The National Ignition Campaign (NIC) team recently conducted the first “convergent ablator” experiment using an X-ray streak camera to measure the velocity of the capsule implosion. The ablator is the plastic material surrounding the fuel in a NIF capsule that blows off (ablates) when heated by the X-rays inside the hohlraum. This causes a rocket-like implosion that compresses and heats the fuel to the conditions required for fusion.
Convergent ablator experiments measure changes in the capsule at various stages of the implosion. A thin metal foil called a backlighter is placed behind the hohlraum to generate X-rays that illuminate the capsule during the implosion, much like a strobe light.
In the March 24 experiment, 181 NIF beams heated the depleted uranium hohlraum with 1.475 megajoules of ultraviolet light, while eight beams heated the x-ray backlighter target located on the opposite side of the streak camera.The backlighter pulse illuminated the imploding capsule, and the ablator edges cast a sharp shadow on the detector.
About the image:
The first continuous streak camera record of a NIF implosion, captured by the diagnostic instrument manipulator insertable streak camera. The broad bright region extending the length of the record is the x-ray backlighter. The dark line in the center is the shadow of a fiducial (reference) wire over the hohlraum diagnostic window. The slanted lines are the shadows cast by the ablator as it implodes. The image shows the ablator moving inward and then exploding after stagnating near the center of the capsule. The bright x-ray self-emission at “bang time,” the peak of the implosion, can be seen at the center.