# Measurement of Light scattered from Density Fluctuations within a Collisionless Shock

## Abstract

WE have previously described a collisionless shock with low Alfvén Mach number (MA), which propagates perpendicular to a magnetic field1–3. The shock is produced by the radial compression of an initial hydrogen plasma within a linear z-pinch. The initial plasma is 85 per cent ionized with electron density $n e 1$ = 6.4 × 1020 m−3, and temperatures $T e 1$ = $T i 1$ = 1.2 eV, and is in an axial magnetic field $B z 1$ = 0.12 Wb m−2. The shock propagates radially inwards through the initial plasma with a steady velocity Vs = 240 km s−1 (MA = 2.5), and with a steady structure of width Ls = 1.4 mm and compression ratio F = 2.5. The measured electron temperature behind the shock, $T e 2$ = 44 eV, implies a resistivity within the shock about two orders of magnitude greater than the classical collisional value4. This demonstrates the collisionless nature of the shock. The effective mean resistivity (η̄), obtained from the power balance equation corresponds to a colision frequency The currently accepted theories of such shocks3,5,6 invoke ion wave turbulence7 to explain the structure and collisionless heating.

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## References

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Paul, J. W. M., Holmes, L. S., Parkinson, M. J., and Sheffield, J., Nature, 208, 133 (1965); Seventh Intern. Conf. Phen. in Ionized Gases (Belgrade), 2, 319 (1965).

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Paul, J. W. M., Goldenbaum, G. C., Iiyoshi, A., Holmes, L. S., and Hardcastle, R. A., Nature, 216, 363 (1967).

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Paul, J. W. M., Scottish Universities Summer School on Plasma Physics, Edinburgh, August 1968 (Oliver and Boyd, in the press).

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Spitzer, L., and Härm, R., Phys. Rev., 89, 977 (1953).

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Sagdeev, R. Z., Rev. Plasma Physics, 4, 23 (1966).

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Krall, N., and Brook, D. L., Phys. Fluids, 12, 347 (1969).

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Bekefi, G., Radiation Processes in Plasmas (Wiley, 1966).

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Paul, J. W. M., Daughney, C. C., and Holmes, L. S., A. P. S. Conference on Plasma Instabilities in Astrophysics, Asilomar, October 1968).

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Rudder, R. R., and Back, D. R., J. Opt. Soc. Amer., 58, 1260 (1968).

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Denisee, J. F., and Delcroix, J. L., Plasma Waves (Interscience, 1963).

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Thompson, W. B., and Hubbard, J., Rev. Mod. Phys., 32, 714 (1960).

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Stringer, T. E., J. Nucl. Energy, C., 6, 267 (1964).

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Reprints and Permissions

PAUL, J., DAUGHNEY, C. & HOLMES, L. Measurement of Light scattered from Density Fluctuations within a Collisionless Shock. Nature 223, 822–824 (1969). https://doi.org/10.1038/223822a0

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