We are now nearing the end of the second decade since the inaugural photoinjector experiments were carried out at Los Alamos National Laboratory in 1985. The interim period has seen a dramatic increase in the number of photoinjectors in operation around the world, in our physical understanding of their operation, and, consequently, the brightness of their beams. For some time now, photoinjectors have been the solution of choice for electron beam applications requiring very high-quality electron beams, such as free-electron lasers (FELs). In recent years, there has been a renewed interest in high-average-power FELs and, as a result, high-average-current photoinjectors. Here, we present an overview of photoinjector development with particular emphasis on high-average-current devices in FEL applications.

Overview of high-brightness, high-average-current photoinjectors for FELs

The drive laser system of the photoelectric injector for the Los Alamos free-electron laser (FEL) is described. This system consists of a 1.053 mu m mode-locked Nd:YLF oscillator, a grating-rhomb pulse compressor, a fast electrooptic shutter to reduce pulse repetition frequency, a pulsed Nd:YLF amplifier chain, and a KTP frequency doubler. The bright electron beams required for high performance FELs demand stringent specifications on drive laser output levels, beam quality, and stability. The design specifications, laser architecture, and operating performance following the most recent round of upgrades are presented. >

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The Los Alamos FEL photoinjector drive laser

The two-screen measurement of emittance is a standard method often used by many accelerator laboratories in the world. In this method forming a beam waist at the location of the screen is a necessary condition. But in many cases it is very difficult to satisfy this condition. Instead, one often uses an approximate method, in which the beam spot at the screen is focused as small as possible (beam spot minimizing). Then the minimum beam spot seems as a beam waist. In this present paper, we give a new two-screen emittance measurement formula which corresponds directly to the condition “beam spot minimizing”. Thus, the approximation problem is solved completely.

Calculation of two-screen emittance measurement

In this thesis, the transverse and longitudinal beam properties of the PHIN photoinjector are characterized. The objective of the research is to demonstrate the reliable and stable production of a 1.3 µs long bunch train, with 2.33 nC charge per bunch and 4.5 µC of total charge, by the PHIN photoinjector. The results of this thesis are the important steps towards the feasibility demonstration of a photoinjector as the Compact Linear Collider's drive beam source. The PHIN photoinjector has been conceptualized by a collaboration between "Laboratoire de l'Accelerateur Lineaire (LAL)", "Rutherford Appleton Laboratory (RAL)" and "Organisation Europeenne pour la Recherche Nucleaire (CERN)". Within this collaboration, LAL and RAL have committed to the design and the construction of the RF gun and laser, respectively. The photocathode production as well as the overall coordination and commissioning were under the responsibility of CERN. The project is in the framework of the second Joint Research Activity PHIN of the European CARE program. The photoinjector has been installed, in 2008, on a dedicated test-stand at CERN. During the commissioning of the PHIN photoinjector, the beam properties have been studied by rigorous systematic measurements and simulations. The maximum charge per bunch yield of the Cs2Te cathode has been found to be 4.4 nC, in agreement with the theoretical limit of 4.7 nC. The transverse normalized emittance, beam energy and energy spread have been measured both in a single shot and the time-resolved manner. The single shot measurements revealed useful information about the shot-to-shot stability of the beam properties, whereas the time-resolved measurements have reflected the stability along the bunch train. The multi-slit emittance measurement set-up, OTR profile monitors with gated CCD cameras and a segmented beam dump have been designed, implemented and utilized for the main measurements. The experimental characterization and the numerical studies clearly indicate the feasibility of a photoinjector within the specifications defined by the existing CLIC test facility. Further simulation and design studies have been conducted based on several modifications of the current PHIN RF gun design. In this successive model an RF gun with the specifications of the CLIC drive beam has been investigated and proposed as the preliminary design for the future CLIC drive beam injector studies.

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Study and Experimental Characterization of a Novel Photo Injector for the CLIC Drive Beam

The usefulness of imaging electro-optics for rf-driven accelerators can be enhanced by synchronizing the instruments to the system fundamental frequency or an appropriate subharmonic. This step allows one to obtain micropulse bunch length and phase during a series of linac bunches or storage ring passes. Several examples now exist of the use of synchroscan and dual-sweep streak cameras and/or image dissector tubes to assess micropulse scale phenomena (10 to 30 ps) during linac and storage ring operations in the U.S., Japan, and Europe. As space permits, selections will be presented from the list of phase stability phenomena on photoelectric injectors, micropulse bunch length during a macropulse, micropulse elongation effects, transverse Wakefield effects within a micropulse, and submicropulse phenomena on a stored beam. Potential applications to the subsystems of the Advanced Photon Source (APS) will be briefly addressed. >

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RF-synchronized imaging for particle and photon beam characterizations

We report our initial measurements of bright (high-charge, low-emittance) electron beams generated at the Los Alamos High Brightness Accelerator FEL (HIBAF) Facility. Normalized emittance values of less than 50 {pi} mm-mrad for charges ranging from 0.7 to 8.7 nC were obtained for single micropulses at a y-waist and at an energy of 14.7 MeV. These measurements were part of the commissioning campaign on the HIBAF photoelectric injector. Macropulse measurements have also been performed and are compared with PARMELA simulations. 5 refs., 8 figs., 3 tabs.

A. H. Lumpkin

Papers

Initial observations of high-charge, low-emittance electron beams at HIBAF (High Brightness Accelerator FEL)