Direct Measurement of Ice-Ablator Interface Motion for Instability Mitigation in Indirect Drive ICF Implosions

Area of Science:

• Physics
• Nuclear Fusion
• Plasma Physics

Background:

• Hydrodynamic instabilities at the ablator-DT fuel interface in indirect-drive ICF implosions can degrade fuel compressibility and hot spot conditions.
• Instability growth injects ablator material into the hot spot, reducing pressure and temperature, which are critical for fusion.
• Simulations and theory suggest that gentle acceleration of this interface can mitigate instability growth during early implosion stages.

Purpose of the Study:

• To experimentally measure the acceleration of the imploding capsule ablator-DT fuel interface in indirect-drive ICF.
• To investigate the effect of laser drive adjustments on interface acceleration.
• To validate theoretical predictions for instability mitigation through controlled acceleration.

Main Methods:

• High-contrast, time-resolved x-ray refraction enhanced radiography (RER) was employed.
• Measurements were performed on indirect-drive implosions at the National Ignition Facility.
• The initial laser drive time history was systematically tweaked to alter interface acceleration.

Main Results:

• Demonstrated a transition from no acceleration to a measured acceleration of 20±1.8 μm ns⁻².
• This controlled acceleration was achieved by adjusting the laser drive parameters.
• The observed acceleration is predicted to reduce initial hydrodynamic instabilities by an order of magnitude at high modes.

Conclusions:

• Experimental control over capsule interface acceleration is achievable in indirect-drive ICF.
• This control offers a viable strategy to significantly reduce hydrodynamic instabilities.
• The findings support the use of gentle acceleration as a key mitigation technique for improving ICF performance.