I.S. Lehrman

Design of a harmonic generation FEL experiment at BNL

We present design parameters of a harmonic generation FEL experiment to be carried out at the Accelerator Test Facility (ATF) at BNL. This experiment out as a proof-of-principle for the proposed UV-FEL Users Facility at BNL. In the experiment we plan to triple the frequency of a CO{sub 2} seed laser by utilizing two superconducting wigglers and a dispersive section. The first wiggler will be used in conjunction with the CO{sub 2} seed laser to generate a ponderomotive force that will bunch the electron beam. The bunching will then be enhanced by the dispersion section. The second wiggler, tuned to the third harmonic of the seed laser will follow. In the beginning of the second wiggler the bunched beam will produce super-radiant emission (characterized by a quadratic growth of the radiated power), then the radiation will be amplified exponentially. The last part of the wiggler will be tapered. We plan to study the evolution of the various radiation growth mechanisms as well as the coherence of the tripled and exponentially amplified radiation. 12 refs.

Design of a high-brightness, high-duty factor photocathode electron gun

Abstract The proposed UV-FEL users facility at Brookhaven National Laboratory will require a phatocathode gun capable of producing short ( ps ) bunches of electrons at high repetition rates (5 kHz), low energy spread ( ), a peak current of 300 A (after compression) and a total bunch charge of up to 2 nC. At the highest charge, the normalized transverse emittance should be less than 7π mm mrad. We are presently designing a gun that is expected to exceed these requirements. This gun will consist of 3 1 2 cells, constructed of GlidCop-15, an aluminum oxide dispersion strengthened copper alloy. The gun will be capable of operating at duty factors in excess of 1%. Extensive beam dynamics studies of the gun were used to determine the effect of varying the length of the first cell, shaping the apertures between cells, and increasing the number of cells. In addition, a detailed thermal and mechanical study of the gun was performed to ensure that the thermal stresses were well within the allowable limits and that copper erosion of the water channels would not occur.

An Algorithm for the Analysis of Inductive Antennas of Arbitrary Cross Section for Heating in the Ion Cyclotron Range of Frequencies

The application of ion cyclotron range of frequency (ICRF) heating to near-ignited plasmas will require launching structures that will be capable of withstanding the harsh plasma environment. The recessed antenna configuration is expected to provide sufficient protection for the structure, but to date no analysis has been done to determine if adequate coupling can be achieved in such a configuration. In this work we present a method for determining the current distribution for the antenna in the direction transverse to current flow and predict antenna loading in the presence of plasma. Antennas of arbitrary cross section are analyzed above ground planes of arbitrary shape. Results from the antenna design code (ANDES) are presented and compared to experimental results.

Design and operation of the Compact Infrared Free-Electron Laser (CIRFEL)

The Compact Infrared Free Electron Laser (CIRFEL) was built as part of a joint collaboration between Northrop Grumman and Princeton University to develop FELs for use by researchers in the materials, medical and physical sciences. The CIRFEL was designed to laser in the Mid-IR and Far-IR regimes with picosecond pulses, megawatt level peak powers and an average power of a few watts. The CIRFEL utilizes an RF photocathode gun to produce high-brightness time synchronized electron bunches. The micropulse separation is 7 nsec which allows a number of relaxation phenomena to be observed. In addition, the photocathode illumination laser can be used in combination with the FEL IR light for pump- probe experiments. The CIRFEL is presently being commissioned and working towards lasing. The present status of the machine is presented.

FIRST LASING OF THE COMPACT INFRARED FREE-ELECTRON LASER

Abstract On 15 May 1996, lasing was achieved at the Compact Infrared Free-Electron Laser (CIRFEL) at 14 μm. The electron beam energy was 11.2 MeV and the micropulse charge was 1 nC. The width of the detuning curve was approximately 50 μm. Since achieving this milestone, we have lased repeatedly from 12.5 to 20 μm with between 0.25 and 1.5 nC of charge. Saturation is typically reached in less than 1.5 μs. The measured FEL spectrum is Gaussian in shape with a line width of 56 nm at 13 μm. Assuming a transform limited pulse, this corresponds to an FEL light pulse of 4.4 ps. The CIRFEL is a photocathode based free-electron laser with a micropulse width of 4–10 ps and a micropulse separation of 7 ns. The macropulse length is between 4 and 6 μs and the repetition rate is between 1 and 5 Hz. We present details of the CIRFEL machine as well as experimental measurements of the FEL radiation.

Beam diagnostics and modeling in CIRFEL

Abstract In this paper results of phase stability measurements of the photocathode drive laser at different points in the drive laser system, as well as the electron current micro-pulse phase stability measurements results are reported. Summary results of CIRFEL bend modeling and emittance measurements are also described. Result of a 5th order perturbation model for wiggler focusing is also reported.

The Northrop Grumman Compact Infrared FEL (CIRFEL)

As part of Northrop Grummans ongoing research in particle accelerators, we have designed and are commissioning a Compact Infrared Free-Electron Laser (CIRFEL) for the study of high-brightness electron beams and free-electron lasers. Besides serving as a tool for FEL development, the CIRFEL laboratory, located at Princeton University, will be used in experiments ranging from basic FEL physics and biophysics to chemistry, materials science and medicine. The CIRFEL is to lase initially in the 10 - 20 /spl mu/m range. The pulse format of this FEL is a train of micropulses, 5 - 10 psec is duration, at a repetition rate of 142.8 MHz. The micropulse energy is in excess of 100 /spl mu/J. The micropulses comprise a macropulse lasting approximately 10 /spl mu/sec. The macropulse repetition rate is 10 Hz, thus the average power of the FEL is on the order of 1.5 W.

Electron transport and emittance diagnostics in CIRFEL

Electron pulses for the Northrop Grumman Compact Free Electron Laser CIRFEL are produced at a repetition rate of up to 10 Hz by the illumination of a Mg photocathode with a photon injector 261 nm seed laser system mode locked to the 20th sub-harmonic of 2.856 GHz. Presently the system is being operated in the 10 to 12 MeV energy range and spontaneous radiation has been observed. We present some preliminary results on electron beam characterization including its energy spread, energy stability, and spontaneous radiation observations.

Design and construction of a Compact Infrared Free Electron Laser CIRFEL

The 5-15 micron Grumman Compact Infra Red Free Electron Laser CIRFEL which will produce extremely short pulses of tunable radiation under construction is described. Electron pulses are produced at a repetition rate of up to 10 Hz by the illumination of a single crystal <001> LaB/sub 6/ photocathode with a photon injector, a 6-10 psec, 349 nm (frequency tripled Nd-YLF) laser mode locked to the 20th subharmonic of 2856 MHz. Photoelectrons are further accelerated and guided to the superconducting microwiggler by a robust beam transport system through an achromatic bend. The /spl sim/10 MeV electrons interact with the optical radiation inside a near symmetric laser cavity. The FEL output will be coupled out through a hole in one of the cavity mirrors. The CIRFEL system is expected to be delivered in 1994.<<ETX>>

ICRF edge modeling studies

Edge plasma models are presented that attempt to explain the behavior observed during Ion Cyclotron Range of frequencies heating experiments. The models calculate particle transport near the antenna. Kinetic modifications to the edge plasma and their implication on material sputtering from the Faraday shield are investigated. The electric fields produced by the antennas are computed. (AIP)