Clifford M. Fortgang

Status of the DARHT phase 2 long-pulse accelerator

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. We intend to produce measurements containing three-dimensional information with sub-millimeter spatial resolution of the interior features of very dense, explosively-driven objects. The facility will be completed in two phases with the first phase having become operational in July 1999 utilizing a single-pulse, 20-MeV, 2 -kA, 60-ns accelerator, a high-resolution electrooptical X-ray imaging system, and other hydrodynamics testing systems. The second phase will be operational in 2004 and features the addition of a 20-MeV, 2-kA, 2-microsecond accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved X-ray images. The second phase also features an extended, high-resolution electro-optical X-ray system with a framing speed of 1.6-MHz. Production of the first beam from the Phase 2 injector will occur this year. In this paper we will present the overall design of the Phase 2 long-pulse injector and accelerator as well as some component test results. We will also discuss the downstream transport section that contains the fast kicker used to separate the long-pulse beam into short bursts suitable for radiography as well as the X-ray conversion target assembly. Selected experimental results from this area of the project will also be included. Finally, we will discuss our plans for initial operations.

Demonstration of ultraviolet lasing with a low energy electron beam

Abstract We report on the design details of the first ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. In our experiment we used a high-current, high brightness electron beam in combination with a wiggler of novel design to produce an FEL that lased at wavelengths from 369 to 380 nm using 45.9–45.2 MeV electrons. In addition we performed a proof-of-principle experiment that demonstrated the first ever photolithography on a photoresist-coated silicon wafer using an FEL light source.

First lasing of the regenerative amplifier FEL

The Regenerative Amplifier Free-Electron Laser (RAFEL) is a high-gain RF-linac FEL capable of producing high optical power from a compact design. The combination of a high-gain and small optical feedback enables the FEL to reach saturation and produce a high optical power and high extraction efficiency without risk of optical damage to the mirrors. This paper summarizes the first lasing of the Regenerative Amplifier FEL and describes recent experimental results. The highest optical energy achieved thus far at 16.3 {micro}m is 1.7 J over an 9-{micro}s macropulse, corresponding to an average power during the macropulse of 190 kW. They deduce an energy of 1.7 mJ in each 16 ps micropulse, corresponding to a peak power of 110 MW.

Initial performance of Los Alamos Advanced Free Electron Laser

Abstract The Los Alamos compact Advanced Free Electron Laser (AFEL) has lased at 4.7 and 5.2 μm with a 1-cm period wiggler and a high-brightness electron beam at 16.8 and 15.8 MeV, respectively. The measured electron beam normalized emittance is 1.3 π mm mrad at a peak current of 100 A, corresponding to a beam brightness greater than 2 × 10 12 A/m 2 rad 2 . Initial results indicate that the AFEL small signal gain is ∼ 8% at 0.3 nC (30 A peak). The maximum output energy is 7 mJ over a 2-μs macropulse. The AFEL performance can be significantly enhanced by improvements in the rf and drive laser stability.

Fabrication of high-field short-period permanent magnet wigglers

Abstract A permanent magnet wiggler is described that has been designed to lase at unusually short wavelengths. Its novel features include the following: all magnets are magnetized parallel to the wigglers axis; only two pairs per period are used; the gap occupied by the electron beam is very small; the magnet arrangement is optimized for lasing on the third harmonic; the assembly of the magnets is carried out during continuous measurements of the field integrals; field gradients are measured with equal care; and residual errors are corrected by gluing small correcting magnets to appropriate places. The assembly, testing, and trimming of this wiggler was accomplished in less than a week. The wiggler has been used to lase successfully at 0.375 μm wavelength.

Status of the dual axis radiographic hydrodynamics test (DARHT) facility

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. We intend to produce measurements containing three-dimensional information with sub-millimeter spatial resolution of the interior features of very dense, explosively-driven objects. The facility will be completed in two phases with the first phase having become operational in July 1999 utilizing a single-pulse, 20-MeV, 2-kA, 60-ns accelerator, a high-resolution electro-optical x-ray imaging system, and other hydrodynamics testing systems. We will briefly describe this machine. The first electron beams will be generated in the second phase of DARHT this year. The second DARHT accelerator consists of a 18.4-MeV, 2-kA, 2-microsecond pulse-width accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved x-ray images. The second phase also features an extended, high-resolution electro-optical x-ray system with a framing speed of about 2-MHz. We will discuss this accelerator by summarizing the overall design of the long-pulse injector and accelerator. We will also discuss the fast kicker used to separate the long-pulse beam into short bursts suitable for radiography.

Measurement and correction of magnetic fields in pulsed slotted-tube microwigglers☆

Abstract Pulsed electromagnetic microwigglers are capable of creating stronger fields with shorter periods than conventional wigglers. Wiggler field parameters of 1 T and 5-mm period are achieved by passing ∼ 25 kA down a slotted 3 mm outer diameter copper tube. The taut-wire technique is used to measure the wiggler magnetic field in a small aperture (∼ 1.7 mm) with the required spatial (0.1 mm) and temporal (∼ 5 μs) resolution. The absence of midplane symmetry, in contrast to permanent magnet wigglers, causes unique problems in our microwigglers. We have measured strong dipole (bending) and quadrupole (defocusing) field errors with both dc and time-dependent parts. The effects on the magnetic field of current redistribution caused by heating in the copper tube has been measured. We have identified causes for the various field errors and their sensitivities to fabrication tolerances. Improvements we have made towards meeting the required tolerances and our field-correction technique will be discussed.

Compact 1-kW infrared regenerative amplifier FEL

This paper describes the design, construction, and initial operation of an infrared FEL designed for 1 kW average power. The experiment utilizes the existing advanced free-electron laser (AFEL) accelerator. An expected 6% extraction efficiency of electron beam power to optical power gives an overall wall- plug efficiency of approximately 1%. The 10 to 20 MeV electron accelerator is 1.2 m long. This regenerative amplifier FEL (RAFEL) relies on a few (less than ten) passes to reach saturation. The technique is similar to a FEL oscillator through the use of optical feedback to reach saturation. However, in this design the feedback is limited to less than 1% in the large signal regime. The chief advantage to this approach is that no mirror is exposed to a high peak intensity, enabling high-average power in a compact optical configuration. To compensate for the low optical feedback, the single-pass gain must be very high. In our design, the single- pass gain is 105 in the small signal regime. RAFEL is presently configured to operate at a wavelength near 16 microns. However, this system is scaleable to shorter wavelengths by increasing beam energy. Present results indicate that a single pass gain of 60 in the infrared has been observed.

Development of a pulsed-microwiggler system

Abstract Pulsed microwigglers can develop unusually high magnetic fields with short periods. They find potential applications in compact system that use low energy accelerators that can generate very bright electron beams, and in systems that lase on high harmonics of the fundamental frequency. In the past many of the unique properties of pulsed wigglers have been addressed. In this paper we will concentrate on our solutions to the practical problems that must be solved to make such a device work. Among other topics we will discuss the following: achieving adequate precision in fabrication; controlling quadrupole fields; “trimming” the field after fabrication; providing structural support, cooling, and vacuum; and coupling to the pulsed power supply. Completed microwigglers of our design will be installed in an FEL this summer to lase on the fundamental in the red part of the spectrum and on the third harmonic in the ultraviolet.

Synchronously injected amplifiers, a novel approach to high-average-power FEL☆

Two new FEL ideas based on synchronously injected amplifiers are described. Both of these rely on the synchronous injection of the optical signal into a high-gain, high-efficiency tapered wiggler. The first concept, called Regenerative Amplifier FEL (RAFEL), uses an optical feedback loop to provide a coherent signal at the wiggler entrance so that the optical power can reach saturation rapidly. The second idea requires the use of a uniform wiggler in the feedback loop to generate light that can be synchronously injected back into the first wiggler. The compact Advanced FEL is being modified to implement the RAFEL concept. We describe future operation of the Advanced FEL at high average current and discuss the possibility of generating 1 kW average power.