Bunching Factor Research Articles

Effect of time-of-flight bunching on efficiency of light-ion-beam inertial-confinement-fusion transport schemes

The Laboratory Microfusion Facility (LMF) has been proposed for the study of high-gain, high-yield inertial-confinement-fusion targets. The light-ion LMF approach uses a multimodular system with applied-B extraction diodes as ion sources. A number of ion-beam transport and focusing schemes are being considered to deliver the beams from the diodes to the target. These include ballistic transport with solenoidal lens focusing, z-discharge channel transport, and wire-guided transport. The energy transport efficiency ηt has been defined and calculated as a function of various system parameters so that point designs can be developed for each scheme. The analysis takes into account target requirements and realistic constraints on diode operation, beam transport, and packing. The effect on ηt of voltage ramping for time-of-flight beam bunching during transport is considered here. Although only 5 mrad microdivergence calculations are presented here, results for bunching factors of ≤3 show that transport efficiencies of ≳50% can be obtained for all three systems within a range of system parameters which seem achievable (i.e., for diode microdivergence within 5–10 mrad, for diode radius within 10–15 cm, and for diode-ion-current density within 2–10 kA/cm2). In particular, the point design for the baseline LMF system using ballistic transport with solenoidal lens focusing and a bunching factor of 2 was calculated to have ηt=84%. Other factors affecting the overall system efficiency, but not included in the analysis, are also identified and estimated.

Observation of coherent transition radiation at millimeter and submillimeter wavelengths.

Coherent transition radiation has been observed in the wavelength range from 0.5 to 5.5 mm emitted from bunched electrons of 42 MeV passing through an Al foil in vacuum. The radiation intensity at \ensuremath{\lambda}=4 mm is enhanced by a factor of 3\ifmmode\times\else\texttimes\fi{}${10}^{6}$ in comparison with incoherent transition radiation. The intensity is proportional to the square of the beam current, and it increases with the emission length, or the length of the trajectory of the electrons between the foil and a mirror to observe the radiation. The divergence of radiation increases with the wavelength and it is qualitatively in agreement with theory. A sheet of Eccosorb is also used as a radiator, and a similar spectrum to that from the Al foil has been observed. The bunch shape is discussed on the basis of a bunch form factor derived from the spectrum.

New RF Exercises Envisaged in the CERN-PS for the Antiprotons Production Beam of the ACOL Machine

The new antiproton collector machine (ACOL)[1] presently under development at CERN, requires the highest possible proton intensity (more than 1013 protons per pulse) in one quarter of the PS circumference, and a guaranteed bunching factor, for its p production beam: possibly less than 25ns bunches, 105 ns apart. The present 26 GeV longitudinal technique [2], cannot meet these specifications, so that new quasi-adiabatic processes had to be imagined [3]. First, before transition energy, a bunch pair merging operation reduces from 10 to 5 the number of bunches, with a moderate blow-up, by slowly bunching the beam on half the initial harmonic number. Then, on the high energy flat top, the length of circumference occupied by the bunches is halved, by a technique tentatively named bunch batch compression. This is basically a slow increase of the RF harmonic number seen by the beam, implemented by operating part of the existing cavities stepwise at increasing harmonic numbers (10, 12, 14, 16, 18 and finally 20). Computer outputs are presented, together with machine experiment results.

Synchrotron Improvements with Shed Waveform

In synchrotrons for ion beam fusion one needs a small bucket to limit the longitudinal emittance, a small synchrotron frequency to avoid synchrobetatron resonance, a large bunching factor to reduce space charge tune depression, and a high synchronous voltage to complete acceleration without ion-ion collisions. It is proposed to use a trigonometric series approximation to a new shed-like waveform to replace the usual pure sine wave (fundamental). The shed waveform divides the interval (0,2?) into two parts with a crossing in between. The right portion contains the particles and is linear, while the left portion is merely an area dump. A fit to a shed with three properly phased sine waves is demonstrated to give almost as good results as the original. In the present application a fundamental frequency range 12.5 to 50.0 MHz is required. Four cavities and four rf systems are utilized to produce the fundamental and two harmonics up to 150 MHz. The frequency range limit of a cavity is imposed by properties of the ferrite, voltage requirement, and the operating frequency. Each cavity covers a factor of 3 in frequency. A dropping down scheme is described so that a single cavity may be used in more than one range.

A Radial Signal Process Method for the BPS Booster LLRF Control System

The function of the Radial Position Processor is to convert rf fundamental frequency components induced by circulating bunches in a Position Pick-up Unit into an analog voltage proportional to the average beam transverse deviation from the synchronous orbit position. Because of the relatively large frequency swing, (2.76-6.31 MHz), some second harmonic components of lower frequencies would be passed by a broad-band system capable of covering the entire frequency range. Because of this, and because of the relatively high signal to noise ratio inherent in swept narrow-band channels, the superheterodyne principle has been chosen for this system. Position error discrimination is developed by amplitude to phase conversion following frequency conversion to 10 MHz. This technique results in high precision radial position error discrimination over a 52 db dynamic range of input signals. The system can, therefore, operate without readjustment over a large range of beam intensity and bunching factor. Detailed system design and test results are described. Since the BPS booster synchrotron is not yet completed, no actual performance details are available.

Bunched Beam Neutralization

One of the steps involved in producing an intense ion beam from conventional accelerators for Heavy Ion Fusion (HIF) is beam bunching. To maintain space charge neutralized transport, neutralization must occur more quickly as the beam bunches. It has been demonstrated at BNL that a 60 mA proton beam from a 750 kV Cockcroft-Walton can be neutralized within a micro-second. The special problem in HIF is that the neutralization must occur in a time scale of nano-seconds. To study neutralization on a faster time scale, a 40 mA, 450 kV proton beam was bunched at 16 MHz. A biased Faraday cup sampled the bunched beam at the position where maximum bunching was nominally expected, about 2.5 meters from the buncher. Part of the drift region, about 1.8 meters, was occupied by a series of Gabor lenses. In addition to enhancing beam transport by transverse focussing, the background cloud of electrons in the lenses provided an extra degree of neutralization. With no lens, the best bunch factor was at least 20. Bunch factor is defined here as the ratio of the distance between bunches to the FWHM bunch length. With the lens, it was hoped that the increased plasma frequency would decrease the neutralization time and cause an increase in the bunch factor. In fact, with the lens, the instantaneous current increased about three times, but the bunch factor dropped to about 10.

A 2ND RF System for Nimrod

An additional high power RF system has been built for Nimrod to provide an 8th harmonic accelerating field i.e. the 2nd harmonic of the main RF. Its purpose is to reduce the bunching factor of the proton beam thus increasing beam acceptance and accelerated intensity. In the accelerating cavity a drift tube is tuned over a frequency range from 2.8 MHz to 16 MHz by two ferrite loaded resonators. The configuration adopted proved to have considerable advantages over a self contained tuned cavity particularly for assembly and access, permitting economy in the use of ferrite. The problems discussed include that of covering a wide tuning range while restricting the magnetic flux in the ferrite, suppressing unwanted modes in the cavity and maintaining accurately the phase and amplitude of the harmonic field relative to the main RF.

A versatile klystron bunching system for a 5.5 MV Van de Graaff accelerator

A special version of the ORTEC Klystron bunching system has been installed on the SUNI 5.5 MV Van de Graaff accelerator. The design considerations are outlined. The realization of the full capabilities of this system has been made possible by two modifications: (a) the incorporation of a cross-field analyser as an integral part of the pulser unit and (b) the installation of a split buncher tube. With the duoplasmatron ion source, continuous current operation was made virtually impossible because of overloading of the accelerator tube by defocussed particles of different masses. The cross-field analyser selects a single mass (or change) particle for acceleration and thus eliminates this problem. The use of the cross-field analyser is also required for optimum utilization of the pulsed beam. The incorporation of the analyser as part of the pulser is described. A buncher tube of fixed length with a bunching voltage of fixed frequency is only optimum for one particle mass. A modified buncher tube is shown to give superior performance for mass 1 to mass 3 beams for which pulses of 1 to 1.5 ns width (fwhm) respectively, have been obtained. Using the original buncher tube the best result achieved with mass 3 beams was 2.6 ns. Effective bunching factors greater than 20 have been achieved with an average current of about 5 μA on the target at a pulse rate of 2 MHz. The energy spread in the bunched proton beam is about 4 keV. The SUNI system is designed to switch readily from continuous to pulsemode operation with protons, deuterons or helium ions.

Bunched nsec beam pulses of heavy ions up to mass 32 with a hvec tandem accelerator

Negatively charged O 2 and H dc ion beams from the HVEC ion source were modulated with rf frequencies of 3 and 10 Mc/s. Beam pulses of 18 MeV O 16 ions nsec wide and of 6 MeV protons 7.0 nsec wide were measured with bunching factors of 13.5 respectively 8.0.

Proposed methods for producing intense pulsed beams of mono-energetic neutrons

These methods utilize time-of-flight and extended targets to produce bunched beams of mono-energetic neutrons. It would appear that bunching factors of several hundred could be achievable by combinations of these techniques.

Bunching of a Beam of 20-keV Neutral Hydrogen Atoms

A beam of 20-keV neutral hydrogen atoms was formed by electron attachment in a hydrogen gas canal. Prior to neutralization, the protons were velocity modulated by applying an accelerating voltage to the neutralizing canal, thereby requiring only a single gap to produce bunching. The flight time of the neutral atoms, between the buncher and a target, was over 2 μsec. A minimum width of 3.7 nsec (full-width at half-height) was observed on a sampling oscilloscope and a bunching factor (peak intensity in the bunch over dc intensity) of 37 was obtained. The minimum bunch width has been related to the time spreads introduced by the energy spread of the protons from the ion source and by energy straggling in the charge-changing process.

Production of high intensity ion pulses of nanosecond duration

A system for producing high intensity ion bursts of nanosecond duration is described. It consists of a Van de Graaff accelerator fitted with a deflection pulser in the terminal and a post-acceleration Mobley magnet with a bunching factor of about 12. Results are given on the overall performance at 3 MeV proton energy, observing the neutrons and gamma rays from the reaction Mn 55(p, n)Fe 55. The system performs in accordance with design expectations and delivers ion bursts of less than 1 ns duration at a peak current of several milliamperes.

An investigation into the design of a gridless low-voltage reflex klystron

The paper describes an investigation to determine the limit of lowvoltage operation of reflex klystrons employing gridless apertures. The problems involved include the design of suitable electron guns and the determination of the maximum useful current.Initial calculations indicated that outputs of several hundred milliwatts should be obtainable from a 3 cm oscillator with a 500-volt 30 mA electron gun if the effective current was assumed to be half the total current and space-charge effects in the reflector space were neglected. Practical values fall far short of this. Some measurements of unbunched effective current and the current-density distribution across the waist have enabled a considerable part of this discrepancy to be removed. The remaining discrepancy is thought to be due mainly to the effects of bunching on the effective current, of space charge on the bunching factor, and of phase aberrations. Further work is required to obtain more information on these points. The measurements of effective current indicate that a maximum useful mean current density exists in the region of 0.5 amp/cm2, beyond which the effective current decreases.The effect of non-uniformity of the beam shape on the beam coupling factor and beam damping is considered in an Appendix. Expressions for the correction factors have been calculated for the case where the current-density distribution follows a Gaussian form, and these are presented, together with some graphs covering a range of the relevant parameters. Graphs giving the beam coupling and beam-damping factors are also included for convenience.