S-P6-2
Advanced Plasma Diagnostics Using Lasers
レーザーを用いた先進プラズマ診断	 
Toshinori Yabuuchi籔内	  俊毅
Graduate School of Engineering, Osaka University
2-1, Yamada-oka, Suita, Osaka 565-0871, Japan
大阪大学大学院工学研究科 〒565-0871	  大阪府吹田市山田丘2-1
Various diagnosing techniques have been developed to investigate plasma characteristics and its behaviors
for studies of high energy density science and fusion science, and fusion reactor engineering.
Developments of high power and high intensity lasers have opened new approaches to diagnose plasmas
using laser-generated high energy particles as a probe. Advanced plasma diagnosing techniques using thelaser-generated particles are introduced and their applications are discussed.

1. Introduction
Diagnosing plasma is essential in mostexperimental researches on plasma science andengineering. A variety of diagnostics have beendeveloped to study the plasma characteristics andits behaviors. For example, the electrostatic probe
and the spectroscopy of plasma emission arecommonly used to measure the temperature anddensity of plasma. Diagnosing techniques using
laser light is also widely used; such as the laserinterferometry or the absorption in plasmas todetermine the plasma density. Laser scattering,
the Thomson scattering for example, is also
common for the characterization purpose. Inaddition to the plasma characterization, laser lighthas been also used to monitor fields around theplasma. Magnetic field measurement with
Faraday rotation of laser light is a good example.
Laser development in recent years opens theway for new approaches to diagnose the plasma
and the fields around plasmas. Examples of such
advanced techniques are introduced in thepresentation and also expected applications in thefusion reactor engineering are discussed.
2. Plasma Diagnostics Using Lasers
2.1 Diagnostics for plasma characterization
The plasma characterization using a laser opticalprobe cannot be used when the transmittance is notenough in the plasma. X ray is another option as abacklighter to investigate such plasmas. Particularly,
K-shell x rays can be a strong solution[1]. Because
the characteristic x ray is monochromatic, it has
great advantages compared to broadband x rays as
similartothecomparison
betweena
monochromatic laser light and a white light.
Characteristic x ray can be generated by theirradiation of an intense laser light on a solid target.

Due to the short pulse duration of the laser light (<ps), the backlight using laser-induced K-shell x ray
can have a good temporal resolution.
Laser-generated broadband x ray is also useful tocharacterize plasma in some techniques. One ofthem is the x-ray line absorption spectroscopy[2].
Plasma (more appropriately, component ion inplasma) absorbs x rays at specific energies
depending on the material and its ionization level,
which is a function of the plasma temperature anddensity. Figure 1 shows sample spectra (shown withdots) of shock-heated aluminum doled CH foam. In
the experiment, the Al-doped CH plasma wasbacklit with broadband x rays emitted fromsamarium dot target, which was irradiated withnanosecond
laser
pulses.
X-ray
streaked
spectrometer was employed to observe temporal
evolution of the absorption lines of Al. As shown in
the figure, the fittings of the spectra using atomic
physics code (solid curves) provide the inferredtemperature and density of plasma in each timewindow.
When sample is opaque for low energy x rays sothat the above techniques cannot be used, thesample may be characterized with the neutron

Fig. 1. Absorbed x-ray spectra of Al doped CH
plasma observed at various timings; (a) 6 ns, (b) 7 ns,(c) 8 ns after the laser pulse irradiation.

resonance spectroscopy[3]. For the technique,neutron absorbers are doped in the sample inadvance. The absorbed spectra due to the dopantreflect the sample temperature. Recent study
indicates that 109 neutrons can be generated with~100 J class ultra-intense laser pulses[4].
2.2 Diagnostics for field measurements
Backlight with laser-generated charged particles
is useful scheme to study electrostatic or magnetic
fields around plasmas. Field information isimportant to understand the plasma dynamics.
Backlight technique with protons (~MeV) has been
well established for the studies of laser-plasmainteractions[5]. Typically, multi-layered film-type
detectors are used in experiments. The detectors
allow observing multiple images of protons withdifferent energies at once. Because of thedifferences in the velocity of protons, the images
can show the spatially and temporally resolved fieldinformation. The temporal resolution can be in theorder of ps.
We have observed proton radiographs of a wireattached cone target after an intense laser light (10ps duration) was focused into the cone[6]. Proton
beam was generated by interactions between 1 ps
laser pulse and a solid foil. Observed image isshown in Fig. 2. The proton distribution wasperturbed by electrostatic fields around the target,
which was excited by fast electron pulse. The
proton radiographs can be used to understand thefast electron beam propagation in the target, which
is important for the high energy density science.
Laser-generated fast electron beam is also a
candidate as a backlight source[7]. Comparing to
the proton backlight, the fast electron backlight has
some advantages; 1) better spatial resolution due toFig. 2. Proton radiograph of laser-irradiatedcone-wire target. Protons (5 MeV) are deflectedaround the target due to electrostatic fields, which isestimated to be 30 kV/µm around the wire tip.

its smaller divergence, 2) better temporal resolution
due to its short pulse duration, and 3) hightransmission in matter.
3. Application for Fusion Reactor Engineering
The plasma diagnostics with lasers can be a goodtool for the research of fusion reactors, particularly
plasma-wall interactions. The diagnosing techniqueof temperature and density described above can beapplied to investigate the condition of ablated
plasma from the wall and the heated wall itself. The
backlight techniques using laser-generated charged
particles are also useful to monitor the dynamics ofablated plasmas, for example. As similar to theapplication in high energy density science, theadvantages of laser-based techniques, such as good
temporal resolutions, are still valid for the reactor
study.
4. Summary
Laser-based plasma diagnostics are introduced
and discussed their advantages for the usage in thehigh energy density science. Development of highpower, high intensity lasers has provided many
advanced techniques to characterize plasma andinvestigate fields surrounding plasma. Thoselaser-based diagnosing techniques are expected tohelp to further understanding of underlying physics
in researches of fusion reactor engineering.
Acknowledgments
Part of this work was supported by NNSA/USDOE under Contracts DE-FG52-09NA29033(NLUF), DE-FC02-04ER54789 (FSC), andDE-FG02-05ER54834 (ACE). Author gratefully
acknowledges useful discussion with Profs. F. N.
Beg (UC San Diego), S. P. Regan (LLE, Univ.
Rochester), and K. A. Tanaka (Osaka Univ.), andalso Drs. H. Sawada (UC San Diego), K. Anderson(LLE, Univ. Rochester), M. S. Wei, R. B. Stephens(General Atomics), and P. K. Patel (LLNL).
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