1. Thomas Jefferson National Accelerator Facility
300kV DC High Voltage Gun with Inverted Insulator Geometry and CsK2Sb Photocathode
Y. Wang, P. Adderley, B. Bullard, J. Benesch, J. Grames, F. Hannon, C. Hernandez-Garcia, J. Hansknecht,
G. Krafft, R. Kazimi, M. Poelker, M. A. Mamun, G. Palacios, R. Suleiman, M. Tiefenback, S. Wijiethunga, S. Zhang
Thomas Jefferson National Accelerator Facility
Introduction
Compact DC Gun / Inverted Insulator
Shed / Round Electrode
Shed reduces high electric
field at joint point of insulator
and electrode.
Round electrode has no sharp edges.
The goal of this research project is to develop a
reliable compact DC high voltage photogun, make alkali-antimonide photocathode, and characterize
beam quality. Inverted insulator, ball-shape
electrode and shed are the key design features. Preliminary studies show very promising results: gun reliably operates at 300kV; shed extends lifetime of the insulator; alkali-antimonide photocathode QE survives bad vacuum conditions and ion back bombardment; beam emittance is very low compared with that from others.
New design (fig. b) employs inverted-insulator
geometry, which reduces interior surface area
Simulation
Poisson & COMSOL
are used to simulate
electric field of gun
Low Field Emission after HV Conditioning
Spent 150 hours to condition gun
Kr HV conditioned to 350kV
Operated at 300kV
Minimal field emission
Five-Meter Beamline
Gun chamber
Lenses & correctors
Harp & viewers
Vacuum pumps & dump
Beam Operations
Operates at 300kV
14-month period
1400 hrs beam time
Insulator Lifetime
Shed protects insulator by reducing
electric field near it.
It lasted 1400 hours with beam operations.
CsK2Sb Photocathode
Photocathodes are made in-house. Active area is
controlled using mask.
Quantum efficiency
survives ion back
bombardment.
Emittance Studies
Photocathode Lifetime
Photocathode lifetime was measured using 4.5A
CW beam for several hours. Lifetime is 164 C.
Beam Profile
Beam profiles are measured using YAG viewer and
wire scanner with low duty factor tune beam.
Beam emittance was calculated by varying lens
strength and measuring corresponding beam sizes.
Studies were conducted
under different conditions:
varying high voltage,laser
size and position, beam
current.
Emittance near damaged
spot was also studied.
Summary & Plan
Designed & built compact DC gun with inverted insulator geometry; HV conditioned gun using Kr gas; operated at 300 kV
Conducted beam emittance studies.
Unprecedented low normalized emittances (~0.1 mm mrad at 0.2 mm laser rms size and 18nC bunch charges) encourages further development
CsK2Sb photocathode is robust: quantum efficiency survives ion back bombardment.
Identified reasons of HV breakdown
Make CsK2Sb photocathodes with different thickness of Sb layers
Quantify how surface roughness of the photocathode affects beam emittance
Redesign the shed to increase the insulator lifetime
Acknowledgement: This work is supported by the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC05-06OR23177