Light-scattering in the Frascati plasma focus

Volume 89A, number 6

PHYSICS LETTERS

24 May 1982

LIGHT-SCATTERING IN THE FRASCATI PLASMA FOCUS

J. EHRHARDT, P. KIRCHESCH, K. HUBNER and J.P. RAGER Associazione EURATOM-CNEN, Centro Frascati, Rome, Italy and lnstitut fftr Angewandte Physik, Universi#it Heidelberg, Fed. Rep. Germany Received 12 January 1982 Revised manuscript received 22 February 1982

Light scattering measurements (k-spectra) have been carried out on the Frascati 1MJ-plasma focus in a neutron optimized working regime (250 kJ, 3 Torr). The time-dependent electron density and electron temperature were measured. Turbulent effects have been found during the implosion phase as well as during the rise-time of the neutron production.

Principle of measurements. As a primary light source we used a ruby laser system (20 ns, 6 J, 1.5 mrad). Special attention was given to the problem of straylight which could be misinterpreted as a scattered signal. Therefore the laser light was focused on a pinhole in order to improve the divergence. Furthermore a set of sharp-edged baffles was mounted inside the vessel. With this arrangement for angles larger than 15° no straylight was detectable. Due to the absence of straylight the minimum detectable signal was given by the noise of the multipliers due to the high level of plasma light. The value was about neS(k ) ~ 1017 cm -3 during maximum compression. The second source of error is the plasma light itself. For that reason an optical differential system was used. The plasma light passed an interference filter (FWHM = 30 A, centered at 6943 A) and was split into two parts. With the help of two crossed polarizers we measured the plasma light polarized both parallel and perpendicular to the primary laser beam. Scattered light can only be observed in the parallel channel and is determined by the difference of both signals. The whole diagnostic system for simultaneous observation of spectral integrated scattered light at different scattering angles (k ± to focus axis) was mounted inside the vacuum chamber, the signals were fed through the vessel by fibre optics. With this setup we achieved good flexibility in the alignment without 0 031-9163/82/0000-0000/$02.75 © 1982 North-Holland

influencing the focus discharge. Angles of observation range from 15 ° to 104° to cover a-values in the range 0.5 < a < 5 during all phases of the plasma development.

Results. The angular distribution of scattered light (k-spectrum) was observed at three different points of the discharge (fig. 1). The dashed lines in the figure indicate the plasma structures at the onset of neutron production. Mach-Zehnder interferograms taken independently give the ne(r , z) values and show high reproducibility of the discharge. Like on the small

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Volume 89A, number 6 '

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Heidelberg plasma focus [ 1 ] we observed three types of spectra: (a) spectra which can be fitted by theoretical curves following the Salpeter approximation; (b) spectra which can be fitted in the same way at large angles but show enhanced scattering at small angles (forward direction); (c) spectra which only show signals in the forward direction. From the spectra (a), (b) it is possible to evaluate the electron density n e and electron temperature T e. The electrcn density corroborates the interferometric data. Type-(c) curves could not be fitted by the Salpeter approximation but by taking n e from the interferometric data it was possible to determine the factor o f enhancement of scattering at 0 -- 15 °. The time development of the plasma parameters is shown in fig. 2 for all observation points. Timing was

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24 May 1982

done with respect to the onset of neutron production, which was chosen as t = 0. At point l (*) a thermal plasma [type-(a) spectra] is observed. At point 2 (A) the plasma shows lower n e and both types of enhanced scattering are found. Type-(c) spectra are observed in front of the sheath, with a maximum enhancement factor of 100, before maximum compression. Type-(b) spectra are observed after maximum compression during the rise-time of neutron production (100 ns). At point 3 (o) in front of the sheath type-(c) spectra are found, n e and T e peak at the maximum compression, reaching the highest observed values n e max = 8 X 1018 cm - 3 , T e max = 2 keV. During neutron production both parameters decrease and turbulence is observed.

Conclusions. Microturbulence after the maximum compression occurs at the outer boundary of the dense pinch region, while the plasma on axis is thermal. This turbulence exists in a region where high density gradients are found and also part of the plasma current is likely to flow. It occurs with good reproducibility during the rise-time of the neutron pulse, therefore it is probable that the neutron-producing 100-keV deuterons observed by neutron energy measurements with nuclear emulsion [2] are produced by these microturbulences. The measured microturbulences before the maximum compression confirm observations reported earlier [1,3] on focus devices of much lower energy level. References [ 1] J. Ehrhardt, P. Kirchesch, R. B/itzner, K. Behler, G. B6ckle, H. Bruhns, K. Ht~bner, K. Steinmetz and N. Wenzel, Light scattering in a plasma focus, measurement of k- and co-spectra, Proc. X Europ. Conf. of Controlled fusion and plasma physics. Moscow 1981. [2] K. Htibner, J.P. Rager and K. Steinmetz, Space-resolved investigations on the plasma focus neutron emission, Proc. X Europ. Conf. of Controlled fusion and plasma physics, Moscow 1981. [3] A. Bernard, J.P. Garconnet, A. Jolas, J.P. Lebreton and J. de Mascureau, Proc. VII Intern. Conf. on Plasma physics and controlled nuclear fusion research, Vol. II (IAEA, Vienna, 1979) p. 159.