Dynamics of Laser Driven, Ablatively Accelerated Targets
Abstract
The characteristics of ablation plasma from planar targets, driven by long Nd:glass laser pulses (4 nsec, < 10 to the 14th power w/sq cm), and the velocity of the ablatively accelerated targets are experimentally studied. Ablation plasma diagnostics include arrays of time-of-flight ion collectors, plasma calorimeters, and ballistic pendula which directly measure the plasma velocity, energy, and momentum. The ballistic pendula have been tested and calibrated in the experimental environment. A novel double-foil technique has been developed and used to study the velocity of the accelerated target. Using measurements of plasma energy, velocity, and momentum, we determined the scaling with irradiance and the absolute magnitudes of the ablation pressure, velocity and ablation depth (or mass ablation rate). These results are insensitive to laser spot-size effects that may introduce error under some experimental conditions. They imply that the hydrodynamic efficiency and initial thickness of targets accelerated to fusion velocities are weak and strong functions of absorbed irradiance respectively. Thus, target thickness and irradiance may be varied as necessary to alter the pellet-aspect-ratio or to increase laser nonuniformity smoothing with only small changes in hydrodynamic efficiency. We have determined that the accelerated target is composed of a high pressure, high density region preceeded by a low pressure, low density plasma. We have measured the velocity of the high density region and found that it agrees with target velocities predicted from the ablation parameters using a simple rocket model. We accelerated targets over distances many times their own thickness to velocities of 100 km/sec with no apparent breakup.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 08, 1981
- Accession Number
- ADA098883
Entities
People
- Jacob Grün
Organizations
- United States Naval Research Laboratory