S. Penner

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.

Analysis of free-electron-laser performance utilizing the National Bureau of Standards' CW microtron

The National Bureau of Standards’ (NBS) cw racetrack microtron (RTM) will be utilized as a driver for a free electron laser (FEL) oscillator. The NBS RTM possesses many exceptional properties of value for the FEL: (i) cw operation; (ii) energy from 20–185 MeV; (iii) small energy spread and emittance; (iv) excellent energy stability; and (v) high average power. The 1D FEL gain formula predicts that the FEL would oscillate at the fundamental approximately from 0.25–10 μm when upgrading the peak current to ≥2 A. In this paper, we present 3D self‐consistent numerical results including several realistic effects, such as emittance, betatron oscillations, diffraction, and refraction. The results indicate that the design value of the transverse emittance is small enough that it does not degrade the FEL performance for intermediate to long wavelengths, and only slightly degrades the performance at the shortest wavelength under consideration. Due to the good emittance, the current density is high enough that focusi...

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.

Proposal for a free electron laser driven by the National Bureau of Standards' CW microtron

We propose a free electron laser (FEL) driven by the racetrack microtron (RTM) which is under development at the National Bureau of Standards (NBS). The design of the NBS RTM has many exceptional properties: (i) normalized transverse emittance of better than 10 mm mrad, comparable to a storage ring, (ii) axial emittance <30 keV deg., much superior to rf linacs, (iii) electron energy tunable from 20 to approximately 200 MeV, (iv) cw operation, (v) energy stability, (vi) compactness, and (vii) high power-conversion efficiency. We propose accelerator upgrades to increase the peak current. It seems possible to obtain peak current larger than 2 A, for an average electron beam power of 100 kW. The wavelength of the fundamental radiation from the FEL can span from 0.25 to 10 μm, and harmonics will be produced in the UV spectral region. Because of the wavelength range, tunability, high average power, continuous beam, and the ability to generate picosecond pulses, this FEL will be suitable for a wide range of applications.

Proposal for FEL experiments driven by the National Bureau of Standards' CW microtron

We propose FEL experiments driven by the race-track microtron (RTM), which is under development at the National Bureau of Standards (NBS). The design of the NBS RTM has many exceptional properties: (i) normalized transverse emittance of 10 mm mrad, comparable to a storage ring, (ii) axial emittance <30 keV-degree, much superior to rf linacs, (iii) electron energy tunable from 20 to approximately 200 MeV, (iv) CW operation, (v) energy stability, (vi) compactness, and (vii) high power-conversion efficiency. We propose accelerator up-grades to increase the peak current and to recover a significant part of the electron beam energy. It seems possible to obtain peak current of Ip = 5−10 A, for electron pulse length of leb ≳ 1.2 mm. We have made conservative estimates based on Ip = 1.0 A in estimating the performance of the FELs to be used with the RTM. Low gain calculations indicate that the FEL can oscillate at the fundamental wavelength ranging from 0.15 to 100 μm. Furthermore, we will outline experiments to generate harmonics in the XUV.

Performance of the 100 keV Chopper/Buncher System of the NBS-Los Alamos RTM Injector

The purpose of the chopper/buncher system for the RTM injector is to chop a 100 keV 5 mA dc electron beam into 60/sup 0/-long pulses at 2380 MHz and then bunch these beam pulses to 10/sup 0/ at insertion into the 5 MeV injector linac. These beam manipulations must contribute a minimum increase in the phase space of the beam such that, at the entrance to the injector linac, the transverse emittance is less than 5..pi.. mm-mrad. Phase-shift measurements on the chopped beam indicate that the bunching fields are sufficient to achieve the required longitudinal compression. Beam envelope measurements, using wire scanners on the chopped and bunched beam, show that the emittance remains within design goals.

A 12-Channel Semiconductor Counter System for the NBS Electron Scattering Spectrometer

A 12-channel array of lithium-drifted silicon detectors for detecting high energy electrons in the focal plane of a magnetic spectrometer is described. The detectors are movable along the focal plane and are backed up by two large stationary plastic scintillators. The scintillators are placed one behind the other, such that a count is recorded when a triple coincidence occurs between both scintillators and a single semiconductor detector. The system has a momentum resolution of 0.036 percent, determined by the detector size and the spectrometer momentum dispersion. Discriminator curves have been obtained which show sufficiently flat plateaus to afford stable counting conditions for electron momentum greater than 20 MeV/C. Detector efficiency measurements have also been made which show that the relative efficiencies of the detectors are stable and known to less than two percent of their values. An electron scattering experiment is now in progress using these detectors.

The hybrid undulator for the NIST-NRL free-electron laser

Abstract The NIST-NRL FEL will use a 3.64 m hybrid undulator that is being constructed at Brobeck Division of Maxwell Laboratories. The undulator has a period of 2.8 cm, a variable gap with a 1.0 cm minimum, a peak magnetic field of 0.54 T, and 130 total periods. The magnetic design uses SmCo permanent magnets and vanadium permendur pole pieces. Provision has been made to operate the undulator at half its length to enhance lasing at longer wavelengths. Remote control of gap and taper will permit on-line tuning of the wavelength and extraction efficiency. Vacuum chambers for both full- and half-length configurations are available. The oval aperture of the vacuum chambers is 0.86 cm by 1.6 cm. A system to measure the magnetic field along the undulator is included in the structure. A full-scale, one-period model of the magnetic design has been tested and the results exceed specifications. The mechanical structure of the undulator is nearly complete as is the control system. Construction of the magnet assemblies is under way.

The NBS-LASL CW Microtron

The NBS-LASL racetrack microtron (RTM) is a joint research project of the National Bureau of Standards and the Los Alamos Scientific Laboratory. The project goals are to determine the feasibility of, and develop the necessary technology for building high-energy, high-current, continuous-beam (cw) electron accelerators using beam recirculation and room-temperature rf accelerating structures. To achieve these goals, a demonstration accelerator will be designed, constructed, and tested. Parameters of the demonstration RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15; final beam energy - 185 MeV; maximum current 550 ¿A. One 450 kW cw klystron operating at 2380 MHz will supply rf power to both the injector linac and the main accelerating section of the RTM. The disk and washer standing wave rf structure being developed at LASL will be used. SUPERFISH calculations indicate that an effective shunt impedance (ZT2) of about 100 M¿/m can be obtained. Thus, rf power dissipation of 25 kW/m results in an energy gain of more than 1.5 MeV/m. Accelerators of this type should be attractive for many applications. At beam energies above about 50 MeV, an RTM should be considerably cheaper to build and operate than a conventional pulsed rf linac of the same maximum energy and time-average beam power. In addition, the RTM provides superior beam quality and a continuous beam which is essential for nuclear physics experiments requiring time-coincidence measurements between emitted particles.