Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.     

Generation of subnanosecond 10-GHz pulses in high peak and high average power mode

The regime of excitation of microwave pulses in a 10-GHz range at a pulse duration of 0.8 ns and a peak power of ～2 GW has been studied in a relativistic backward wave oscillator with an extended periodic slow-wave system. A pulsed electron accelerator generating high-current electron beams (3 GW, ～600 keV, ～5 kA, 7 ns) at a repetition rate of 700 Hz and a pulse train width of 1 s has been developed based on a high-voltage generator with inductive energy storage, a semiconductor current interrupter, and a pulse-sharpening hydrogen-filled discharge gap. Optimization of the regime of the field-particle interaction allowed an average microwave power of 2.5 kW to be obtained at a transport magnetic field strength reduced below the cyclotron resonance value.     

Post-acceleration bunching system for a CN-type Van de Graaff accelerator

The 1.5 ns fwhm pulses delivered with 2.5 MHz repetition rate by the CN Van de Graaff accelerator of CBNM have been further compressed. The time compression is based on a two-gap buncher powered with a frequency of 105 MHz. The synchronization with a pickup electrode signal is realized by a gated phase lock device which is described as well as the electronics used to perform time resolution measurements. Pulses with fwhm ⩽ 0.2 ns were obtained.     

High-intensity multi-bunch beam generation by a photo-cathode RF gun

A multi-bunch photo-cathode RF gun system has been developed as an electron source for the production of intense quasi-monochromatic X-rays based on inverse Compton scattering. The desired multi-bunch beam is 100 bunches/pulse with a total charge of 500 nC and a bunch spacing of 2.8 ns. We modified the gun cavity of a ‘BNL-type IV’ RF gun to allow a CsTe cathode plug in the end plate. The system uses a four-dipole chicane beam line to allow the injection of laser light normal to the cathode surface. We compensate the gun cavity beam loading caused by the high-intensity multi-bunch electron beam by injecting the laser pulse before RF power has filled the cavity. We have achieved a total intensity of 220 nC in 100 bunches with a bunch-to-bunch energy spread under 1.3% (peak-to-peak). This paper concentrates on experiments to generate the high-intensity multi-bunch beam with compensation of the bunch-to-bunch energy spread due to heavy beam loading.     

The ion optical properties of a travelling-wave chopper

The ion optical properties of a travelling-wave chopper and its influence on energy and time spread of bunched low energy heavy ion beams are investigated. An arrangement where the deflection plates are separated by grounded plates is found to be most effective in generating beam pulses with good time structure. Pulse widths of 0.45 ns (fwhm) for protons and deuterons and 1.9 ns for ¹²C and ¹⁹F are routinely achieved.     

Application of a trochoidal electron monochromator/mass spectrometer system to the study of environmental chemicals.

A trochoidal electron monochromator has been interfaced to a mass spectrometer to perform electron capture negative ion mass spectrometric (ECNIMS) analyses of environmentally relevant chemicals. The kinetic energy of the electron beam can be varied from 0.025 to 30 eV under computer control. No reagent gas is used to moderate the electron energies. An electron energy spread of +/- 0.1 to +/- 0.4 eV full width at half-maximum (fwhm) can readily be obtained at a transmitted current of 2 x 10(-6) A, improving to +/- 0.07 eV at 5 x 10(-7) A. Comparisons of ECNI results from the electron monochromator/mass spectrometer system with those from a standard instrument that uses a moderating gas show similar spectra for heptachlor but not for the s-triazine herbicides, as for example, atrazine. This compound shows numerous adduct ions by standard ECNIMS that are eliminated by using the electron monochromator to generate the mass spectra. Isomeric tetrachlorodibenzo-p-dioxins show distinct differences in the electron energies needed to produce the maximum amount of parent and fragment anions. Multiple resonance states resulting in stable radical anions (M.-) are easily observed for nitrobenzene and for polycyclic aromatic hydrocarbons. Ionic products of dissociative electron capture invariably occur from several resonance states.     

Principles of high current electron beam acceleration

High current pulsed electron accelerators operate with beam currents exceeding 1 kA, pulse lengths from 20 ns to 1 μs, and output energies up to 50 MeV. Potential applications include pulsed radiography, intense microwave generation, free electron laser drivers, directed energy for defense, and industrial radiation processing applications. This paper gives a tutorial on the principles of high current electron accelerators. It is divided into four sections: (a) high current sources, (b) space charge dominated extractors, (c) beam transport with strong self-fields, and (d) methods of high power acceleration. In addition to discussions of conventional technology, such as the linear induction accelerator, promising new approaches to beam generation and acceleration are outlined. These include laser driven photocathodes, ion channel focusing, and high power rf accelerators.     

Pulsed energy conversion with a dc superconducting magnet

A generator system for pulsed power is described which employs a dc superconducting magnet in a magnetic flux compression scheme. Experience with a small-scale generator together with projections of numerical models indicate potential applications to fusion research and commercial power generation. When the system is large enough pulse energy can exceed that stored in the magnet and pulse rise time can range from several microseconds to tens of milliseconds.     

Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.     

Studies on a pulse shaping system for fast coincidence with very large volume HPGe detectors

A variant of the leading edge timing (LET) has been proposed which compensates the “walk” due to risetime spread in very large volume (∼ 100 cm³) HPGe detectors. The method - shape compensated leading edge timing (SCLET) — can be used over a wide dynamic range of energies with 100% efficiency and has been compared with the LET and ARC methods. A time resolution of 10 ns fwhm and 21 ns fwtm has been obtained with ²²Na gamma rays and two HPGe detectors of 96 and 114 cm³ volume. This circuit is easy to duplicate and use and can be a low cost alternative to commercial circuits in experiments requiring a large number of detectors.     

Approaching the resolution limit of nanometer-scale electron beam-induced deposition

We report the writing of very high resolution tungsten containing dots in regular arrays by electron beam-induced deposition (EBID). The size averaged over 100 dots was 1.0 nm at fwhm. Because of the statistical spread in the dot size, large and small dots are present in the arrays, with the smallest having a diameter of only 0.7 nm at fwhm. To date these are the smallest features fabricated by EBID. We have also fabricated lines with the smallest having a width at fwhm of 1.9 nm and a spacing of 3.2 nm.     

Design, measurement and correction of a pair of novel focusing undulators for the ALPHA-X project

ALPHA-X is a UK collaboration to develop and build a prototype laser-plasma accelerator. The ALPHA-X beam line generates an electron bunch with energy 0.1-1 GeV; this is injected into a pair of permanent magnet undulators to generate a short pulse of radiation. The design of the undulators is detailed in this paper.The undulators have a requirement to focus the beam along their length. Several focusing schemes were analysed. The scheme used involved magnet blocks with a 5×1 mm slot cut into the top to provide a sextupole-type field.The undulators were built and tested at STFC's Daresbury Laboratory. Some block swapping was carried out for each one to reduce the overall field integrals and optimise the beam transport. The first field integral was reduced using this technique to less than 0.1 T mm for both undulators.     

Development of a high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is useful for high frequency operation, because the high beam efficiency is kept even at high harmonic of electron-cyclotron resonance. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already succeeded in operation at the third harmonic and the fourth harmonic resonances. The operation frequencies are 89.3 and 112.7 GHz, respectively. Operation cavity modes are TE₃₁₁ and TE₄₁₁. The permanent magnet system consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in cavity and electron gun regions. The magnetic field at the cavity region can be varied from 0.97 to 1.18 T. At the optimum condition of the magnetic field intensity, the output power at the third harmonic operation is 2.5 kW. The operation is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy and current are 40 kV and 1.2 or 1.3 A. Starting current, beam efficiency and emission pattern also have been measured. In this paper, the operation results of the gyrotron and comparison with the computer simulation results are described.     

Fast microwave detection system for coherent synchrotron radiation study at KEK: Accelerator test facility

A fast room temperature microwave detection system based on the Schottky Barrier-diode detector was created at the KEK ATF (Accelerator Test Facility). It was tested using Coherent Synchrotron Radiation (CSR) generated by the 1.28 GeV electron beam in the damping ring. The speed performance of the detection system was checked by observing the CSR from a multi-bunch (2.8 ns bunch separation time) beam. The theoretical estimations of CSR power yield from an edge of bending magnet as well as new injection tuning method are presented. A very high sensitivity of CSR power yield to the longitudinal electron distribution in a bunch is discussed.     

Diode-end-pumped electro-optically Q-switched Nd:YLF slab laser

We report a very compact Nd:YLF slab laser that is end pumped by a quasi-continuous-wave diode stack. A hybrid resonator is used to generate high output power in a near-diffraction-limited beam (i.e., a beam propogation M2 factor of less than 1.2). A pulse energy of 14.3 mJ was obtained with a pulse width of 8.5 ns at a repetition rate of 500 Hz, which corresponds to a Q-switched peak power of 1.68 MW.     

Comparative study of generation characteristics of CdS crystal laser pumped by low- and high-energy electron beams

A CdS crystal has been excited by radially converging pulsed electron beam with an electron energy of up to E ≈ 18 keV, a beam current density at the target of up to 4 kA/cm², and a pulse full width at half maximum within τ ≈ 12–100 ns. For comparison, the same crystal was pumped by electrons with E = 170 keV at τ = 2.2 ns. A specific distinctive feature of the excitation by powerful low- and high-voltage electron beams is the appearance of previously unreported long-wavelength generation with a peak at λ ≈ 535 nm in the electron-hole plasma band. In the region of the interband transition with a peak at λ ≈ 515 nm, the generation spectra are similar to those known from the literature.     

Time-dependent photon number discrimination of InGaAs/InP avalanche photodiode single-photon detector

We investigated the photon-number-resolving (PNR) performance of the InGaAs/InP avalanche photodiode (APD) as a function of the electric gate width and the photon arrival time. The optimal electric gate width was around 1 ns for PNR measurements in our experiment, which provided a PNR capability up to three photons per pulse when the detection efficiency was ~20%. And the dependence of the PNR performance on the arrival time of the photons showed that the photon number could be better resolved if the photons arrived on the rising edge of the electric gate than on the falling edge. In addition, we found that with the increase of the electric gate width, PNR performance got worse. The observation would be helpful for improving the PNR performance of the InGaAs/InP APD in the gated mode.     

High-power subnanosecond 38-GHz microwave pulses generated at a repetition rate of up to 3.5 kHz

An original relativistic backward tube (BWT) for a 38 GHz range is developed and tested. The BWT is capable of generating stable pulses of ～250 ps duration and a peak power of ～250 MW in trains with a length of up to 1 s at a repetition rate of 1–3.5 kHz. The BWT design implements an inhomogeneous slow-wave structure of increased cross section with a band reflector. A pulsed electron beam (～270 keV, ～2 kA, 0.9 ns) was injected by a high-current accelerator based on a high-voltage generator with an inductive energy store, a semiconductor current interrupter, and a pulse-shaping hydrogen-filled discharge gap. A focusing magnetic field of 2 T was generated by a cooled pulsed solenoid power-supplied from a special stabilized current source.     

Method for stabilizing beam intensity and energy in the SPring-8 linac

In any electron accelerator facilities, radio frequencies (RF) for a linear accelerator and a circular accelerator that includes a booster synchrotron ring and a storage ring are completely different. There is not necessarily a sub-harmonic relation of two RFs between a linear accelerator and a circular accelerator. It is, however, indispensable to obtain a synchronous relation between both RFs and the timing of the gun trigger signal with the increasing beam current and shortening of beam time width from an electron gun in a linear accelerator. For a synchronous timing system in any electron accelerator facilities, there is no choice but to assemble a complicated system with the frequency dividers and multipliers in order to realize the synchronous relation between both RFs and the gun trigger signal. To simplify the complicated synchronous timing system, we have developed a new synchronization method for the RFs of both linear and circular accelerators. The new synchronization system has been installed into the synchrotron radiation facility, SPring-8 (Super Photon ring 8 GeV), which consists of a 1-GeV linac, an 8-GeV booster synchrotron and an 8-GeV storage ring. A 2856 MHz RF for the 1-GeV linac was generated by the 508.58 MHz RF of the 8-GeV storage ring with the new synchronous timing system, and the emission and acceleration under the condition of the shortening the beam time width in the linac was carried out. Since the synchronous relation between both the RFs and the gun trigger signal was realized by using the new synchronous timing system, the time jitter between the gun trigger signal and the RF phase of 2856 MHz was significantly reduced and resulted in beam energy stabilization. The new synchronous timing system has been used for usual beam operations at SPring-8. This timing system has achieved time jitters of 3.5 ps (rms) and beam energy stability of 0.009% (rms) under the condition of completely synchronized two RFs and the gun trigger signal.     

Optical properties of carbon-containing titanium oxide nanocomposites obtained by the pulsed plasma chemical method

This paper presents the results of an experimental investigation on the optical properties of the TiO₂ and Ti_xC_yO_z nanopowders, produced by the pulsed plasma chemical method. Pulsed plasma chemical synthesis is realized on the laboratory stand, including a plasma chemical reactor (6 l) and TEA-500 electron accelerator. The parameters of the electron beam are as follows: 400–450 keV electron energy, 60 ns half-amplitude pulse duration, up to 200 J pulse energy, and 5 cm beam diameter. In TiO₂ sample, obtained using the pulsed plasma chemical method, the particles can be divided into two groups: 100–500 nm large spherical particles and tiny complex particles (sized less than 100 nm). For TixCyOz sample, the morphology of the particles is mainly presented with irregular fragment shape. The average size of the particles is ranged from 200 to 300 nm. The band gap for all synthesized samples is within 2.94–3.35 eV.