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Electron-Impact Selection Rules for Rotational Temperature Measurement of Molecular Hydrogen in the PFRC

Author: David R. Farley
Requested Type: Poster Only
Submitted: 2009-12-04 11:49:03

Co-authors: S.A. Cohen

Contact Info:
Princeton Plasma Physics Laboratory
P.O. Box 451
Princeton, NJ   08543
USA

Abstract Text:
Knowledge of gas temperatures is needed in the Princeton Field-Reversed Configuration (PFRC) device to understand fueling, heat transfer, and impurity generation. Spectroscopic molecular rotational temperature measurement techniques are often used in molecular gas dynamics and generally can be related to the gas translational temperature since rotational and translational modes equilibrate within several collisions. To analyze the data, many authors assume a low-mass electron cannot disturb the molecular rotational-level distribution, enforcing a rotational selection rule delta-K = 0 during electron-impact excitation. Others assert that optical selection rules apply (delta-K = 0,±1). In either case, these selection rules allow for the use of simple Boltzmann plots to estimate the rotational temperature. However, rotational transitions for delta-K > 1 are in fact allowed by molecular symmetry selection rules. It has been found through this study that non-optical selection rules (delta-K = ±2) applied to the hydrogen Fulcher-alpha emission (600-640 nm) can substantially affect the predicted rotational distributions. For example, the K = 5 upper rotational state can differ by up to 40%, resulting in inferred rotational temperatures being 100-250 degrees lower than would be obtained with simple Boltzmann plots and optical selection rules. A LN2-cooled copper cylinder has been built and inserted into the vacuum chamber of the PFRC to test these electron-impact excitation selection rules. The cold cylinder effectively lowers the temperature of hydrogen molecules through collisions with the cylinder wall. The Fulcher-alpha emission is collected through a small quartz window at the cylinder mid-plane, and routed to a 0.5-m iCCD-equipped spectrometer. Measured rotational spectra are fitted with a least-squares criterion using both optical and non-optical selection rule spectroscopic models. Also, ratios of the Q-branch Q3/Q1 and Q3/Q2 rotational lines provide clearly different rotational temperature results depending upon whether optical or non-optical selection rules are used. Initial results indicate that non-optical selection rules fit measured rotational spectra better than optical selection rules. Inferred temperatures from both spectral fits and line ratios are consistently cryogenic (< -200 C) with the non-optical model, whereas room temperatures are obtained using the optical model. Further analysis is being conducted to determine whether the hydrogen molecules within the cooled cylinder are indeed significantly cooled or not, which will then provide an answer as to which selection rules are correct.

Characterization: A3,E11

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