Optical Plasma Phenomena
Though not one of the main areas of experimental research in the Swarthmore Magnetofluids Lab, optical phenomena of plasmas are easily studied given the current setup. It also happens to be an area of personal expertise. The vaccuum vessel is fitted with a transparent window in one of the ports through which light generated by the plasmas can escape.
The project has three stages.
- Single-band observations at high time resolution - Using a small, commercial detector fitted with a broadband filter at 6563 angstroms (Hydrogen alpha), we obtained photometric data with a time resolution of about 10 micro-seconds of the spheromak plasmas as it emerged through the hole in the small flux conserver.
- Spectrographic observations of spheromak plasmas - The next stage is to obtain spectrographic observations of the plasmas using a low resolution spectrograph borrowed from the Swarthmore Astronomy Department. The spectrograph is fitted with a SU200 CCD camera and a neon comparison lamp so the spectra can be calibrated. The camera resolution is about 6.5 angstroms per pixel giving us a visible range of a bit over 3000 angstroms or most of the visible range.
- Fast Multi-band photometry - I am building a new detector composed of three UDT020 UV photodetectors clustered together. Using stock components, we have achieved time resolution better than 3 microseconds and we hope to improve this figure with high-quality capacitors and resistors. A filter holder will allow each detector to look at a different part of the visible spectrum. Looking at three excitation levels of hydrogen would allow us to probe the temperature of the plasma whereas looking at emission lines from hydrogen, carbon, and oxygen ions would tell us things about pollutants on the vacuum vessel walls.
The first few firings with DD1 (Danforth Detector 1), the commercial single-channel detector, show interesting results. The data followed expected trends with a rapid rise in intensity followed by an exponential fall-off probably due to radiative cooling of the plasma. There is a region between the rapid rise and the cooling phases where the intensity continues to rise. My hypothesis is that this is the phase where the plasma leaks out of the spheromak and diffuses through the hole in the small flux conserver.
An unexpected find is that DD1 is an excellant diagnostic as to whether or not a spheromak was formed. In the case where the plasma disconnects from the stuffing flux, a great deal of plasma leaked through the hole in the flux conserver (red line). In the case where the gun did not "unstuff", most of the plasma was trapped between the electrodes and almost no light was produced (blue line).
After having fixed the shutter assembly, it has been determined that there is something wrong with the CCD camera's imaging capability. Despite our best efforts, the camera won't work for us. It has been returned to Photometrics for repairs. We expect it back in a few weeks at which point it can once again be hooked up to the spectrograph and data can be taken. An unfortunate setback.
After several days of work DD3 (Danforth Detector 3) has been completed. It is based around three photodiode detector chips with on-board amplification and is powered by two 9-volt batteries. The filter holder is adapted to use standard astronomical one-inch diameter filters up to 3/8" thick. I look forward to first light on this third phase of the project. Unfortunately, with the current lack of functioning spectrograph, we will have to wait on ordering the filters.