Thermal Analysis of LLE FBit and High Flux Calorimeters

Ethan Burnham-Fay, Department of Mechanical Engineering, University of Rochester

Hopeman 224

Calorimeters are used in numerous applications at the Laboratory for Laser Energetics (LLE), perhaps most importantly in beam power measurements and beam balancing. The calorimeters consist of a number of layers including an NG9 absorbing glass layer to absorb beam energy, and a layer of 20 thermoelectric (TE) generators. The TE's generate an electrical signal in response to a temperature differential across the elements; in practice beam power is absorbed by the NG9 layer and transformed into heat, which is then seen on one side of the TE's. To calibrate the TE output for beam power measurements, eight electrical heaters are used to input a set amount of energy into the system, and the response of the TE's is recorded. A decay constant is found from this calibration testing, which is then used with beam load measurements to determine beam power. Variability in these calibration tests was historically treated as random noise, but it is now believed that the calibration tests should be highly repeatable. In an attempt to improve performance, a layer of polyethylene insulation was added to some of the calorimeters. The result was more repeatable calibration testing results, but the reason for this improvement was not well understood. The purpose of this work is to investigate the source of the original variation in results, and why the insulation layer improved performance. Additionally, the results can help motivate a refinement of the 20+ year old calorimeter design, calibration methods, and operational parameters.