Geoffrey A. Krafft

Narrow-band emission in Thomson sources operating in the high-field regime.

We present a novel and quite general analysis of the interaction of a high-field chirped laser pulse and a relativistic electron, in which exquisite control of the spectral brilliance of the up-shifted Thomson-scattered photon is shown to be possible. Normally, when Thomson scattering occurs at high field strengths, there is ponderomotive line broadening in the scattered radiation. This effect makes the bandwidth too large for some applications and reduces the spectral brilliance. We show that such broadening can be corrected and eliminated by suitable frequency modulation of the incident laser pulse. Furthermore, we suggest a practical realization of this compensation idea in terms of a chirped-beam-driven free electron laser oscillator configuration and show that significant compensation can occur, even with the imperfect matching to be expected in these conditions.

Magnetized electron beam for the JLEIC re-circulator cooler ring

The ion beams of the proposed Jefferson Lab Electron Ion Collider (JLEIC) must be cooled to achieve the required collision luminosity. In general, cooling is accomplished when an electron beam co-propagates with an ion beam moving at the same average velocity, but with different temperature, where the energy of chaotic motion of the ion beam is transferred to the cold electron beam. The cooling rate can be improved by about two orders of magnitude if the process occurs inside a solenoidal magnetic field – so-called magnetized cooling - that forces the electrons to follow small helical trajectories thereby increasing the interaction time with ions and improving the cooling efficiency. However, one of the challenges associated with implementing this cooling technique relates to the fringe field of the cooling solenoid which imparts a large unwanted azimuthal kick onto the electron beam that prevents the electron beam from traveling in the desired tight, well-defined volume within the solenoid. As proposed by Derbenev, the ill-effect of this fringe field can be cancelled if the electron beam is born in a similar field and encountering a fringe field upon exiting the electron gun that produces an azimuthal kick in the opposite direction, such that the two kicks cancel. Besides requiring magnetized beam, the JLEIC re-circulator cooler design requires an electron beam with very high average current and high bunch charge: 140 mA and with nanoCoulomb bunch charge. This contribution describes the latest milestones of a multiyear program to build a magnetized electron beam source based on a 350 kV DC high voltage photogun with inverted insulator geometry.

Compensation of non-linear bandwidth broadening by laser chirping in Thomson sources

A new laser chirping prescription is derived by means of the phase-stationary method for an incident Gaussian laser pulse in conjunction with a Lienard-Wiechert calculation of the scattered radiation flux and spectral brilliance. This particularly efficient laser chirp has been obtained using the electric field of the laser and for electrons and radiation on axis. The frequency modulation is somewhat reduced with respect to that proposed in the previous literature, allowing the application of this procedure to lasers with larger values of the parameter a0. Numerical calculations have been performed using mildly focused and narrow bandwidth laser pulses, confirming a larger efficiency of the chirp prescription here introduced. The chirp efficiency has been analysed as a function of the laser parameter and focusing.A new laser chirping prescription is derived by means of the phase-stationary method for an incident Gaussian laser pulse in conjunction with a Lienard-Wiechert calculation of the scattered radiation flux and spectral brilliance. This particularly efficient laser chirp has been obtained using the electric field of the laser and for electrons and radiation on axis. The frequency modulation is somewhat reduced with respect to that proposed in the previous literature, allowing the application of this procedure to lasers with larger values of the parameter a0. Numerical calculations have been performed using mildly focused and narrow bandwidth laser pulses, confirming a larger efficiency of the chirp prescription here introduced. The chirp efficiency has been analysed as a function of the laser parameter and focusing.

ELIC at CEBAF

We report on the progress of the conceptual development of the energy recovering linac (ERL)-based electron-light ion collider (ELIC) at CEBAF that is envisioned to reach luminosity level of 1033-1035/cm2s with both beams polarized to perform a new class of experiments in fundamental nuclear physics. Four interaction points with all light ion species longitudinally or transversally polarized and fast flipping of the spin for all beams are planned. The unusually high luminosity concept is based on the use of the electron cooling and crab crossing colliding beams. Our recent studies focused on the design of low beta interaction points, exploration on raising the polarized electron injector current to the level of 3-30 mA with the use of electron circulator-collider ring, forming a concept of stacking and cooling of the ion beams, and specifications of the electron cooling facility.

Injection Options for 12 GeV CEBAF Upgrade

Jefferson Lab is planning a major upgrade of CEBAF accelerator from 6 to 12 GeV. The injection energy needs to be increased accordingly from 67 MeV to 123 MeV. While the present 100 keV electron gun and beam formation up to 5 MeV would remain unchanged, the accelerating SRF modules in the current injector cannot provide the desired energy increase. Two options for attaining the energy increase have been considered: (1) replacing the present injector SRF modules with new, higher gradient modules, or (2) re-circulating the electron beam through the existing cryomodules to achieve the necessary energy gain in two passes. In this paper we present computer simulation studies for these two options of the injector upgrade and list their advantages and disadvantages.