E. R. Lindstrom

Progress Report on the NBS/LOS Alamos RTM

The NBS-Los Alamos 200 MeV Racetrack Microtron (RTM) is being built under a program aimed at developing the technology needed for high-current intermediateenergy CW electron accelerators. In this report we give an overview of the present status of the project. Recent progress includes: (1) completion of testing of the 100 keV chopper-buncher system demonstrating a normalized emittance well under the design goal of 2.6 ¿ mm mrad at currents exceeding the design goal of 600 ¿A; (2) operation of the rf structures comprising the 5 MeV injector linac at power levels up to 50 kW/m, resulting in an accelerating gradient at ß=1 of 2 MV/m (compared to a design goal of 1.5 MV/m). The measured shunt impedance is 82.5 Mn/m; (3) construction and installation of the 30 ton end magnets of the RTM. Field mapping of one magnet has been completed and its uniformity exceeds the design goal of ±2 parts in 104; (4) performance tests (with beam) of prototype rf beam monitors which measure current, relative phase, and beam position in both transverse planes. (5) Installation and initial operation of the primary control system.

NBS-LANL RTM Injector Installation

The injector for the NBS-LANL CW racetrack microtron consists of a 100 KeV electron gun and beam transport line followed by a 5 MeV linac. The function of the gun and transport line, which have been installed at NBS, is to provide a chopped and bunched 100 KeV and up to 0.67 mA dc or pulsed beam of very low transverse emittance for matched insertion into the linac. In this paper the authors present both the design and construction details of the 100 KeV system and the results of preliminary beam tests. The tests conducted thus far show the gun and transport system to be performing well within design specifications.

Progress on the NBS-LANL CW Microtron

The NBS-LANL racetrack microtron (RTM) currently under construction at the National Bureau of Standards is a demonstration accelerator to determine the feasibility of, and to develop the technology necessary for building high-energy, high-current, continuous beam (CW) electron accelerators using beam recirculation through room temperature rf accelerating structures. Parameters of the RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15 or 16; final beam energy - 185-197 MeV; maximum current - 550 ¿A; rf frequency - 2380 MHz. At present, the electron gun and 100 keV beam transport line are operational, and most other major subsystems are in the construction or installation phase. Exceptions are the rf structure (under development), the 5 MeV beam transport line (in engineering design), and the extraction beam line (in conceptual design). Our studies of the original candidate accelerating structure, the disk-and-washer, have led to the discovery of beam steering modes which render this structure unsuitable for the RTM without at least substantial further development beyond the scope of the project. The most promising alternate for meeting the design goal of CW operation at 1.5 MeV/m is the side-coupled structure. A shunt impedance of 80 M¿/m has been measured in a test section of side-coupled structure at 2380 MHz, adequate cooling has been designed, and a 2.7 m long section of this design is under construction. The electron optics of the RTM have been studied in detail.

The NBS-LANL RTM End-Magnet Field Mapper

A computer-controlled magnetic field mapper is under construction at the National Bureau of Standards to map the end magnets of the NBS-LANL racetrack microtron (RTM). The mapper consists of a large, two-dimensional translation stage which simultaneously positions a nuclear magnetic resonance (NMR) magnetometer probe in the 55 cm × 135 cm uniform field region and a temperature-compensated Hall effect probe in the fringe field region. A computer-based control system automatically positions the probes at points on a selected grid and records the measured field values and positions in computer memory. In this paper we describe the field mapping requirements, the mapper, its operation, and the field measurements and analysis that are to be performed.

End Magnets for the NBS-LOS Alamos Racetrack Microtron

Two end magnets have been designed and constructed for the 185 MeV NBS-Los Alamos racetrack microtron. The field has been measured in the first magnet and is uniform over a 0.62 m2 area to within ±2×10-4 at 1 T. The magnet meets all performance specifications. Field measurements are underway on the second magnet. In this paper, design and construction details which play an important role in magnetic performance are described, and the measured fields are compared with calculations.

The NIST/NRL Free-Electron Laser Facility

A free-electron laser (FEL) user facility is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory. The FEL, which will be operated as an oscillator, will be driven by the electron beam of the racetrack microtron (RTM) that is nearing completion. Variation of the electron kinetic energy from 17 MeV to 185 MeV will permit the FEL wavelength to be tuned from 200 nm to 10 pm. Performance will be enhanced by the high brightness, low energy spread, and continuous-pulse nature of the RTM electron beam. We are designing a new injector to increase the peak current of the RTM. A 3.6-m undulator is under construction, and the 9-m optical cavity is under design. The FEL will emit a continuous train of 3-ps pulses at 66 MHz with an average power of 10-200 W, depending on the wavelength, and a peak power of up to several hundred kW. An experimental area is being prepared with up to five stations for research using the FEL beam. Initial operation is scheduled for 1991.

Design of a high-current injector for the NIST-NRL free electron laser

Abstract The electron beam of the NIST-LANL racetrack microtron (RTM) is to be used to drive a free electron laser and for other applications. We have designed a new injector for the RTM to provide the 3.5 ps long, 7–14 pC electron beam pulses required for lasing. The new injector consists of a pulsed, 120 keV thermionic electron gun, a subharmonic chopping and bunching system, and the existing 5 MeV RTM injection linac. It has been designed to produce a continuous train of electron pulses at 5 MeV with a normalized transverse emittance of 5 mm mrad and a longitudinal emittance of 20 keV deg for 95% of the beam (for 7 pC) at repetition rates of 66.111 or 16.528 MHz. These two frequencies are subharmonics of the fundamental accelerator frequency of 2380 MHz. The new injector will also produce lower-emittance beams at lower peak currents at the fundamental frequency.

NIST-NRL free-electron laser facility

A free-electron laser facility (FEL) is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory (NRL) . The FEL will be driven by the electron beam from the NIST racetrack microtron (RTM). The anticipated performance of the FEL is: (1) wavelength variability from 200 run to 10 tim; (2) continuous train of 3-ps pulses at 66 MHz; and (3) average power of 10 W to 200 W. This excellent performance will be achieved primarily because of the unique characteristics of the RTM. This accelerator will provide a continuously pulsed electron beam with high brightness and low energy spread at energies from 17 MeV to 185 MeV. For FEL operation high peak current is required and a new injector for this purpose has been designed. The undulator for the project is 3.64-m long with 130 periods and a peak field of 0.54 T. The construction of the undulator is nearly complete and delivery is expected shortly. The 9-m optical cavity has been designed and is under construction. An experimental area is being prepared for FEL users which will have up to six stations. Initial operation of the FEL is scheduled for 1991. The NIST-NRL FEL will provide a powerful, tunable light source for research in biomedicine, materials science , physics , and chemistry.

NBS-LASL Racetrack Microtron

The NBS-LASL racetrack microtron (RTM) is a joint project of the National Bureau of Standards (NBS) and the Los Alamos Scientific Laboratory (LASL). This is a new accelerator research project whose goal is to determine the feasibility of building a high-energy, high-current, cw electron accelerator using beam recirculation and room-temperature rf acceleration structures. The NBS-LASL RTM is being designed and built to develop the required technology for a large national 1 to 2 GeV cw accelerator for nuclear physics research and to prove experimentally that high currents can be accelerated successfully in an RTM. Some of the parameters of the NBS-LASL RTM are 185 MeV final energy, 550 μA maximum current, 15 passes, 12 MeV one-pass energy gain, and 2380 MHz frequency. One 450 kW cw klystron will supply rf power to both the 5 MeV injector and the 12 MeV linac in the RTM.