24 research outputs found
Comparison of DC and SRF Photoemission Guns For High Brightness High Average Current Beam Production
A comparison of the two most prominent electron sources of high average
current high brightness electron beams, DC and superconducting RF photoemission
guns, is carried out using a large-scale multivariate genetic optimizer
interfaced with space charge simulation codes. The gun geometry for each case
is varied concurrently with laser pulse shape and parameters of the downstream
beamline elements of the photoinjector to obtain minimum emittance as a
function of bunch charge. Realistic constraints are imposed on maximum field
values for the two gun types. The SRF and DC gun emittances and beam envelopes
are compared for various values of photocathode thermal emittance. The
performance of the two systems is found to be largely comparable provided low
intrinsic emittance photocathodes can be employed
Thermal emittance measurements of a cesium potassium antimonide photocathode
Thermal emittance measurements of a CsK2Sb photocathode at several laser
wavelengths are presented. The emittance is obtained with a solenoid scan
technique using a high voltage dc photoemission gun. The thermal emittance is
0.56+/-0.03 mm-mrad/mm(rms) at 532 nm wavelength. The results are compared with
a simple photoemission model and found to be in a good agreement.Comment: APL 201
Ultracold electrons via Near-Threshold Photoemission from Single-Crystal Cu(100)
Achieving a low mean transverse energy or temperature of electrons emitted
from the photocathode-based electron sources is critical to the development of
next-generation and compact X-ray Free Electron Lasers and Ultrafast Electron
Diffraction, Spectroscopy and Microscopy experiments. In this paper, we
demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled
(100) surface of copper using near-threshold photoemission. Further, we also
show that the electron energy spread obtained from such a surface is less than
11.5 meV, making it the smallest energy spread electron source known to date:
more than an order of magnitude smaller than any existing photoemission, field
emission or thermionic emission based electron source. Our measurements also
shed light on the physics of electron emission and show how the energy spread
at few meV scale energies is limited by both the temperature and the vacuum
density of states
Demonstration of Low Emittance in the Cornell Energy Recovery Linac Injector Prototype
We present a detailed study of the six-dimensional phase space of the
electron beam produced by the Cornell Energy Recovery Linac Photoinjector, a
high-brightness, high repetition rate (1.3 GHz) DC photoemission source
designed to drive a hard x-ray energy recovery linac (ERL). A complete
simulation model of the injector has been constructed, verified by measurement,
and optimized. Both the horizontal and vertical 2D transverse phase spaces, as
well as the time-resolved (sliced) horizontal phase space, were simulated and
directly measured at the end of the injector for 19 pC and 77 pC bunches at
roughly 8 MeV. These bunch charges were chosen because they correspond to 25 mA
and 100 mA average current if operating at the full 1.3 GHz repetition rate.
The resulting 90% normalized transverse emittances for 19 (77) pC/bunch were
0.23 +/- 0.02 (0.51 +/- 0.04) microns in the horizontal plane, and 0.14 +/-
0.01 (0.29 +/- 0.02) microns in the vertical plane, respectively. These
emittances were measured with a corresponding bunch length of 2.1 +/- 0.1 (3.0
+/- 0.2) ps, respectively. In each case the rms momentum spread was determined
to be on the order of 1e-3. Excellent overall agreement between measurement and
simulation has been demonstrated. Using the emittances and bunch length
measured at 19 pC/bunch, we estimate the electron beam quality in a 1.3 GHz, 5
GeV hard x-ray ERL to be at least a factor of 20 times better than that of
existing storage rings when the rms energy spread of each device is considered.
These results represent a milestone for the field of high-brightness,
high-current photoinjectors.Comment: Physical Review Special Topics - Accelerators and Beams 16, 073401
(2013
Experimental characterization of photoemission from plasmonic nanogroove arrays
Metal photocathodes are an important source of high-brightness electron
beams, ubiquitous in the operation of both large-scale accelerators and
table-top microscopes. When the surface of a metal is nano-engineered with
patterns on the order of the optical wavelength, it can lead to the excitation
and confinement of surface plasmon polariton waves which drive nonlinear
photoemission. In this work, we aim to evaluate gold plasmonic nanogrooves as a
concept for producing bright electron beams for accelerators via nonlinear
photoemission. We do this by first comparing their optical properties to
numerical calculations from first principles to confirm our ability to
fabricate these nanoscale structures. Their nonlinear photoemission yield is
found by measuring emitted photocurrent as the intensity of their driving laser
is varied. Finally, the mean transverse energy of this electron source is found
using the solenoid scan technique. Our data demonstrate the ability of these
cathodes to provide a tenfold enhancement in the efficiency of photoemission
over flat metals driven with a linear process. We find that these cathodes are
robust and capable of reaching sustained average currents over 100 nA at
optical intensities larger than 2 GW/cm with no degradation of performance.
The emittance of the generated beam is found to be highly asymmetric, a fact we
can explain with calculations involving the also asymmetric roughness of the
patterned surface. These results demonstrate the use of nano-engineered
surfaces as enhanced photocathodes, providing a robust, air-stable source of
high average current electron beams with great potential for industrial and
scientific applications.Comment: 9 pages, 9 figure
Photocathode Behavior During High Current Running in the Cornell ERL Photoinjector
The Cornell University Energy Recovery Linac (ERL) photoinjector has recently
demonstrated operation at 20 mA for approximately 8 hours, utilizing a
multialkali photocathode deposited on a Si substrate. We describe the recipe
for photocathode deposition, and will detail the parameters of the run.
Post-run analysis of the photocathode indicates the presence of significant
damage to the substrate, perhaps due to ion back-bombardment from the residual
beamline gas. While the exact cause of the substrate damage remains unknown, we
describe multiple surface characterization techniques (X-ray fluorescence
spectroscopy, X-ray diffraction, atomic force and scanning electron microscopy)
used to study the interesting morphological and crystallographic features of
the photocathode surface after its use for high current beam production.
Finally, we present a simple model of crystal damage due to ion
back-bombardment, which agrees qualitatively with the distribution of damage on
the substrate surface.Comment: 20 pages, 15 figure