P. Piot

First Lasing of the Jefferson Lab IR Demo FEL

As reported previously [1], Jefferson Lab is building a free-electron laser capable of generating a continuous wave kilowatt laser beam. The driver-accelerator consists of a superconducting, energy-recovery accelerator. The initial stage of the program was to produce over 100 W of average power with no recirculation. In order to provide maximum gain the initial wavelength was chosen to be 5 mu-m and the initial beam energy was chosen to be 38.5 MeV. On June 17, 1998, the laser produced 155 Watts cw power at the laser output with a 98% reflective output coupler. On July 28th, 311 Watts cw power was obtained using a 90% reflective output coupler. A summary of the commissioning activities to date as well as some novel lasing results will be summarized in this paper. Present work is concentrated on optimizing lasing at 5 mu-m, obtaining lasing at 3 mu-m, and commissioning the recirculation transport in preparation for kilowatt lasing this fall.

High power beam profile monitor with optical transition radiation

A simple monitor has been built to measure the profile of the high power beam (800 kW) delivered by the CEBAF accelerator at Jefferson Lab. The monitor uses the optical part of the forward transition radiation emitted from a thin carbon foil. The small beam size to be measured, about 100 /spl mu/m, is challenging not only for the power density involved but also for the resolution the instrument must achieve. An important part of the beam instrumentation community believes the radiation being emitted into a cone of characteristic angle 1//spl gamma/ is originated from a region of transverse dimension roughly /spl lambda///spl gamma/; thus the apparent size of the source of transition radiation would become very large for highly relativistic particles. Our monitor measures 100 /spl mu/m beam sizes that are much smaller than the 3.2 mm /spl lambda//spl gamma/ limit; it confirms the statement of Rule and Fiorito (1991) that optical transition radiation can be used to image small beams at high energy. The present paper describes the instrument and its performance. We tested the foil in, up to 180 /spl mu/A of CW beam without causing noticeable beam loss, even at 800 MeV, the lowest CEBAF energy.

A multislit transverse-emittance diagnostic for space-charge-dominated electron beams

Jefferson Lab is developing a 10 MeV injector to provide an electron beam for a high-power free-electron laser (FEL). To characterize the transverse phase space of the space-charged-dominated beam produced by this injector, we designed an interceptive multislit emittance diagnostic. It incorporates an algorithm for phase-space reconstruction and subsequent calculation of the Twiss parameters and emittance for both transverse directions at an update rate exceeding 1 Hz, a speed that will facilitate the transverse-phase-space matching between the injector and the FELs accelerator that is critical for proper operation. This paper describes issues pertaining to the diagnostics design. It also discusses the acquisition system, as well as the software algorithm and its implementation in the FEL control system. First results obtained from testing this diagnostic in Jefferson Labs Injector Test Stand are also included.

First results on energy recovery in the Jefferson Lab IRFEL

A recirculating, energy-recovering linac is used as driver accelerator for Jefferson Labs high average power FEL. CW beam of 5 mA design current is transported from the superconducting RF (SRF) linac to the wiggler for lasing, and then recirculated back to the linac for deceleration and energy recovery. About 75% of the beam power is extracted before the beam is transported to the beam dump. Energy recovery reduces power consumption, RF equipment capital costs, and beam dump shielding requirements. It is arguably essential as FEL technology is scaled to higher average power levels. To date, 4 mA of CW beam has been energy recovered successfully. There is no evidence of RF instabilities due to the energy aperture of the transport system, momentum compaction or the phase of the decelerating beam. HOM power from the beam has interfered with the operation of the IR interlock detectors, designed to protect the warm waveguide window from thermal runaway. Installation of copper screens appears to have solved the problem. More detailed studies of the HOM spectra and their correlation to the beam properties are planned.

Performance of the electron beam diagnostics at Jefferson Lab's high power free electron laser

We describe the performance and current status of the electron beam diagnostic complement for Jefferson Labs IR-FEL oscillator. In addition measurements for the driver-accelerator are presented. Beam diagnostics devices include optical transition radiation profile monitors, multi-slit beam emittance measurement, coherent transition and synchrotron radiation based bunch length monitors, both strip-line and button antenna BPMs and pick-up cavities for longitudinal transfer function measurement. All device are controlled via the EPICS control system.

Real-Time Transverse Emittance and Phase-Space Monitor

A real-time multislit [1] transverse-emittance monitor has been developed for diagnosing the space-charge-dominated beam in the 10MeV injection line of the FEL at Thomas Jefferson National Accelerator Facility (formerly CEBAF). It gives emittance, Twiss parameters, and phase-space contours (without any symmetry assumptions) at the update rate of 1Hz. It reduces measurement noise in real-time, and incorporates a special algorithm for constructing the phase-space matrix, which yields more accurate results by sweeping the beam across the slits. In this paper we will discuss issues relevant to the software design and implementation. Experimental results obtained from a 250keV photocathode gun will also be presented and compared with other methods and with PARMELA simulations.