2011 Particle Accelerator Conference - List of Authors (BastaniNejad, M.)

Paper

Title

Page

RF Breakdown Studies Using Pressurized Cavities

184

R. Sah, A. Dudas, R.P. Johnson, M.L. Neubauer
Muons, Inc, Batavia, USA
M. BastaniNejad, A.A. Elmustafa
Old Dominion University, Norfolk, Virginia, USA
J.M. Byrd, D. Li
LBNL, Berkeley, California, USA
M.E. Conde, W. Gai
ANL, Argonne, USA
A. Moretti, M. Popovic, K. Yonehara
Fermilab, Batavia, USA
D. Rose
Voss Scientific, Albuquerque, New Mexico, USA

Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE Contract DE-AC02-07CH11359
Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved, and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual gas and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of radiofrequency and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) has been built, and a series of detailed experiments is planned at the Argonne Wakefield Accelerator. These experiments will be followed by additional experiments using a second test cell operating at 402.5 MHz.

WEODS3

CEBAF 200 kV Inverted Electron Gun

1501

J.M. Grames, P.A. Adderley, J. Clark, J. Hansknecht, M. Poelker, M.L. Stutzman, R. Suleiman, K.E.L. Surles-Law
JLAB, Newport News, Virginia, USA
M. BastaniNejad
Old Dominion University, Norfolk, Virginia, USA
J.L. McCarter
UVa, Charlottesville, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In addition, DOE-HEP funds this work in support of the ILC R&D program.
Two DC high voltage GaAs photoguns have been built at Jefferson Lab based on a compact inverted insulator design. One photogun provides the polarized electron beam at CEBAF and operates at 130 kV bias voltage. The other gun is used for high average current lifetime studies at a dedicated test facility and has been operated at bias voltage up to 225 kV. The advantages of higher DC voltage for CEBAF include reduced space-charge emittance growth and the potential for prolonged photocathode lifetime. However, a consequence of operating at higher voltages is the increased likelihood of field emission or breakdown, both of which are unacceptable. Highlights of the R&D studies leading toward a production 200keV GaAs photogun for CEBAF will be presented.

Slides WEODS3 [1.360 MB]

WEP287

Field Emission Measurements from Niobium Electrodes

2020

M. BastaniNejad
Old Dominion University, Norfolk, Virginia, USA
P.A. Adderley, J. Clark, S. Covert, J. Hansknecht, C. Hernandez-Garcia, R.R. Mammei, M. Poelker
JLAB, Newport News, Virginia, USA

Increasing the operating voltage of a DC high voltage photogun serves to minimize space charge induced emittance growth and thereby preserve electron beam brightness, however, field emission from the photogun cathode electrode can pose significant problems: constant low level field emission degrades vacuum via electron stimulated desorption which in turn reduces photocathode yield through chemical poisoning and/or ion bombardment and high levels of field emission can damage the ceramic insulator. Niobium electrodes (single crystal, large grain and fine grain) were characterized using a DC high voltage field emission test stand at maximum voltage -225kV and electric field gradient > 10MV/m. Niobium electrodes appear to be superior to diamond-paste polished stainless steel electrodes.

Paper	Title	Page
MOP046	RF Breakdown Studies Using Pressurized Cavities	184
	R. Sah, A. Dudas, R.P. Johnson, M.L. Neubauer Muons, Inc, Batavia, USA M. BastaniNejad, A.A. Elmustafa Old Dominion University, Norfolk, Virginia, USA J.M. Byrd, D. Li LBNL, Berkeley, California, USA M.E. Conde, W. Gai ANL, Argonne, USA A. Moretti, M. Popovic, K. Yonehara Fermilab, Batavia, USA D. Rose Voss Scientific, Albuquerque, New Mexico, USA
	Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE Contract DE-AC02-07CH11359 Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved, and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual gas and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of radiofrequency and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) has been built, and a series of detailed experiments is planned at the Argonne Wakefield Accelerator. These experiments will be followed by additional experiments using a second test cell operating at 402.5 MHz.

WEODS3	CEBAF 200 kV Inverted Electron Gun	1501
	J.M. Grames, P.A. Adderley, J. Clark, J. Hansknecht, M. Poelker, M.L. Stutzman, R. Suleiman, K.E.L. Surles-Law JLAB, Newport News, Virginia, USA M. BastaniNejad Old Dominion University, Norfolk, Virginia, USA J.L. McCarter UVa, Charlottesville, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In addition, DOE-HEP funds this work in support of the ILC R&D program. Two DC high voltage GaAs photoguns have been built at Jefferson Lab based on a compact inverted insulator design. One photogun provides the polarized electron beam at CEBAF and operates at 130 kV bias voltage. The other gun is used for high average current lifetime studies at a dedicated test facility and has been operated at bias voltage up to 225 kV. The advantages of higher DC voltage for CEBAF include reduced space-charge emittance growth and the potential for prolonged photocathode lifetime. However, a consequence of operating at higher voltages is the increased likelihood of field emission or breakdown, both of which are unacceptable. Highlights of the R&D studies leading toward a production 200keV GaAs photogun for CEBAF will be presented.
	Slides WEODS3 [1.360 MB]

WEP287	Field Emission Measurements from Niobium Electrodes	2020
	M. BastaniNejad Old Dominion University, Norfolk, Virginia, USA P.A. Adderley, J. Clark, S. Covert, J. Hansknecht, C. Hernandez-Garcia, R.R. Mammei, M. Poelker JLAB, Newport News, Virginia, USA
	Increasing the operating voltage of a DC high voltage photogun serves to minimize space charge induced emittance growth and thereby preserve electron beam brightness, however, field emission from the photogun cathode electrode can pose significant problems: constant low level field emission degrades vacuum via electron stimulated desorption which in turn reduces photocathode yield through chemical poisoning and/or ion bombardment and high levels of field emission can damage the ceramic insulator. Niobium electrodes (single crystal, large grain and fine grain) were characterized using a DC high voltage field emission test stand at maximum voltage -225kV and electric field gradient > 10MV/m. Niobium electrodes appear to be superior to diamond-paste polished stainless steel electrodes.