T. Houck

Prototype microwave source for a relativistic klystron two-beam accelerator

A test facility is established at Lawrence Berkeley National Laboratory (LBNL) to study RF power sources for linear colliders based on the relativistic klystron two-beam accelerator (RK-TBA) concept. A 24-m long prototype source, the RTA, will be constructed in this facility to study physics, engineering, and cost issues related to RK-TBAs. The RTA will generate 200-ns 180-MW RF (11.4-GHz) pulses from each of eight output ports. The major components of the RTA include a 2.8-MeV 1.2-kA induction injector, transverse beam modulator, adiabatic compressor, and RF extraction section. The beam energy is increased to 4 MeV and the RF bunch length is shortened from 240/spl deg/ to 110/spl deg/ in the adiabatic compressor. The 8-m long extraction section includes 40 induction accelerator cells to maintain beam energy at an average 4 MeV, eight equally spaced RF output structures, and a ppm quadrupole focusing system. In this paper, we describe the RTA and present results of component testing and computer simulations.

Measured and theoretical characterization of the RF properties of stacked, high-gradient insulator material

Recent high-voltage breakdown experiments of periodic metallic-dielectric insulating structures have suggested several interesting high-gradient applications. One such area is the employment of high-gradient insulators in high-current, electron-beam, accelerating induction modules. For this application, the understanding of the RF characteristics of the insulator plays an important role in estimating beam-cavity interactions. In this paper, we examine the RF properties of the insulator comparing simulation results with experiment. Different insulator designs are examined to determine their RF transmission properties in gap geometries.

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.

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.

Relativistic Klystron Two-Beam Accelerator studies at the RTA test facility

A prototype rf power source based on the Relativistic Klystron Two- Beam Accelerator (RK-TBA) concept is being constructed at LBNL to study physics, engineering, and costing issues. The prototype, called RTA, is described and compared to a full scale design appropriate for driving the Next Linear Collider. Specific details of the induction core test and pulsed power system are presented. Details of the 1-MeV, 1.2-kA induction gun currently under construction are described.

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. >

Insulator surface flashover due to UV illumination

The surface of an insulator under vacuum and under electrical charge will flashover when illuminated by a critical dose of ultra-violet (UV) radiation - depending on the insulator size and material, insulator cone angle, the applied voltage and insulator shot-history. A testbed comprised of an excimer laser (KrF, 248 nm, ∼16 MW, 30 ns FWHM,), a vacuum chamber, and a negative polarity dc high voltage power supply (≤ −60 kV) were assembled to test 1.0 cm thick angled insulators for surface-flashover. Several candidate insulator materials, e.g. High Density Polyethylene (HDPE), RexoliteR 1400, Macor™ and Mycalex, of varying cone angles were tested against UV illumination. Commercial energy meters were used to measure the UV fluence of the pulsed laser beam. In-house designed and fabricated capacitive probes (D-dots, ≫12 GHz bandwidth) were embedded in the anode electrode underneath the insulator to determine the time of UV arrival and time of flashover. Of the tested insulators, the +45 degree Rexolite insulator showed more resistance to UV for surface flashover; at UV fluence level of less than13 mJ/cm2, it was not possible to induce a flashover for up to −60 kV of DC potential across the insulators surface. The probes also permitted the electrical charge on the insulator before and after flashover to be inferred. Photon to electron conversion efficiency for the surface of Rexolite insulator was determined from charge-balance equation. In order to understand the physical mechanism leading to flashover, we further experimented with the +45 degree Rexolite insulator by masking portions of the UV beam to illuminate only a section of the insulator surface; 1) the half nearest the cathode and subsequently, 2) the half nearest the anode. The critical UV fluence and time to flashover were measured and the results in each case were then compared with the base case of full-beam illumination. It was discovered that the time for the insulator to flash was earlier in time for the cathode-half beam illumination case than the anode-half illumination case which led us to believe that the flashover mechanism for the UV illumination is initiated from the cathode side of the insulator. Qualitatively stated, the testing revealed that the shielding of the cathode triple point against UV is more important than the anode triple junction in the design of vacuum insulators and electrodes.

UV Induced Insulator Flashover

Insulators are critical components in high-energy, pulsed power systems. It is known that the vacuum surface of the insulator will flashover when illuminated by ultraviolet (UV) radiation depending on the insulator material, insulator cone angle, applied voltage and insulator short-history. A testbed comprised of an excimer laser (KrF, 248 nm, ~2 MW/cm2, 30 ns FWHM,), a vacuum chamber (low 1.0E-6 torr), and dc high voltage power supply (<60 kV) was assembled for insulator testing to measure the UV dose during a flashover event. Five in-house developed and calibrated fast D-Dot probes (>12 GHz, bandwidth) were embedded in the anode electrode underneath the insulator to determine the time of flashover with respect to UV arrival. A commercial energy meter were used to measure the UV fluence for each pulse. Four insulator materials high density polyethylene, Rexolitereg 1400, Macortrade and Mycalex with side-angles of 0, plusmn30, and plusmn45 degrees, 1.0 cm thick samples, were tested with a maximum UV fluence of 75 mJ/cm2 and at varying electrode charge (10 kV to 60 kV). This information clarified/corrected earlier published studies. A new phenomenon was observed related to the UV power level on flashover that as the UV pulse intensity was increased, the UV fluence on the insulator prior to flashover was also increased. This effect would bias the data towards higher minimum flashover fluence.

Double pulse experiment with a velvet cathode on the ATA injector

Double pulse transport experiments were conducted on the front end of the ATA accelerator to obtain data on the capability of a velvet cloth cathode to produce two successive pulses. Pulses of approximately 3 kA were extracted from the cathode with interpulse spacings varying from 150 ns to 2.8 /spl mu/s using an anode-cathode voltage of about 1 MV. Analysis of the current and voltage waveform data from the injector indicate that the effects of cathode plasma on the second pulse of a two-pulse burst is minimal.

Relativistic klystron research for two-beam accelerators

We have tested a high-power 11.4-GHz rf generator which consists of a 5.7-GHz transverse modulating system and two 11.4-Ghz traveling-wave output structures. The device was designed to generate 500 MW of pulsed rf power when driven by a 1-kA, 3-MeV induction accelerator beam. Transverse beam instability due to rf coupling between the two output structures has limited the width of the rf output pulse for currents above 600 amperes. Short rf pulses of total output power of up to 420 MW have been produced. Using a single output structure, rf output pulses with stable phase (< +/- 2 degree(s)) and amplitude (< +/- 2%) have been achieved for widths comparable to the beam width. We have modified and tested an output structure to decrease the growth of fields causing transverse instabilities. During the next year our experimental program will include both studies of rf power extraction and reacceleration of modulated electron beams. In support of reacceleration experiments, we are developing a time dependent computer code for the simulation of transverse instabilities due to dipole modes in the rf structures, and are upgrading the induction beam to 5 MeV.