Fast Resistive Bolometry

A bolometer with microsecond scale response time is under construction for the Caltech spheromak experiment to measure radiation from a $\sim $20 $\mu $s duration plasma discharge emitting $\sim $10$^{2}$---10$^{3}$ kW/m$^{2}$. A gold film several micrometers thick absorbs the radiation, heats up, and the consequent change in resistance can be measured. The film itself is vacuum deposited upon a glass slide. Several geometries for the film are under consideration to optimize the amount of radiation absorbed, the response time and the signal-to-noise ratio. We measure the change in voltage across the film for a known current driven through it; a square pulse (3---30A, $\sim $20 $\mu $s) is used to avoid Joule heating. Results from prototypes tested with a UV flashlamp will be presented. After optimizing the bolometer design, the final vacuum-compatible diagnostic would consist of a plasma-facing bolometer and a reference in a camera obscura. This device could provide a design for fast resistive bolometry.