7. FY06 HIGH POLARIZATION ELECTRON
SOURCE/ACCELERATOR MATERIALS DEVELOPMENT
by Bob Kirby and Takashi Maruyama
Appendix B Self-Evaluation FY2006
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The Surface and
Materials Science Dept. (SMS) contributes to SLAC's accomplishments in a number of areas, by using
vacuum and materials expertise to support the development of novel electron
sources, detectors and accelerating structures. Current areas of focus include
a high polarization-high current electron source for the ILC, metal photocathodes
for the LCLS photo-injector, and surface-analytical research and development
on methods for suppressing collective electron instabilities in high current
positron/proton storage rings.
SMS engages in a continuing
research collaboration with Linear Collider Detector Group, the Accelerator
Technology Group's Sources and Polarization Group, and the University of
Wisconsin on the development of high-polarization
high-current semiconductor electron sources,
originally for E-122, then for the SLC and End Station A experiments, and
currently for the ILC. After several years of DOE SBIR programs, strained-superlattice
photocathodes based on GaAsP and GaAs have been developed in collaboration
with SVT Associates, who grow such wafers using molecular-beam-epitaxy (MBE).
The strained superlattice structures consisting of very thin quantum well
layers alternating with lattice-mismatched barrier layers are excellent
candidates for achieving higher polarization. Due to the difference in the
effective mass of the heavy- and light-holes, a superlattice exhibits a
natural splitting of the valence band, which adds to the strain-induced
splitting. In addition, each of the superlattice layers is thinner than
the critical thickness for strain relaxation. Spin polarization as high
as 86% is reproducibly observed with the quantum efficiency (QE) exceeding
1%.
Although the GaAsP/GaAs strained-superlattice
structure is considered as the leading candidate for the ILC polarized electron
source, the polarization appeared saturated at about 85% and is independent
of the valence band energy splitting. This is a strong indication of a spin-depolarization
mechanism in the GaAsP/GaAs structure. To characterize the spin-depolarization
mechanism, three structures are under investigation:
1) Biased photocathodes −
The spin depolarization apparently takes place during transport in the conduction
band and in the band bending region. By applying a bias voltage, the electron
drift velocity can be controlled and the band bending can be altered. The
bias across the device is achieved through a metallic grid photolithographically
grown atop the emitting GaAs surface and a back contact to the substrate
GaAs. Supported by a DOE STTR Phase II program, spin-polarized photoemission
from metal-gridded cathodes has been investigated in collaboration with
Saxet Surface Science. When the surface is positively biased, the QE increases
as much as 100% as a result of the lower vacuum level. The polarization
is also observed to increase by 5% (∆Pe/Pe).
2) AlInGaAs/GaAs strained-superlattice
structure −
The aluminum content determines the formation of a barrier in the conduction
band, while adding indium leads to conduction band lowering, so that the
conduction band offset can be completely compensated by an appropriate choice
of the aluminum and indium contents. As a result, a higher vertical electron
mobility and a lower spin relaxation rate can be achieved. The structure
has been investigated in collaboration with a St. Petersburg Technical University
group. Polarization as high as 90% has been observed from two wafers. However,
such a high polarization does not seem to be reproducible partly because
the quaternary structure is more difficult to grow. Furthermore, the high
polarization can be observed only when the cathode heat-cleaning temperature
is significantly lower than the standard 600°C.
The low temperature heat-cleaning technique using atomic hydrogen source
will be investigated.
3) InGaP/GaAs strained-superlattice
structure −
This is a structure similar to the GaAsP/GaAs strained-superlattice structure,
but with the GaAsP barrier layers replaced by InGaP. The GaAs layers are
quantum wells and continue to be strained. The spin-orbit interaction in
InGaP is three times smaller than in GaAsP, and the spin depolarization
is expected to be smaller. Since the band gap energy in InGaP is larger
than in GaAsP, higher QE is also expected. Five wafers have been grown through
a DOE SBIR Phase I program, and cathode characterization is in progress.
Electron cloud
disruption of positively-charged beams
is a significant problem in high-current positron and proton rings, and
is expected to be a problem in the LHC main ring and the ILC positron
Damping Ring. Heating
by very low energy secondary electrons endangers the LHC beam chamber cryogenic
budget. SMS’s X-ray photoelectron spectrometer (XPS) makes secondary electron
yield (SEY) measurements down to 10 eV primary electron energy. In FY06,
secondary yield and XPS surface chemical valence measurements continued
on yield-suppressing coatings on grooved surfaces of aluminum (Al). Particularly
interesting was grooving plus TiN coating, which have a cumulative yield-lowering
effect. Various grooving profiles were measured, bare or with TiN coating,
with coated values less than one, before electron conditioning (which lowers
the yield further during ring commissioning).
Ion bombardment of the coatings
from beam- and surface-ionized residual gas was measured
using an ion gun with H2
and N2
feed gases. The measured conditioning efficiency of ions over electrons
was several thousand times higher, with ion mass as a secondary effect.
In early FY07, coated pieces of flat and grooved chamber wall will be inserted
into the PEP-II ring to determine the effect of photon scrubbing
on SEY. The samples will be transported, after exposure and under vacuum,
to the XPS chamber for yield measurement.
The Linear Coherent
Light Source (LCLS) injector
is scheduled to commission with a metal photocathode having a quantum photoefficiency
(QE) of > 2x10-5
at the exciting laser wavelength of 255 nm. Cathodes are now prequalified
for installation by measuring the QE in the SMS Cathode Qualification System,
after processing the cathode to maximize the QE. Then, after installation
at the injector, only a modest bake-out is be required to remove atmospheric-adsorbed
water and hydrocarbons. The first LCLS cathode is now prequalified and installed
in the Sector 20 rf gun.
In vacuo
process cleaning of the copper cathode surface is done by bombardment with
1-3 keV H2 ions, which previous laboratory
measurements showed does not increase surface roughness. Several hours of
initial bombardment is needed to remove the amorphous machining layer from
the surface then, after air exposure, only a brief re-bombardment is required
to restore good QE. |
SLAC National
Accelerator Laboratory, Menlo Park, CA