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

iments

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

RLNR, TIT

U.S.-Japan Workshop on Heavy Ion Fusion and High Energy Density Physics, Sep 30,

2005Academia Hall, Utsunomiya University

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.

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Density (g/cm3)

= 1

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WDM

Thin FoilDischarge 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.

Nonlinear effects on plasma stopping power strongly depend on the projectile

velocity.

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γ=3ZeffΓee

3 / 2

(1+ (v 2 /v th2 ))3 / 2

⇒ Nonlinear stopping

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ee =e2

4πε0akTPlasma parameter:

Beam plasma coupling coefficient:

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γ≥1

Zeff ~ 10, ee ~ 1, v/vth ~ 10 ⇒ γ ~10–5 !!

Typical beam energy in our beam-plasma experiment:

4.3 MeV/u ⇒ v/vth ~ 176 MeV/u ⇒ v/vth ~ 21

Nonlinear effects are negligible!

By using fully-stripped ions as projectile, we can fix the effective charge of the projectile in plas

ma 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.

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0 5 10 15 20 25 30 35 40Atomic Number

fully stripped

6 MeV/u

4.3 MeV/u

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−dE

dx=e4Zeff

2

4πε02mv p

2N t qtL f + (Zt −qt )Lb[ ]Plasma stopping power:

From the enhancement of the stopping power, we can extract mean ion charge of target plasma.

Principle of Thin-Foil-Discharge (TFD) plasma generation

• 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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A Half of Foil Width (mm)

Foil width >> Beam Diam.

For the first order estimation of TFD plasma parameters, we used a 1D plasma expansion model with SE

SAME 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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umax = 4γ /(γ −1)ε

Preliminary experiment on TFD plasma generation.

CurrentTransformer

High Voltage

ThinFoil Discharge

Current

G.S.Capacitors

Thin Foil

0.3 µF0.3 µF

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

Time evolution of TFD plasma（ Aluminum, 12 µm）

800 ns

750 ns600 ns 650 ns 700 ns

820ns800 ns750 ns 870 ns

Thin foil550 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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Time (ns)

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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Umax (calc.) = 2.8 ×104m/s

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2.6 ×104m/s

In case of carbon (18µm), only the surface was heated and ionized by

discharge.

6.2 µs2.2 µs 10.2 µs

Electrical conductivity of carbon (graphite)

2.9×104 S/m at 0 ˚C

1.1×105 S/m at 2500 ˚C

• 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)

Cold core

Preheating of the foil is needed.

A newly developed TFD plasma generator.

Target holder

Dischargeelectrodes

Beam axis

Thin Foil

Electrodes

Multiple foil target enabled us to change foil without breaking vacuum.

Thin foil

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

• T~3 eV• Initial foil thickness ~ 0.8 µm• Capacitor voltage is determined

to be 25 kV.0

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Density(Mg/m3)

2.2 eV

2.8 eV 3.5 eV

Time evolution of thin foil discharge plasma

(Al, 0.8 µm)

Current

Thin Foil 230 ns 280 ns

330 ns 430 ns 480 ns

25 kV

Energy deposited to the foil was evaluated from voltage and current waveforms.

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Time (µs)

Obtained G value is much lower than expected. Energy input efficiency

• Only 1~2% 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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ee

Zi

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

• Projectile: O8+

• Incident Energy: 4.3 MeV/u

• TOF distance: < 3.5 m

• Stop detector: MCP

Beam

MCP

TFD plasma chamber

Drift tube

Preliminary results of energy loss measurement.(O8+, 4.3 MeV/u -> Al, 0.8 µm)

• T < 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.0

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Time (ns)

Cold Eq.

4.3 MeV/u O8+ -> Al

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.

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.