"Fast high resolution echelle spectroscopy of a laboratory plasma"
C.D. Cothran, J. Fung, and M.R. Brown

Summary:

Ion Doppler spectroscopy (IDS) is perhaps the most widely used method for characterizing laboratory plasmas. It involves using thermal Doppler broadening and Doppler shift of an emission line to measure ion temperature and line-of-sight average flow velocity. The Swarthmore Spheromak Experiment (SSX) IDS instrument incorporates an echelle diffraction grating and multi-anode photomultiplier tube (PMT). It's high spectral resolution (λ / δλ ≈ 2.5 x 10^4) and time resolution (~ 1 μs) are ideal for investigating radial and toroidal midplane flows and ion heating during counter-helicity spheromak merging experiments. The IDS can analyze spectral lines from the UV through the visible.

While it is the only detector array technology with sufficiently fast time response for measuring SSX lineshapes, the multi-anode PMT is difficult to use in a high-resolution spectrograph because of the large (~ 1 mm) spacing between its channels. The echelle grating overcomes this difficulty operating at 25th order and therefore providing a large dispersion (numerically smaller dλ/dx) across this PMT. The optimal design for a spectrometer with a large detector has a large étendue, which is the product of the entrance slit area and the accepted solid angle. Given expected ion temperatures of 15-20 eV and a typical Alfvén speed of 100 km/s, the SSX IDS system is well-equipped to observe the expected Doppler shifts and line broadening.

When operating in the UV range the instrument uses a set of UV-grade fused silica (UVGFS) lenses in the collection, input, and output optical systems. A set of BK7 lenses with the same index of refraction in the visible range that the UVGFS have in the UV range are substituted for observing in the visible. The collection optics include an interference filter, which is necessary to observe the desired spectral line since there are multiple solutions to the diffraction grating equation for a given grating angle. The spectrometer is a McPhersonModel 209 spectrometer that has been modified by the installation of an echelle grating with a 316/mm groove density and blaze angle of 63.43 degrees. The grating angle can be adjusted manually or by a stepper motor controller for greater precision.

The IDS system was calibrated by observing a total of 11 lines, some at multiple orders. After subtracting systematic errors, the calibration error in flow velocity was less than ± 6 km/s, and the absolute wavelength calibration error was ± 0.009 nm for the CIII 229.687 nm line at 25th order. Upon calibration, the IDS system was used to analyze the shape of the CIII 229.687 nm emission line during counter-helicity spheromak merging experiments.

Applications to my research:

I will be using IDS to study SSX plasmas this summer, so Chris's description of how the diagnostic works is of direct relevance to my project. The first simulation I did involved the line strength ratio that we should expect to see between the CIII 229.687 nm line mentioned above and the CIV 155.072 nm and 154.813 nm doublet, and as the summer progresses I will do simulations for other impurity emission lines. Comparing these simulations to the actual line ratios that we observe in SSX will be invaluable for understanding what the observational results mean in terms of plasma properties.