Diagnostics In H 2 Discharges Using A Tunable 193nm Excimer Laser

Abstract

ABSTRACT An H' source for high brightness H° beams should have the lowest possible internal temperature T, because the beam-power density at a distance scales as 1/T. We determine local T inside a discharge with the use of a tunable ArF excimer laser that measures the populations of the lowest four rotational states of H2 by means of (2+1) resonance enhanced multiphoton ionization (REMPI). The number of laser- created ions causes a proportional change of the discharge's electrical impedance* A calibration in room temperature H2 (without a discharge) showed that the REMPI works well. Another objective is to develop a monitor for the vibrationally excited molecules, H2(v), that are believed to be the primary origin of H within volume-type ion sources. We use the same apparatus and approach, but add a Raman shifter to change the laser wavelengths to those appropriate for (2+1) REMPI of H2(v). 1 . PRINCIPLE OF TFMPKKATTTRE MEASTTKKMKNTS Our approach is to measure a rotational T in various locations within a discharge via (2+1) resonance enhanced multiphoton ionization [i.e., (2+1) REMPI] in H2-1* 2 The (2+1) here means a two-photon excitation from a rotational level i in the ground (X) electronic and vibrational (v=0) states to levels of the E,F electronic state and an additional photon that ionizes from the E,F state. A given state yields a number of ions that is proportional to the density of i, n^. Therefore in a neutral gas only a simple current measurement is needed. In a discharge these ions produce a proportional impedance change that can be easily measured: this is an optogalvanic effect.For two different i, that have energies E^ and rotational quantum numbers j^, we convert the corresponding n^ to T by means of the Boltzmann equation. This assumes local thermal equilibrium.If the experimental ion signals S^ are proportional to the n^, then Si/S2 - [(2j1+l)(2T1+l)]/(2J2+l)(2T2+l)]exp-[(E1-E2)/kT], orT = [(E2-E1)/k] ln{[S2(2j1+l)(2T1+l)]/[S1(2j2+l)(2T2+l)]},where T^ is the nuclear spin degeneracy and is 1 and 3 for even and odd values of Ji, respectively.This example used only two states. The use of more rotational states improves the accuracy and provides a consistency check.Two-photon excitation occurs only where the light is focussed, and at the time of the laser pulse. The result is that temperature is measurable at desired locations within a discharge as well as at specific times (e.g., relative to the ion source pulse times). The measured rotational temperatures should be in equilibrium with the H temperature that is of direct interest. Even if there were