Data on the emission of energetic ions produced in laser–matter interactions have been analyzed for a wide variety of laser wavelengths, energies, and pulse lengths. Strong correlation has been found between the bulk energy per AMU for fast ions measured by charge cups and the x-ray-determined hot electron temperature. Five theoretical models have been used to explain this correlation. The models include (1) a steady-state spherically symmetric fluid model with classical electron heat conduction, (2) a steady-state spherically symmetric fluid model with flux limited electron heat conduction, (3) a simple analytic model of an isothermal rarefaction followed by a free expansion, (4) the lasnex hydrodynamics code [Comments Plasma Phys. Controlled Fusion 2, 85 (1975)], calculations employing a spherical expansion and simple initial conditions, and (5) the lasnex code with its full array of absorption, transport, and emission physics. The results obtained with these models are in good agreement with the experiments and indicate that the detailed shape of the correlation curve between mean fast ion energy and hot electron temperature is due to target surface impurities at the higher temperatures (higher laser intensities) and to the expansion of bulk target material at the lower temperatures (lower laser intensities).

Experiments are reported on helical plasma equilibrium and stability in the Scyllac toroidal θ-pinch sectors (120°) which have major radii of 2.375 and 4.0 m with coil arc lengths of 5.0 and 8.4 m, respectively. In these experiments the outward toroidal drift force was compensated by a combination of ℓ = 1 helical and ℓ = 0 bumpy fields which are generated by shaping the inner surface of the compression coil or by driven ℓ = 1 windings. Time-resolved measurements were made of the gross plasma-column motion, the plasma radius, the magnetic flux excluded by the plasma, the external magnetic field, the plasma density, the electron and ion temperatures, and the plasma β at axial locations of minimum and maximum plasma radius. These data are used to study the approach to the theoretically predicted toroidal equilibrium (including axial pressure equilibrium). The plasma column remained in stable equilibrium for 7 – 10 μs in the 8-m sector compared with 4 – 7 μs in the 5-m experiment, at which times the onset of a terminating m = 1, k ≈ 0 sideways motion occurred. The results show that the plasma achieved axial pressure equilibrium (nkT = const) in 4 – 6 μs, while maintaining equilibrium in the toroidal plane for 10 μs or longer. The measurements of the plasma radius, β and magnetic field in the various experiments have confirmed in detail the stable toroidal equilibrium observed in the streak photographs during the first 4-10 μs of the discharge. The observed toroidal equilibria of the high-β, θ-pinch plasma are in quantitative agreement with MHD sharp-boundary theory and confirm the theoretical scaling of the equilibrium field between the 5-m and the 8-m sector experiments.

^> z: o -> OQ cr hco Q T O (T ECT UJ \ X = 7cm \ \ a NO He 1 b 0.5% He V * \ i \ « V\ b a \\ i i 1..1L 1 -J: TF 20 10 0 (a) NO He i t 1 >V ' ' 50 20 VELOCITY (5.9 x 10 cm/sec) f L f 0 f U (b) 0.5% He FIG. 4. Effects of the nonresonant upper-sideband mode u) l/ =oj 0 +a; i , k t /=k 0 + k i on the electron distribution (a) before adding He and (b) after 0.5% of He is added. Here V Q is the phase velocity of the pump wave and VJJ that of the upper-sideband wave. participation of Mr. J. Matsumoto are gratefully acknowledged.The spatial and temporal evolution of the plasma density distribution and its relation to the magnetic-piston-field structure during a theta-pinch implosion have been experimentally investigated. With a deuterium fill density of 0.7 *10 15 atoms cm" 3 , evidence of particle reflections from the imploding piston field is indicated.The transient interaction between the imploding plasma and driving magnetic piston field in lowdensity (10 12^/ 0^6 xi0 13 cm" 3 ) fast-magneticcompression #-pinch devices has received considerable investigation. 1 " 3 In these experiments the implosion behavior can generally be described, depending on the density range, by either snowplow or free-particle models. However, in Scyllac-type devices 4 which operate at fill densities of approximately 10 15 cm" 3 the plas-409

An intensity dependence of the absorption of 10-/u m laser light on C02-laser-fusion targets has been observed. Absorption on gold spheres increases from 25%--30% at 10^^ W/cm^ to 50%-60% at 10^^ W/cm^, with most of the variation occurring above 10^^ W/cm^ Concurrently, hot-electron temperature scales as T^ot ^I^'^^ over the entire range. The absorption variation is interpreted as enhanced resonant absorption. It is su^ested that as intensity is increased, the critical surface in the irradiated region becomes increasingly unstable, thereby permitting greater surface distortion and more favorable coupling conditions for resonant absorption.

Flute instabilities have been observed during the implosion of an ≈1×1014 cm−3 initial electron density theta pinch. The flutes occupy only a fraction of the plasma self-luminosity pattern and disappear after formation of the plasma column. The total structure of the luminosity patterns has been correlated with the imploding plasma density and magnetic field distribution.