(2) Gesellschafl fief Schwerionenforschung (GSI) - W-64291 Darmstadt, Germany. (3) Max-Planck-Institut fi~r Quantenoptik (MPQ) - W-85748 Garching, ...
IL NUOVO CIMENTO
VOL. 106A, N. 12
Dicembre 1993
Diagnostics Methods for Beam-Plasma Experiments (*). H. D. (1) (2) (3)
WETZLER(1), W. SEELIG(1), E. BOGGASCH(2) H. H. HOFFMANN(2) and A. TAUSCHWITZ(3) Institut fi~r Angewandte Physik, TH Darmstadt - W-64289 Darmstadt, Germany Gesellschafl fief Schwerionenforschung (GSI) - W-64291 Darmstadt, Germany Max-Planck-Institut fi~r Quantenoptik (MPQ) - W-85748 Garching, Germany
(ricevuto il 25 Maggio 1993; approvato 1'1 Giugno 1993)
Summary. - - We report the first optical diagnostics of the dense plasma of a Z-pinch helium plasma which is used as a plasma target for investigation of beam-plasma interaction. Information about dynamics of the plasma and the relevant plasma parameters was obtained by spatially, temporally and spectrally resolved measurements of its visible light emission. The Stark broadening of the HeII P~ and P~ line yielded the number density of free electrons, temperature was deduced from the line-to-continuum intensity ratio of this lines. Maximal densities up to 1.14.1019cm-S at a temperature around 23 eV have been determined. PACS 28.50.Re - Fusion reactors and thermonuclear power studies.
1. -
Introduction.
The investigation of interaction phenomena of heavy ions with hot ionized matter is of fundamental interest for heavy-ion-beam-driven inertial confinement fusion (ICF). For the design of high-gain fusion pellets the knowledge of the amount of energy deposited by heavy ions in plasma is crucial. In order to compare the energy loss data to the plasma parameters, precise plasma diagnostics are important. Energy loss measurements of heavy ions in a hydrogen Z-pinch plasma have been carried out in the energy range between 1.4 MeV/u and 6 MeV/u for a large variety of ion species ranging from Ar to U[1, 2]. The experimental results demonstrated the rising energy loss during the density increase of the plasma pinch phase. The observed energy loss in fully ionized matter was about three times higher than in cold matter. The plasma density diagnostics was performed with laser interferometry in Mach-Zehnder geometry along the Z-pinch axis, the plasma temperature was determined by spectroscopy of the emitted H~ line emission [3]. However it was not
(*) Paper presented atthe Internation~ Symposium on Heavy Ion Inertial Fusion, Frascafi, May 25-28, 1993. 1851
1852
H. WETZLER, W. SEELIG, E. BOGGASCH, ETC.
possible to evaluate plasma parameters at pinch time because of absorption and scattering of the laser beam and missing visible line emission in a fully ionized hydrogen plasma. The highest measured density of free electrons 0.5 ~s after pinch time was 3.10 is cm -s. In the pinch plasma energy loss data suggested a density of about 1.5.1019cm -3. In a next step the investigation has to be extended to higher Z gases. Spectroscopical determination of plasma parameters for a helium plasma have been performed recently and will be presented here. Since helium emits line radiation also from its first ionization state and consequently at higher plasma density and temperature than hydrogen, it was possible to measure plasma parameters during all phases of the Z-pinch discharge including the most dense pinch plasma.
2. - E x p e r i m e n t a l
set-up.
The helium plasma is produced in a Z-pinch device which has been designed and constructed especially for beam-plasma experiments[4]. The discharge vessel consists of a cylindrical quartz tube with copper electrodes at both ends. Spark gaps are used as fast high power switches. This plasma target is integrated into the ion-beamline system and the beam passes on the axis of symmetry through small apertures (1.5mm radius) in the electrodes. Its characteristic data are sumTABLE I. - Characteristic data of Z-pinch. Discharge vessel
Electrical circuit capacity inductance charging voltage peak current stored energy
4 ~F 15.4nH 32.5 kV 400 kA 2.1 kJ
length diameter discharge gas pressure
203 mm 104 mm helium I mbar
marized in table I. With a spectrometer and a system of streak and CCD camera, a time-resolved spectroscopy of the radial (side-on) and along the axis (end-on) emitted plasma light was possible. Beyond it, the plasma light was recorded with two-dimensional spatial resolution.
3. - P l a s m a
dynamics.
In fig. 1 the radial distribution of plasma emission is displayed as a function of time. After triggering the spark gaps the discharge starts and skin effect forms a thin current sheet at the inner surface of the quartz tube. Due to the pinch effect this current cylinder is compressed and creates a shock wave, which heats and ionizes the helium while moving inward with a velocity of 5.3.104m/s. When the shock front reaches the axis, 1.22 ~s after ignition, a hot and dense plasma column with a radius of 3 mm is formed. After pinch time the column expands leading to a gradual decrease in plasma density and temperature. During all phases of the Z-pinch discharge high symmetry and stability was observed. The small spread of measuring points reveals the high reproducibility of the plasma created in the Z-pinch device.
1853
DIAGNOSTICS METHODS FOR BEAM-PLASMA EXPERIMENTS
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9 measuring points.
parameters.
The Stark broadening of the H e I I P~ (468.6 nm) and P~ (320.2 nm) line is a good means for the evaluation of the plasma density due to its weak t e m p e r a t u r e dependence. F o r these lines two semi-empirical relationships exist, which relate S t a r k broadening AAstark and n u m b e r density of free electrons ne (5" 1016 cm -3 ~< ne ~< ~< 1019 cm -3) [5-7]: P~ : n e = 3.308.1017 (A~ Stark/nm) 1'21c m - 3 , P/~: ne = 9.179" 1016(AA Stark/nm) 1"35cm -3 9 Electron t e m p e r a t u r e T e was deduced from the line-to-continuum intensity ratio of the two H e I I lines, which can be found in [6, 8]. Figure 2 shows the measured profiles of the P~ and Pz line during the pinch time. The continuum emission was fitted by a
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280 300 320 340 360 2 8 0 ' 3 0 0 ' 3 8 9 440 460 480 500 A (nm) Fig. 2. - Profile of the PZ line end-on, t = 1.33 ~s (a)), the Pz line side-on, t = 1.27 ~s (b)) and the P~ line side-on, t = 1.22~s (c)). 121 - I1Nuovo Cimento A
1854
H. WETZLER~ W. SEELIG, E. BOGGASCH~ ETC. I
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Fig. 3. - Number density of free electrons n e and temperature Te. end-on. 0.6
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Fig. 4. - Redshift of the P~ line.
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polynomial of third degree and was then substracted from the spectrum. The P~ line was completely reabsorbed in the plasma during end-on measurements along the pinch axis. Therefore it was only possible to perform plasma diagnostic measurements along the beam plasma interaction region using the Pz emission line.
DIAGNOSTICS METHODS FOR BEAM-PLASMA EXPERIMENTS
1855
The lines were well fitted by Lorentz profiles. Correcting its full half-width for an instrumental broadening of 1.3 nm yielded the Stark broadening. Other broadening mechanisms were negligible. Using the above-mentioned relations, density and temperature of the plasma were deduced during more than 1 ~s around the pinch time (fig. 3). Both lines and more than 100 shots revealed corresponding results. The maximum density was 1.14.1019cm -3 and the maximum temperature 23eV. The differences between side-on and end-on in the later time were caused by inhomogeneities during plasma expansion. During its most dense phase the plasma was nearly homogeneous. With known temperature the Stark shift of the P~ line offers an alternative n e diagnostic method using the experimentally established relation [9] ~ = 10_19- no - [ cm -3
1 + -5 ] nm -To/eV (Te/eV)2 + 16 4 (5" 1017 cm -3 ~< n e ~< 2.4.10 is cm -3 , 4 eV ~< Te ~
