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100 MeV LINAC

A photoinjector is a high gradient electron accelerator used to produce a short pulse, high peak current, high quality relativistic electron beam. High quality is defined in this case in terms of the ability to focus the electron beam into a very tight focal spot. The presently described application for the photoinjector is acceleration of the electron pulses to high energy by a linear accelerator and interaction with a high peak power laser pulse produced by the FALCON laser system.

Basically, the goal is to put as many highly relativistic electrons in the laser focus as possible, defining the requirements for focal spot size, pulse length, and synchronization. The photoinjector and Thomson scattering project represent a new operating regime for the LLNL RF linac. The current application of the linac is the production of high average current using a thermionic injector optimized for the production of the worlds highest intensity positron beam .

The relativistic electrons scatter the laser photons, shifting their wavelength to the x-ray range [THOMSON SCATTERING WEB LINK]. Since the x-ray photons are produced by the FALCON laser pulse, the x-ray pulse is well synchronized with the optical pulse, leading to a wide array of pump-probe type experiments requiring short x-ray probe pulses synchronized within less than one picosecond.

The key challenges for this project include production of the laser pulse, production of the electron pulse, and synchronization of the two systems in the sub-picosecond regime. The photoinjector system requirements and development are described here.

Photoinjector

In essence, a photoinjector is a high gradient standing wave microwave cavity. The LLNL photoinjector is powered by 10 MW of pulsed microwave energy from a Klystron with a frequency of 2.8545 GHz.

The resonant cavity builds up an accelerating field over several hundred nanoseconds until the peak gradient on axis is of the order 100 MV/m. Electrons are produced through photoemission at the peak of this accelerating field. Solenoidal focusing provides a means of minimizing transverse emittance in the electron beam by utilizing a known correlation between beam divergence and longitudinal position within the electron bunch [1]. Emittance is defined as a product of the minimum electron spot size and divergence at a focus.

Photoemission by a short pulse of ultraviolet light produces a beam that has high peak current density, short duration, small spot size, and low energy spread. The UV photons have sufficient energy to overcome the work function of the cathode material (pure Copper in our case), and release the electrons from the material surface in the presence of a high-gradient accelerating field. The electrons become relativistic over the scale of a few centimeters.

Due to the properties of special relativity, the electrostatic forces which would normally blow apart the high charge electron bunch are reduced quickly as the energy of the electrons reach the Mega electron Volt range. This fact leads to a significant reduction of space charge induced emittance growth.

The photoinjector developed for LLNL is configured with two resonant microwave cavities, coupled on axis through an iris, which also allows the electron beam to pass through the linear field region. The longitudinal resonant mode of the structure has a field null in the iris region, so that the fields in the second cell are 180 degrees out of phase with the first. This arrangement is known as a "p mode" cavity field-distribution.

Another important feature of the LLNL accelerator structure is the fact that the coupling to the RF drive system is accomplished through an iris in the second cavity alone. The first cavity has no RF coupling, except for the azimuthally symmetric iris on the axis of the electron beam. This feature is preferred since the RF coupling introduces a dipole type accelerating term, which degrades the beam emittance. The adverse effect of this field component is worse when the electrons have low energy. Since the electrons have low energy in the first cell, the dipole field effect is reduced. This design was established through a joint project between SLAC, BNL, and UCLA [2].

Bill's New Collimator

Photoemission not only produces a higher quality electron beam than more conventional means, it also directly synchronizes the electron pulses with the FALCON laser system. The UV photons used to produce the electron beam are derived from optical pulses originating from the FALCON laser. This fact does not, however completely overcome the synchronization issue[3].

High resolution electron beam diagnostics are required to properly characterize and optimize the electron beam produced by the photoinjector and accelerated by the linac. The previous high average current application of the linac did not depend strongly on the transverse beam characteristics of the linac. The emittance and spot profile were not as important as the average current and beam transmission. Due to these factors, the only diagnostics on the linac were current transformers and Faraday cups. Recently, we have implemented both beam profile and emittance diagnostics (using optical transition radiation or "OTR").