Work function dependence of surface produced H/sup -/ in the presence of a plasma Page: 4 of 9
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creates the plasma. A magnetic "bucket" geometry is created by
six rows of samarium-cobalt permanent magnets placed around the
15 cm diameter chamber.? The filaments are maintained at -90 V
with respect to the '■hamber walls and the discharge current is
controlled by adjustments in the filament temperature as the Hj
gas and Cs vapor densities are varied. The hydrogen pressure is
usually kept at ~1 mTorr while the Cs coverage on the Mo
converter is increased by heating the Cs oven, which puts more Cs
into the discharge. Once introduced, the Cs remains in the dis-
charge with a long time constant. The rotatable converter is not
cooled, so the Cs coverage is in dynamic equilibrium with the
plasma and depends on plasma temperature, density, Cs fraction
and bias on the converter.
The negative ions from the converter must traverse 7.5 cm of
plasma before entering the first slit at the exit of the
chamber. A second slit completes the collimation of the beam
before it enters a small magnetic analyzer. A slitted Faraday
cup at the focus of the analyzer is used to measure the partial
current and provides a momentum resolution of about 1.5'.
Previous experiments^-4 have established that the maximum
H- yield occurs when the surface work function is at a mini-
mum. This corresponds, also, to a maximum yield in the low energy
portion of the spectrum. Thus by monitoring the low energy H-
peak as Cs is introduced into the plasma, the minimum work func-
tion conditions can be found. Then, with small adjustments in
the oven temperature and discharge parameters, the conditions can
be stabilized so that the H- yield is constant over several
For the results reported here, the converter bias was main-
tained at -150 V. The plasma parameters were ne = 3 x lO^O
cm~3 and Te = 2.5 eV, as measured with a small Langmuir probe.
The normal to the converter was initially aligned with the
coihmation to the analyzer. After the plasma had been stabili-
zed, the analyzer magnet was swept and the H- energy spectrum
recorded. The converter was then rotated by 5* and the procedure
repeated. This process continued until the converter had been
rotated by 45*, and then repeated once more at the 0* position to
confirm the constancy of the H- yield which was generally with-
in 2% of the original measurement. The entire proces required
about 4 minutes to complete.
The angle could be set to about 0.5’ whjle the collimation ad-
mitted ions over a range of about 1.40*. Repetition of the
experiment while rotating the converter in the opposite direction
gave essentially the same results.
A typical raw data result at 0", with the Cs coverage near
optimum is shown in Fig. 2. This result is very similar to ones
measured by Leung and Ehlers.3 Mote the sharp rise at the
lowest Hall probe values, and the shoulders at the larger Hall
probe values. The width of the curve corresponds in energy with
H- ions originating at the surface with nearly zero-energy,
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Wada, M.; Pyle, R.V. & Stearns, J.W. Work function dependence of surface produced H/sup -/ in the presence of a plasma, article, November 1, 1983; [Berkeley,] California. (https://digital.library.unt.edu/ark:/67531/metadc1067485/m1/4/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.