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