G. Westenskow

Emission from ferroelectric cathodes

We have recently initiated an investigation of electron emission from ferroelectric cathodes. Our experimental apparatus consisted of an electron diode and a 250 kV, 12 Ω, 70 ns pulsed high voltage power source. A planar triode modulator driven by a synthesized waveform generator initiates the polarization inversion and allows inversion pulse tailoring. The pulsed high voltage power source is capable of delivering two high voltage pulses within 50 μs of each other and is capable of operating at a sustained repetition rate of 5 Hz. Our initial measurements indicate that emission current densities above the Child-Langmuir space charge limit, JCL, are possible. We explain this effect to be based on a non-zero initial energy of the emitted electrons. We also determined that this effect is strongly coupled to relative timing between the inversion pulse and application of the main anode-cathode pulse. We also have initiated brightness measurements of the emitted beam and estimate a preliminary lower bound to be on the order of 109 A/m2rad2. As in our previous measurements at this Laboratory, we performed the measurement using a pepper pot technique. Beamlet profiles are recorded with a fast phosphor and gated cameras. We describe our apparatus and preliminary measurements.

A 17.1 GHz free-electron laser as a microwave source for TeV colliders

Abstract We describe the proposed developement of a 17.1 GHz microwave source based on an induction-linac-driven free-electron laser that is expected to produce peak power levels at the several-GW level. This system is being designed to serve as the power source for a TeV linear collider or for other high-gradient accelerators. We will discuss the power-source requirements for TeV colliders and present the initial design of an FEL that will produce the desired power. In addition, we shall discuss efficiency enhancement schemes such as a relativistic klystron “afterburner”, the requirements for beam reacceleration, and concepts for extraction of the microwave power while preserving key properties of the beam such as emittance and phase.

High current density beamlets from a rf argon source for heavy ion fusion applications

In a new approach to develop high current beams for heavy ion fusion, beam current at about 0.5 ampere per channel can be obtained by merging an array of high current density beamlets of 5 mA each. We have done computer simulations to study the transport of high current density beamlets and the emittance growth due to this merging process. In our radio frequency (rf) multicusp source experiment, we have produced a cluster of 61 beamlets using minimum gas flow. The current density from a 0.25 cm diameter aperture reached 100 mA/cm2. The normalized 4 rms emittance of 0.0186π mm mrad corresponds to an equivalent ion temperature of 2.08 eV. These results showed that the rf argon plasma source is suitable for producing high current density beamlets that can be merged to form a high current high brightness beam for heavy ion fusion application.

The beamline for the second axis of the Dual Axis Radiographic Hydrodynamic Test Facility

During normal DARHT II operation, the beam exiting the accelerator will be well characterized by its nominal design parameters of 20-MeV, 2000-Amperes, 2-/spl mu/sec-pulse length, and 3 cm-mr unnormalized emittance. Normal operation will have the beam delivered to a beam dump via several DC magnets. A 2-way kicker magnet is used to deflect portions of the beam into the straight ahead beamline leading to either a diagnostic beamline or to the converter target beamline. During start up and or beam development periods, the beam exiting the accelerator may have parameters outside the acceptable range of values for normal operation. The Enge beamline must accommodate this range of unacceptable beam parameters, delivering the entire 80 KiloJoule of beam to the dump even though the energy, emittance, and/or match is outside the nominal design range.

Beam-target interaction experiments for bremsstrahlung converter applications

For multi-pulse radiography facilities, we are investigating the possible adverse effects of (1) backstreaming ion emission from the bremsstrahlung converter target and (2) the interaction of the resultant plasma with the electron beam during subsequent pulses. These effects would primarily manifest themselves in a static focusing system as a rapidly varying X-ray spot. To study these effects, we are conducting beam-target interaction experiments on the ETA-II accelerator (a 6.0 MeV, 2.5 kA, 70 ns FWHM pulsed, electron accelerator) by measuring spot dynamics and characterizing the resultant plasma for various configurations.

Negative halogen ions for fusion applications (invited)

Over the past quarter century, advances in hydrogen negative-ion sources have extended the usable range of hydrogen-isotope neutral beams to energies suitable for large magnetic confinement fusion devices. Recently negative halogen ions have been proposed as an alternative to positive ions for heavy-ion fusion drivers in inertial confinement fusion, because electron accumulation would be prevented in negative-ion beams, and if desired, the beams could be photodetached to neutrals. This article reports an experiment comparing the current density and beam emittance of Cl+ and Cl− extracted from substantially ion-ion plasmas with that of Ar+ extracted from an ordinary electron-ion plasma, all using the same source, extractor, and emittance scanner. At similar discharge conditions, the Cl− current was typically 85%–90% of the positive chlorine current, with an e−∕Cl− ratio as low as 7 without grid magnets. The Cl− current was as much as 76% of the Ar+ current from a discharge with the same rf drive. The minim...

Stacked insulator induction accelerator gaps

Stacked insulators, with alternating layers of insulating material and conducting film, have been shown to support high surface electrical field stresses. We have investigated the application of the stacked insulator technology to the design of induction accelerator modules for the Relativistic-Klystron Two-Beam Accelerator program. The RF properties of the accelerating gaps using stacked insulators, particularly the impedance at frequencies above the beam pipe cutoff frequency, are investigated. Low impedance is critical for Relativistic-Klystron Two-Beam Accelerator applications where a high current, bunched beam is transported through many accelerating gaps. An induction accelerator module designs using a stacked insulator is presented.

A chopper driven 11.4-GHz traveling-wave RF generator

A high-power 11.4-GHz RF generator which consists of a 5.7-GHz chopping system and two 11.4-GHz traveling-wave output structures has been tested. The device was designed to generate about 500 MW of pulsed RF power at 11.4 GHz when driven by a 1-kA, 3-MeV induction beam. Problems with beam breakup in the output structures have limited the width of the RF output pulse for currents above 600 A. Short RF pulses up to 420 MW have been produced. Modifications are being made to decrease the growth of the beam-breakup fields in the output structures, and the chopping section will be used to study various extraction structures and the reacceleration of a bunched beam by induction cells. >

Downstream transport system for the second axis of the dual-axis radiographic hydrodynamic test facility

This paper presents physics design of the DARHT-II downstream system, which consists of a diagnostic beam stop, a novel, fast, high-precision kicker system and the x-ray converter target assembly. The beamline configuration and its beam parameter acceptance, the transverse resistive wall instability modeling, the ion hose instability in the presence of the background gas, and the simulations of beam spill are discussed. We also present the target converter assemblys configuration, and the simulated x-ray spot sizes and doses based on the radiation hydrodynamics code LASNEX and the Monte Carlo radiation transport code MCNP.

The DARHT-II downstream transport beamline

This paper describes the mechanical design of the downstream beam transport line for the second axis of the Dual Axis Radiographic Hydrodynamic Test (DARHT II) Facility. The DARHT-II project is a collaboration between LANL, LBNL and LLNL. DARHT II is a 18.4-MeV, 2000-Amperes, 2-/spl mu/sec linear induction accelerator designed to generate short bursts of X-rays for the purpose of radiographing dense objects. The downstream beam transport line is approximately 22-meter long region extending from the end of the accelerator to the bremsstrahlung target. Within this proposed transport line there are 12 conventional solenoid, quadrupole and dipole magnets; as well as several speciality magnets, which transport and focus the beam to the target and to the beam dumps. There are two high power beam dumps, which are designed to absorb 80-kJ per pulse during accelerator start-up and operation. Aspects of the mechanical design of these elements are presented.