Documenting the Effects of Cryogenic Target Stalk Angle on Fusion Parameters

May 2017
Laboratory for Laser Energetics

 
Photograph of a cryogenic target on SiC stalk just prior to fusion shot. 

Photograph of a cryogenic target on SiC stalk just prior to fusion shot. 

 
 

Cryogenic targets at the Laboratory for Laser Energetics are currently held on a stalk of silicon carbide attached to a polyimide tube. The SiC stalk has a tendency to bend and move targets out of focus. 

 
 
Silicon carbide and polyimide tubes that hold spherical cryogenic targets are prone to bending and casting a shadow onto the targets, causing asymmetries in the incident laser beams. 

Silicon carbide and polyimide tubes that hold spherical cryogenic targets are prone to bending and casting a shadow onto the targets, causing asymmetries in the incident laser beams. 

 
 

The lasers can be resighted to aim at the target, but the bend in the support stalk will block some portion of the incident laser beams. As this creates an asymmetry in the compression of the target, we might expect that the neutron yield, ion temperature, or other parameters of the experiment would also show asymmetry. 

It was found that the asymmetries created by the stalk blocking laser beam 25 were not significant enough to be detected. They were hidden behind the noise of much greater uncertainties, such as cross beam energy transfer. Across all cryogenic target shots from 2015 to mid 2017, there is only a slight increase in the standard deviation of the deuterium-tritium ion temperature as a function of the bending angle of the stalk.

 
 
 
 

Isolating the data to shots with the same pulse shape, there was still not a great enough effect to attribute any measured effects to the stalk angle bending. The bending effect itself is thought to be a result of manufacturing defects. When the SiC fibers are produced, they are spooled, then straightened. In 2018, the Lab will move to a new manufacturing technique and it will be interesting to see if the bending issues persist.