Free-electron laser results from the Advanced Test Accelerator

PALADIN is a 10.6-..mu..m FEL amplifier experiment operating at the Lawrence Livermore National Laboratory's Advanced Test Accelerator, an induction linear accelerator designed to produce a 45-MeV, 10-kA electron beam. With a 15-m long wiggler, PALADIN demonstrated 27 dB of exponential gain from a 14-kW input signal. With a 5-MW input signal, the amplifier saturated after 10 dB of gain. The exponentially growing signal in the unsaturated amplifier was clearly seen to be gain guided by the electron beam. 7 refs., 8 figs

Intense microwave generation using free-electron lasers

In this paper, I will describe a free-electron laser amplifier which operated in the microwave regime. This device, called the Electron Laser Facility (ELF), used an electron beam generated by a Linear Induction Accelerator (LIA). ELF operated as a single pass amplifier at 35 and 140 GHz. Because the device had no cavity, we could study the FEL physics independent of cavity considerations. With a sufficiently large input signal, growth of the signal from noise on the beam did not influence the performance. This device demonstrated significant gain and allowed us to investigate such FEL phenomenon as saturation and synchrotron oscillation of the electrons trapped in the ponderomotive well. We were also able to study the phase shift of the radiation due to the real part of complex gain of the FEL. Because the interaction takes place in a waveguide, the FEL can couple to several spatial modes at a given frequency. The bunched electrons can radiate at harmonics of the fundamental and in this experiment we studied the evolution of the third harmonic. In this paper, I will describe the Electron Laser Facility. I will discuss the FEL performance with regard to gain, saturation, phase evolution, mode coupling and harmonic generation, I will briefly discuss a switching technique which allows the LIA to run at high average power. When driven by such a device, and FEL can produce high average power radiation. We will present the design for such a device which can be used to heat a tokamak plasma. This device is designed to operate at 250 GHz and produce an average power of 2 MW

Dynamics of gas-filled hohlraums

In order to prevent high-Z plasma from filling in the hohlraum in indirect drive experiments, a low-Z material, or tamper is introduced into the hohlraum. This material, when fully ionized is typically less than one-tenth of the critical density for the laser light used to illuminate the hohlraum. This tamper absorbs little of the laser light, thus allowing most of the laser energy to be absorbed in the high-Z material. However, the pressure associated with this tamper is sufficient to keep the hohlraum wall material from moving a significant distance into the interior of the hohlraum. In this paper the authors discuss measurements of the motion of the interface between the tamper and the high-Z hohlraum material. They also present measurements of the effect the tamper has on the hohlraum temperature

Formation and control of plasma potentials in TMX upgrade

The methods to be employed to form and control plasma potentials in the TMX Upgrade tandem mirror with thermal barriers are described. ECRH-generated mirror -confined electron plasmas are used to establish a negative potential region to isolate the end-plug and central-cell celectrons. This thermal isolation will allow a higher end-plug electron temperature and an increased central-cell confining potential. Improved axial central-cell ion confinement results since higher temperature central-cell ions can be confined. This paper describes: (1) calculations of the sensitivity of barrier formation to vacuum conditions and to the presence of impurities in the neutral beams, (2) calculations of microwave penetration and absorption used to design the ECRH system, and (3) techniques to limit electron runaway to high energies by localized microwave beams and by relativistic detuning

Experimental results of beam brightness experiments at the Advanced Test Accelerator (ATA)

Experimental data show improved brightness of Lawrence Livermore National Laboratory's Advanced Test Accelerator both at the injector and at the high-energy output. The effects of matching onto a laser-produced ion channel have been demonstrated, and an improved matching technique is now being used

Sixty keV D/sup -/ beams using double charge-exchange system

A D/sup -/ beam with current greater than 100 ma was accelerated to 60 kV. The beam, with pulse length 10 ms, was generated by charge-exchange in cesium vapor. The physics of generation, propagation at low energy, and acceleration is discussed

Novel probe for determining the size and position of a relativistic electron beam

In order to determine the size and position of a relativistic electron beam inside the wiggler magnetic field of a Free Electron Laser (FEL), we have developed a new probe which intercepts the electron beam on a high Z target and monitors the resulting bremsstrahlung radiation. The probe is designed to move along the entire three meters of the wiggler. This FEL is designed to operate in the microwave region (2 to 8 mm) and the interaction region is an oversized waveguide with a cross section 3 cm x 9.8 cm. The axial probe moves inside this waveguide. The probe stops the electron beam on a Tantalum target and the resulting x-rays are scattered in the forward direction. A scintillator behind the beam stop reacts to the x-rays and emits visible light in the region where the x-rays strike. An array of fiber optics behind the scintillator transmits the visible light to a Reticon camera system which images the visible pattern from the scintillator. Processing the optical image is done by digitizing and storing the image and/or recording the image on video tape. Resolution and performance of this probe will be discussed

Negative beam generation, transport, and acceleration

A negative ion beam, generated by double charge exchange, has been accelerated to 60 keV. The components of the beamline are described, and an analysis of the flow and the effect of the beam-generated plasma is presented. It is shown that plasma flow along the beamline is limited by transverse ambipolar transport; acceleration of electrons can therefore be minimized by employing a drift space to control the flow of electrons from the cesium cell to the accelerator entrance