Designs for LIAs (linear induction accelerators) stem primarily from physics concerns about stable beam transport and emittance preservation; these concerns lead to specific design features of the induction cores and injectors, stringent requirements on energy regulation, and specification of voltage gradient and precision magnetic alignment. Further challenges unique to HAP (high average power) operation (e.g. transient suppression, thermal management) heighten requirements on switching, reset regulation, jitter, power regulation/compensation, diagnostic sensing, and active control. The authors review how the HAP test stands and the total system integration on the ETA-II (Experimental Test Accelerator II) are developing the technologies needed to satisfy these requirements

Experimental work is underway to investigate the feasibility of using relativistic klystrons as a power source for future high-gradient accelerators. The aim is to develop a high-power (500-MW) short-wavelength (2.6-cm) relativistic klystron with beam kinetic energy greater than 1 MeV. Two different relativistic klystron configurations have been built and tested: a high-gain multicavity klystron at 11.4 GHz and a low-gain two-cavity subharmonic buncher driven at 5.7 GHz. In both configurations power is extracted at 11.4 GHz. In order to understand the basic physics issues involved in extracting RF from a high power beam, both a single resonant cavity and a multicell traveling-wave structure were used for energy extraction. A previously reported problem of high-power RF pulse shortening was overcome, and peak RF power levels of 170 MW have been achieved with the RF pulse of the same duration as the beam current pulse.<>

An experiment was conducted at the Advanced Test Accelerator (ATA) in the spring/summer of 1988 to study the transport of the ''laser guided'' electron beam through the accelerator and beyond. The electron beam was guided from the /approximately/3.75 MeV point through the accelerator on an ion channel created by firing a KrF laser (248 nm, P approx.100--400 mJ) into a benzene filled (p approx..1 ..mu..) beamline. Beam transport was documented at several locations with a variety of diagnostics for various beam parameters; channel, gas, and laser parameters; and for various machine configurations. The focus of this experiment was to document, understand, and then alter (remove) the time variation in the observed accelerated beam parameters. The temporal variation of the beam manifests itself in several ways as has been documented. Beam radius versus time, R(t), from both bow probe and optical data shows the beam radius expanding by factors of up to several over the 40 ns of beam pulse. When the beam is threaded through any device with a limited acceptance the radius variation is parlayed into a current variation, I(t). This has been documented for transport through the fusible link, rise time sharpener, slits, bends, emittance selectors, and duringmore » free expansion. The beam temporal variation in time may be viewed as radius increase or blowup, brightness degradation, or emittance increase. The observations of blowup predate the introduction of the collimator. The observations prior to this experiment were made in the post accelerator transport sections. 9 refs., 4 figs.« less

ETA-II is a high current, high-repetition-rate induction linac designed for use in free-electron laser (FEL) research. ETA-II design is to produce 7.5-MeV, 2-kA, 70-ns pulses at a brightness of 2 /times/ 10/sup 9/A/(m-rad)/sup 2/ with a repetition rate of up to 100 Hz continuous or 5 kHz in burst mode. The pulse power is generated by pulse compression using magnetic switching. The accelerator consists of a 1.5-MeV injector using an M-type dispenser cathode followed by 60 induction cells. Each cell adds 100 keV to the beam ETA-II will be used for brightness transport studies needed for the design of higher energy accelerators for visible-light FELs. ETA-II will also drive a microwave FEL at 140 GHz and 250 GHz for plasma heating experiments in a tokamak. This report will describe the design parameters and initial operation of ETA-II. 1 ref., 9 figs., 2 tabs.

The multikiloampere peak currents available from linear induction accelerators make high-gain, free-electron-laser-amplifier configurations feasible. High-extraction efficiencies in a single pass of the electron beam are possible if the wiggler parameters are appropriately ''tapered,'' as recently demonstrated at millimeter wavelengths on the 4-MeV ELF facility. Magnetic pulse power systems enable high-repetition-rate operation of the accelerator for high-average-power applications. Key issues involved in extending the technology to shorter wavelengths and higher average power are described.

Laser-ion guiding is currently deployed on the Advanced Test Accelerator (ATA). Beam profiles are measured as a function of time by detecting optical emissions from foils inserted into the beam path. The beam size is observed to grow with time into the pulse. Two other experimental measurements support this observation: (1) vacuum expansion of the beam shows a loss of current in the latter part of the pulse; (2) beam transport through a pipe of reduced diameter results in a similar loss of current in the tail. These observations of increasing beam size are contrary to expectations based on increasing focus strength due to beam-induced ionization. Possible explanations will be presented.

The Advanced Test Accelerator (ATA) beam is guided in a low density ion channel produced in benzene gas by a KrF laser. The strength of the ion channel has been measured by deflecting the electron beam in a steady transverse magnetic field. After emerging from the magnetic field, the beam oscillates transversely inside the channel with a few mm amplitude. The transverse position of the beam is measured at many positions using the wall current monitors. The wavelength of the oscillation is measured from a plot of transverse position vs propagation distance. With a 0.5 J laser pulse and 4 x 10/sup -4/ Torr benzene pressure the wavelength was 1 m corresponding to 90 esu/cm/sup 3/ with ..gamma.. = 84.

Relativistic electron beam currents in excess of 10 kA have been propagated successfully for 64 m through the Advanced Test Accelerator (ATA), and up to 31 m beyond that, by means of an electro-static column of benzene ions generated by a KrF laser pulse. The immediate consequence was a dramatic reduction in transverse rf beam displacements to <0.1 mm. The absence of any magnetic focusing or steering necessitated stringent controls on the pointing, focusing, spatial uniformity and temporal stability of the ionizing laser. The control and monitoring of a benzene pressure profile in a hostile radiation environment have been addressed but still require constant attention. The subtleties of matching the electron beam between ion columns and magnetic transport out of the injector, at bends in the beam line, and at other transition zones are not yet fully understood. The consequences of temporal, spatial, and angular mismatch impact the electron beam output rise time, pulse shape, pulse length, total current, energy spread and emittance.

The Advanced Test Accelerator (ATA) has solenoidal magnets to focus and guide the electron beam. The transverse position of the center of mass of the beam is measured at many positions along the accelerator using the wall current monitors. With the steering coils off, the beam drifts off axis a few cm in the length of the machine. The drift is independent of several parameters: field strength, beam current, and beam energy. Further measurements to locate the source of the misalignment will be discussed. 1 ref., 4 figs.

The recently demonstrated technique of laser guiding has been used to propagate successfully a 10-kA relativistic electron beam 95 m through the Advanced Test Accelerator and postaccelerator beamline. The maximum transverse displacement of the beam at the end of the Advanced Test Accelerator was 1 mm while the maximum beam breakup amplitude was 0.1 mm. The use of laser guiding constitutes a breakthrough in accelerator technology in that it is able to reduce greatly or even suppress the beam breakup instability, which is the most serious obstacle to high-current beam transport in linear induction accelerators.

with an A-K gap of about 13 cm. There was no grid used during the experiment. The cathode was surrounded by a Pierce correcting shroud and the typical gap voltage was about 2.5 MeV. Our initial tests of the field emission cathodes were done using a woven carbon yarn that was laced through a fine mesh screen and then trimmed to a uniform height. Using these “tufted” cathodes, it was easy to vary the number of emission sites per square centimeter. We also varied the geometry of these cathodes by giving the screen a slight convex shape so that the center of the cathode was about 1 cm closer to the anode plane than the edge of the cathode.

During the last year we re-configured the ATA injector to accommodate field emission cathodes. The injector is now run as a diode machine with a 7 cm radius cathode, an A-K gap of 12.9 cm and a field stress of 190 kV/cm. The advantage of using field emission cathodes is we have increased the injector brightness by a factor of ten above the level we were able to reach using the low density plasma cathodes.

Heuristic relationships such as the Lawson-Penner criterion, used to scale free electron laser (FEL) amplifier gain and efficiency over orders of magnitude in beam current and brightness, have no fundamental basis. The brightness of a given source is set by practical design choices such as peak voltage, cathode type, gun electrode geometry, and focusing field topology. The design of low emittance, high current electron guns has received considerable attention at Livermore over the past few years. The measured brightnesses of the experimental test accelerator (ETA) and advanced test accelerator (ATA) guns are less than predicted with the EBQ [1] gun design code; this discrepancy is due to plasma effects from the present cold, plasma cathode in the code. The EBQ code is well suited to exploring the current limits of gridless relativistic Pierce columns with moderate current density (< 50 A/cm2) at the cathode. As EBQ uses a steady-state calculation it is not amenable for study of transient phenomena at the beam head. For this purpose, a Darwin approximation code, DPC, has been written. The main component in our experimental cathode development effort is a readily modified electron gun that will allow us to test many candidate cathode materials, types and electrode geometries at field stresses up to 1 MV/cm.

The first gas propagation experiment on ATA is planned to be conducted in a 1-foot diameter tank of up to 10 m length. The primary objectives are to measure beam parameters at injection to determine whether the desired beam conditioning is achieved, and to observe how such conditioned beams propagate in air and neon.

Spectral emissions of the Experimental Test Accelerator beam in 500-Torr synthetic air have been measured in the wavelength range 250–700 nm for beam currents of 4.5 and 8 kA. Intense emissions are identified as radiation from nitrogen. Relative intensities agree well with Franck–Condon factors. Wet air results in a dramatic decrease (by a factor of 2–3) in intensity over the scanned range. This effect is presently attributed to increased hose motion. Near 700 nm, emissions that are not observed in dry air, are identified as emanating from water vapor, nitric oxide, and oxygen. The pressure dependence of emitted intensities at 337.1 and 391.4 nm were measured from 80 μ to 500 Torr of nitrogen as well as dry synthetic air. Results obtained for an 8-kA beam are not explained by a time-dependent Boltzmann air chemistry code which predicted very well previous measurements for a 1–kA beam.