionization. WARP (below) and BEST simulations
  indicate incipient halo formation and
  electron-ion two-steam effects begin with
  electron fractions of a few percent.

Ion Halo
Ion Beam (core)
Electron Fraction (extreme case)
0
2 ? 2
10 ? 10

• Random focusing magnet errors Gradient and
  displacement errors can also create halos and
  beam loss.

11
Example of critical physics issue drift
compression of bunch length by factors of 10 to 30
Induction acceleration is most efficient at
tpulse 100 to 300 ns
Target capsule implosion times require beam drive
pulses 10 ns
Bunch tail has a few percent higher velocity than
the head to allow compression in a drift line
Final Focus
Drift compression line
Perveance
The beam must be confined radially and compressed
longitudinally against its space-charge forces

• Physics issues that need more study and
  experiments
• Balance beam focusing and space-charge forces
  during compression.
• Beam heating due to compression (conservation of
  longitudinal invariant)
• Chromatic focus aberrations due to velocity
  spread

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Example of critical physics issue plasma
neutralization of beam space charge in focusing
chamber
Example simulations of time histories of a
driver Xenon beam radius at selected points over
a 6 meter focal length
by plasma
With
by plasma
No
Target
Without plasma in the chamber, the ion kinetic
energy and linac voltage, length and cost would
have to increase by 2 to 3 x to recover the 2 mm
focal spot for the target
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Status of heavy ion fusion research

• Past research (prior to FY01) validated
  fundamental beam dynamics with low current
  (mA-scale) beams with correct energy/current
  ratios for relevant space-charge regimes.
• Research since FY01 has completed initial phase
  of experiments on injection (STS), transport
  (HCX) and focusing (NTX) at higher currents (25
  to 250 mA) where non-ideal effects can be
  studied, such as gas and electron effects, and
  neutralization of beam space charge with plasma.
• More research is needed and planned (FY04-06 ) to
  complete high current experiments, and to study
  longitudinal physics, including drift
  compression.
• An integrated beam experiment to study beam
  brightness evolution from the source through
  acceleration, drift compression and focusing to
  the target is the appropriate (proof-of-principle)
  next step.

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Past research (prior to FY01) validated
fundamental beam dynamics with low current (few
mA scale) beams
Some examples Single-Beam Transport
Experiment (SBTE) Verified simulations of
transport over 86 electric quadrupoles with
negligible emittance growth.
Multiple-Beam Experiment with 4 beams (MBE-4)
Studied 200-900 keV acceleration, gt5 x current
amplification in drift compression, longitudinal
confinement, and multiple-beam transport
Final-Focus Scaled Experiment showed ballistic
focusing at 1/10 scale, and neutralizing
electrons from a hot filament could reduce the
focal spot size
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Source-Injector Test Stand (STS operating at
LLNL)
(Recent paper submitted for publication in Review
of Scientific Instruments. Simulation published
Jan 2003 Phys. Rev Special Topics-Accelerators
and Beams)
Injector Brightness source brightness,
aberration control with apertures, beamlet
merging effects
Merging-beamlet simulation
ex, ey (p-mm-mrad)
Beamlet brightness measurement meets IFE
requirement
16
High Current Experiment (HXC- operating at LBNL)

• Low en 0.5 p mm-mr (negligible growth as
  simulations predict)
• Envelope parameters within tolereances for
  matched beam transport

Marx
ESQ injector
Matching and diagnostics
10 ES quads
(Recently submitted for publication in Physical
Review Special Topics-Accelerators and Beams)
End Diagnostics
Propagation of longitudinal perturbation launched
at t 0.
New Gas-Electron Source Diagnostic (GESD) shows
secondary electrons per ion lost follows theory
(red curve)
Four magnetic quadrupoles and additional
diagnostics have been recently added to study gas
and secondary electron effects
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Neutralized Transport Experiment (NTX- operating
at LBNL)
400 kV Marx / injector
Space charge blow-up causes large 1-2 cm focal
spots without plasma.
Focusing magnets
Drift tube
Pulsed arc plasma source
Smaller 1 to 2 mm focal spot sizes with plasma
are consistent with WARP/LSP PIC
simulations. (Submitted for publication in
Physical Review Special Topics- Accelerator and
Beams)
Scintillating glass
. Envelope simulation of NTX focusing with and
without plasma
18
Small-scale experiments are available to study
long-path transport physics such as slow
emittance growth
Construction of the University of Maryland
Electron Ring experiment (UMER) is nearing
completion. UMER uses electrons to study HIF-beam
physics with relevant dimensionless space charge
intensity.
The Paul Trap Simulator Experiment at PPPL uses
oscillating electric quadrupole fields to confine
ion bunches for 1000s of equivalent lattice
periods (many kilometers).
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A key goal is an integrated, detailed, and
benchmarked source-to-target beam simulation
capability

• Track beam ions consistently along entire system

Study instabilities, halo, electrons, ..., via
coupled detailed models
Systems code IBEAM for synthesis, planning
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Understanding how the beam distribution evolves
passing sequentially through each region requires
an integrated experiment
?The beam is collisionless, with a long
memory ?Its distribution function --- and its
focusablity --- integrate the effects of
applied and space-charge forces along the entire
system
NOW
NEXT
A source-to-target integrated beam experiment
(IBX) which sends a high current beam through
injection, acceleration, drift compression, and
final focus
STS- injection
Combine these elements and add acceleration and
drift compression
HCX- transport
NTX-focusing
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Ion accelerators provide a complementary tool to
lasers for High Energy Density Physics

• Intense accelerator beam physics is itself part
  of the broad field of high energy density
  physics.
• Accelerator-produced ion beams can be tailored in
  velocity spread and at energies near the Bragg
  peak to provide a tool to control and improve
  deposition uniformity in thin foil targets. How
  much uniformity is possible and how much it
  improves equation-of-state measurement accuracy
  needs further exploration. Future accelerators
  could drive large volume targets.
• Ion-driven high energy density physics benefits
  from the same accelerator and beam-plasma physics
  base needed for inertial fusion.
• Laserproduced ion beams such as LOasis _at_ LBNL
  may also allow near-term studies of collective
  effects of intense ion beams in regimes relevant
  to heavy-ion fusion.
• There are excellent opportunities for
  collaboration in ion-driven high energy density
  physics at GSI.

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Two ion dE/dx regimes are available to obtain
uniform ion energy deposition in 1 to few eV
warm-dense matter targets
Linacs with 1 J of ions _at_ 0.3 MeV/u would work
best at heating thin foils near the Bragg peak
where dE/dx 0. ? 3 uniformity possible
(Grisham, PPPL). Key-physics issue can lt 300 ps
ion pulses to avoid hydro-motion be produced?
dE/dx
z
3 mm
3 mm
Heavy ion beams of gt300 MeV/u at GSI must heat
thick targets with ions well above the Bragg
peak? kJ energies required _at_ lt300 ns to achieve ?
15 uniformity.
23
Key scientific issue for ion accelerator-driven
HEDP limits of beam compression, focusing and
neutralization to achieve short (sub-nanosecond)
ion pulses with tailored velocity distributions.
Recent HIF-VNL simulations of neutralized drift
compression of heavy-ions in IBX are encouraging
a 200 ns initial ion pulse compresses to 300 ps
with little emittance growth and collective
effects in plasma.

• Areas to explore to enable ion-driven HED
  physics
• Beam-plasma effects in neutralized drift
  compression.
• Limits and control of incoherent momentum spread.
• Alternative focusing methods for high current
  beams, such as plasma lens.
• Foil heating (dE/dx measurements for low range
  ions lt 10-3 g/cm2) and diagnostic development.

(LSP simulations by Welch, Rose, Olson and
Yu June 2003)
Ion driven fast ignition possibility ?
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Conclusions

• Space-charge-dominated beam regimes and
  associated collective phenomena distinguishes
  much of heavy-ion fusion beam science from that
  of higher energy particle accelerators, and poses
  the primary scientific challenges transport,
  compress and focus heavy-ion beams onto targets.
• High current experiments in injection (STS),
  transport (HCX) and focusing (NTX) are underway
  at higher currents ( 25 to 250 mA) where
  non-ideal effects can be studied, such as gas and
  electron effects, and neutralization of beam
  space charge by background plasma.
• An integrated beam experiment to study beam
  brightness evolution from the source through
  acceleration, drift compression and focusing to
  the target is the appropriate (proof-of-principle)
  next step.
• Accelerator-produced ion beams can be tailored in
  velocity spread and at energies near the Bragg
  peak to provide a tool to control and improve
  deposition uniformity in thin foil targets. How
  much uniformity is possible and how much it
  improves equation of state measurement accuracy
  need further exploration.