Development of Thin Foil Plasma Target for Beam-Plasma Interaction Experiments

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Development of Thin Foil Plasma Target for
Beam-Plasma Interaction Experiments
U.S.-Japan Workshop on Heavy Ion Fusion and High
Energy Density Physics, Sep 30, 2005 Academia
Hall, Utsunomiya University

• J. Hasegawa, S. Hirai, H. Kita, Y. Oguri, M.
  Ogawa
• RLNR, TIT

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Thin-foil-discharge was adopted to generate a
plasma target in warm-dense-matter (WDM) regime.

• We have so far examined plasma effects on
  stopping power using a ideal plasma target
  (z-pinch plasma, laser-produced plasma)
• Theory of plasma stopping well reproduced
  experimental results.
• EOS and conductivity model in WDM regime has not
  been established.
• Diagnostic of WD plasma by conventional methods
  is very difficult.
• Energetic ion beam can penetrate dense (optically
  thick) plasma.

? 0.01
? 0.1
WDM
? 1
Thin Foil Discharge Plasma
Can we use a heavy ion beam as a diagnostic tool
for WD plasma?
Yes, but we have to care nonlinear effects on
stopping.
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Nonlinear effects on plasma stopping power
strongly depend on the projectile velocity.
Plasma parameter
Beam plasma coupling coefficient
? Nonlinear stopping
Zeff 10, ?ee 1, v/vth 10 ? ? 105 !!
Typical beam energy in our beam-plasma
experiment 4.3 MeV/u ? v/vth 17 6 MeV/u
? v/vth 21
Nonlinear effects are negligible!
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By using fully-stripped ions as projectile, we
can fix the effective charge of the projectile in
plasma target.

• Equilibrium charge of projectile in a plasma is
  larger than that in cold matter because of
  suppression of recombination process.
• Zeff in plasma becomes the same as that in cold
  matter.
• In such a situation, the enhancement of the
  stopping can be attributed to an increase in
  Coulomb logarithm due to plasma free electrons.

Plasma stopping power
From the enhancement of the stopping power, we
can extract mean ion charge of target plasma.
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Principle of Thin-Foil-Discharge (TFD) plasma
generation
Foil width gtgt Beam Diam.

• Areal density keeps constant in the early stage
  of discharge.
• (before rarefaction waves reaches to the center
  of the foil.)
• High density is easily available. ( 0.01
  nsolid)
• Plasma effects on stopping power are directly
  observable.

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For the first order estimation of TFD plasma
parameters, we used a 1D plasma expansion model
with SESAME EOS library.

• The LCR circuit solver includes the change of the
  plasma resistance.
• SESAME- EOS, Mean ion charge, and electrical
  conductivity are used.
• When temperature exceeds the vaporization point,
  the plasma starts its expansion with the maximum
  escape velocity
• Plasma density distribution is not considered.
  (Uniform)

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Preliminary experiment on TFD plasma generation.

• Charged voltage 10 kV
• Discharge current 10kA
• Thin foils Al (12 µm), C (18 µm)

0.3 µF
0.3 µF
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Time evolution of TFD plasma(Aluminum, 12 µm)
Thin foil
600 ns
650 ns
700 ns
550 ns
750 ns
800 ns
820ns
800 ns
750 ns
870 ns

• The foil plasma expands with time.
• Until 750 ns, the plasma boundary looks stable.
• At 820 ns or later, the surface became jaggy.

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The 1D plasma expansion model well reproduced the
observed plasma expanding velocity.

• Expansion velocity used in the 1D model is
  reasonable.
• We used this model to estimate the TFD plasma
  parameters.

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In case of carbon (18µm), only the surface was
heated and ionized by discharge.
Cold core
6.2 µs
2.2 µs
10.2 µs

• Inhomogeneous heating due to a skin effect
  increase the surface temperature.
• Electrical conductivity increases at surface.
• Discharge current selectively flows near the
  surface and deposits the energy on the surface by
  Joule heating. (Positive feedback)

Electrical conductivity of carbon
(graphite) 2.9104 S/m at 0 C 1.1105 S/m at
2500 C
Preheating of the foil is needed.
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A newly developed TFD plasma generator.
Multiple foil target enabled us to change foil
without breaking vacuum.
Thin foil
Target holder
Beam axis
Thin Foil
Discharge electrodes
Electrodes
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Required conditions for TFD aluminum plasma

• Enhancement of stopping power due to plasma
  effects is assumed to be 10
• Mean ion charge (Al) 1.3 determined by the
  plasma stopping fomula.
• n 0.01-0.001nsolid
• T3 eV
• Initial foil thickness 0.8 µm
• Capacitor voltage is determined to be 25 kV.

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Time evolution of thin foil discharge plasma(Al,
0.8 µm)
25 kV
Current
Thin Foil
230 ns
280 ns
330 ns
430 ns
480 ns
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Energy deposited to the foil was evaluated from
voltage and current waveforms.
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Obtained G value is much lower than expected.
Energy input efficiency

• Only 12 of the stored energy was deposited at
  330 ns.
• Mean ion charge was only 0.35.
• Energy deposition was not efficient.

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Beam-plasma interaction experiment was performed
using TFD plasma targets.

• Projectile O8
• Incident Energy 4.3 MeV/u
• TOF distance lt 3.5 m
• Stop detector MCP

MCP
Drift tube
TFD plasma chamber
Beam
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Preliminary results of energy loss
measurement.(O8, 4.3 MeV/u -gt Al, 0.8 µm)

• T lt 300 ns, energy loss is constant.
• T 300 ns, when the rarefaction wave reaches to
  the center of the foil, the energy loss began to
  decrease with time.
• Plasma effect could not be observed. Higher
  ionization degree will be needed.

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Summary

• A TFD plasma generator has been developed for
  beam-plasma interaction experiments.
• One dimensionally expanding TFD plasmas were
  successfully produced with Al foils.
• In case of using carbon foils, inhomogeneous
  plasma heating occurred and TFD plasma was not
  produced successfully. However, we expect that
  preheating of the foil will solve this problem.
• We succeed in measuring energy loss of 4.3-MeV/u
  oxgen ions in a TFD Aluminum plasma.
• Due to low ionization degree of the plasma
  target, enhancement of the energy loss has not
  been observed, yet. More efficient energy
  deposition is needed for increase the ionization
  degree.

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Future plan

• The discharge driving circuit will be upgraded.
• 1D-MHD code using more sophisticated EOS and
  conductivity models will be developed soon.
• Spectroscopic measurement will be performed to
  determine surface temperature of TFD plasma.