FEL/Duke University, Durham, North Carolina
BINP SB RAS, Novosibirsk
Funding: This work is supported by the US DoE grant #DE-FG02-01ER41175
A booster-injector synchrotron has been recently built and commissioned at Duke University to provide for the top-off injection into the storage ring in the energy range of 0.24 - 1.2 GeV. Booster injection kicker was designed with a pulse length of 18 out of 19 booster separatrixes, assuming a long train of electron bunches to be injected from the existing linac. Such scheme required a major linac upgrade from single bunch photo emission mode to a multibunch thermionic mode. A major disadvantage of the latter was much higher radiation levels in the facility. Since commissioning, the booster could only operate with one or two bunches limited by both long kicker pulse and single bunch injection from the linac. The consequent limitation of the injection rate restricted the capability of production of the Compton gamma rays in the loss mode, i.e. production of gammas with energy above 20-25 MeV, to about 5*108 photons per sec. Update of the linac for the repetition rate of up to 10 Hz, and modification of the injection kicker for 15 nS pulse length allowed us to developed an alternative multibunch injection scheme with a significant increase of the injection rate into storage ring.
FEL/Duke University, Durham, North Carolina
Funding: This work is supported by US Air Force Office of Scientific Research medical FEL grant FA9550-04-01-0086.
The paper presents a novel technique of measuring the gain of a storage ring based FEL oscillator. As opposed to the conventional technique of measuring the FEL gain from its macro-pulse envelope, this new technique is based upon the measurement of pass-by-pass FELμpulses. To record the growth of the optical energy in the FEL micro-pulse train, we use fast photo-diodes and photo-multiplier tubes (PMTs). PMTs are usually employed at the very beginning of the FEL lasing development, while the photodiodes are used at the latter stages when the FEL power is fully developed and saturated. This new gain measurement technique provides a powerful tool to study the details of the FEL gain process starting from spontaneous radiation to saturation. It allows us to investigate five to seven orders of magnitude of the FEL energy growth. As fast photo-detectors with a sub-nanosecond time response become available, this new technique can be adopted for many oscillator FELs, including those driven by super-conducting linacs. Special attention is paid to the dynamic non-linearity issues of the photodiodes and PMTs associated with short length of FEL pulses.
FEL/Duke University, Durham, North Carolina
BINP SB RAS, Novosibirsk
Funding: Supported by US DoE grant #DE-FG02-01ER41175
The Duke FEL/HIGS (Free electron laser/High Intensity Gamma-ray source) facility has recently undergone through a series of major upgrade. As a part of this upgrade, a kicker system was designed to provide reliable injection from the booster into the storage ring at any energy chosen from the range of 240 MeV to 1.2 GeV. Relatively new and not sufficiently studied switching device has been selected as a basic component to build a set of nanosecond resolution high-voltage generators. So called Pseudo-Spark Switch (PSS), also known as a cold cathode thyratron, has the same or slightly better jitter, reasonable range of switched high voltages and significantly lower heater power as compared to the traditional “hot” thyratrons. Despite of the fact that it requires more complicated triggering system, this device still seems very attractive as a driver for short pulse kickers. Almost three years of operation of the Duke FEL facility has revealed number of advantages and challenges related to the thyratrons of this type. In this paper we depict design features of the kicker system, discuss some accomplished improvements and summarize our three year experience.
FEL/Duke University, Durham, North Carolina
TUNL, Durham, North Carolina
Funding: This work is supported by US Air Force Office of Scientific Research medical FEL grant FA9550-04-01-0086 and by US Department of Energy grant DE-FG02-01ER41175.
Since 2008, the upgraded High Intensity Gamma-ray Source (HIGS) at the Duke FEL Lab has provided users with gamma beams of unprecedented quality for scientific research. The recently completed accelerator upgrades include a HOM-damped RF cavity, a full-energy top-off booster injector, redesigned storage ring straight sections, and two new FELs. The HIGS facility is now capable of producing a high intensity gamma beam in a wide energy range (1 - 100 MeV) using commercial FEL mirrors. It has achieved an exceptionally high flux, up to ~1010 g/s total (< 20 MeV), making it the world's most powerful Compton gamma source. It produces almost 100% polarized gammas, either linear or circular. At the HIGS, the gamma energy can be changed rapidly within a factor of three in minutes. Operated by Triangle Universities Nuclear Laboratory since summer 2008, the HIGS is a dedicated Compton gamma source, capable of producing more than 2,000 h of gamma beam time per year with a five-day, two-shift schedule. Future development at the HIGS includes higher energy gamma beams toward the pion threshold and a rapid switch of circular polarization.
