Experimental program
1) Polarization Studies
a) Polarization vs. Atomic Hydrogen Exposure
i) H-clean a Spire wafer like Paul did; make repeated
measurements of polarization after exposure to atomic hydrogen; 0 minutes,
15 minutes, 30 minutes, 60 minutes, 120 minutes total exposure time. QE
scan for each test to provide accurate site reference. Keep sample orientation
the same.
A new Spire wafer has been cut on May 21, 2001(sample
labels).
NEW!!!!Photos from the lucky wafer removed after 7 months of cooperation (02-20-02)
ii) Take one wafer and clean with atomic hydrogen
in portable H station. Use a moveable mask and vary the exposure on a single
wafer.
iii) Test a Russian wafer. Does polarization depend
on H exposure?
iv) Anodize and strip a wafer the way SLAC prepares
wafers for their guns. Compare QE and polarization to our technique.
v) Look at GaAs surface under microscope (what kind
of scope?). Strained layer samples - no H cleaning, small H cleaning (15
minutes), and excessive H cleaning (> 60 minutes). Similarly, look at the
surface of bulk GaAs after exposure of atomic H.
vi) Repeat polarization vs. H-exposure using a bulk
GaAs sample. Do we see the same effect?
b) Polarization vs. sample thickness and dopant
density.
i) Repeat Peter's measurements but pay close attention
to keeping H exposure constant and small. Perhaps Peter's results were
inconclusive because he inadvertently "roughened" the surface of the material
with too much atomic hydrogen. ("roughened" = whatever mechanism hurts
polarization). I think we have unused material to make a complete reevaluation
of all Peter's data. Perhaps we find we need to anodize and strip the samples
the way SLAC prepares wafers for gun use.
ii) Use the portable H cleaner as an H ion source
and implant a bulk wafer with ions of different energy. The idea is to
create different sample thicknesses at each ion implant zone. Use one wafer
with a mask to vary the sample thickness. Need QE scans.
iii) Purchase more SPIRE strained layer wafers with
different active layer thicknesses; 50 nm, 75 nm. We already have 100 nm
thick samples. Can we get beam polarization > 80% in this manner?
c) Continue to characterize new material
i) New Spire material
ii) Russian wafers (we have at least 4 more samples).
d) Higher beam polarization via two-photon absorption.
2) Lifetime Studies
a) Lifetime and the effects of ambient light
i) Extract constant beam current (e.g., 100 microA)
and measure lifetime versus laser wavelength. Use bulk GaAs and ti-sapphire
laser located near the gun (i.e., eliminate the fiber). Use Brian Bevins'
current lock to drive an attenuator and keep current constant throughout
test. Use a Coulomb counter to log charge delivered to dump. Record ion
pump current with floating picoammeter for each wavelength.
ii) Measure lifetime vs laser radial position on
cathode. Use unanodized bulk GaAs. Need QE scans after each position. Record
ion pump current with floating picoammeter. Measure laser spot size (you
get this indirectly with each qe scan).
iii) Measure lifetime vs. active area diameter.
Pick three (or more) anodized donut sizes; e.g., 9 mm, 5 mm, 3 mm.
b) Lifetime as a function of gas species.
For this we are using the thickness of the active layer as a "diagnostic".
Ions of various species and energy will be implanted in the active area
where they reduce QE or they are implanted behind the active layer where
they do no harm to QE. Information from these studies may help us engineer
better vacuum canisters.
i) Measure lifetime versus base pressure of different
gas species; H2, CH4, CO, CO2, Ar, etc. QE damage will depend on the following;
pressure, radial location of laser spot, ionization cross section of residual
gas species, and stopping depth. We are confident we are drawing the correct
conclusions if we see QE scans that make sense. For example, when we poison
the gun vacuum with H2, we should see QE damage at the electrostatic center
and at laser spot location (no through). When we poison the gun vacuum
with CH4, we should see a QE "trough" from electrostatic center to laser
spot location.
c) Lifetime vs. laser spot size.
i) Keep current constant but vary the size of the
laser spot at the cathode. We suspect lifetime will grow with laser spot.
Is this true? Are we justified quoting lifetime in units of charge/area?
3) Cathode Analyzing Power Studies
Need to define these studies. Or get in the habit of making analyzing
power measurements for every sample and see what information falls out.
It may be worthwhile to modify QE-Tool so that we "divide" two data sets;
pockel cell off and pockel cell at halfwave voltage.
4) Helicity Correlated Beam Studies
Need to define these tests. Will require adding beamline diagnostics.
Using the test cave gun is probably not necessary if Joe's tunnel/injector
work starts to bear fruit.
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