Showing papers on "Linear particle accelerator published in 1990"

Radioisotope production facility for use with positron emission tomography

Abstract: A radioisotope production facility (12) produces radioisotopes having application to Positron Emission Tomography. The radioisotopes produced include 18 F, 13 N, 15 O, and 11 C, and are produced by irradiating a selected target material (40) with a high energy 3 He++ beam accelerated in a radio frequency quadruple (RFQ) linear accelerator (34). The facility includes, in addition to the RFQ linear accelerator and the selected target, a source of 3 He++ ions (30), low energy transport means (32) for focusing the 3 He++ beam into the RFQ linear accelerator, and a high energy transport means (36) for directing the accelerated 3 He++ beam at the selected target. Further included is a target subsystem (16) that holds the target, automatically prepares precursors containing the 18 F, 13 N, 15 O, and 11 C radioisotopes, and an automated radiopharmaceutical subsystem (22) that prepares suitable radiopharmaceuticals from the desired precursors.

Monte Carlo and convolution dosimetry for stereotactic radiosurgery.

Abstract: The dosimetry of small photon beams used for stereotactic radiosurgery was investigated using Monte Carlo simulation, convolution calculations, and measurements. A Monte Carlo code was used to simulate radiation transport through a linear accelerator to produce and score energy spectrum and angular distribution of 6 MV bremsstrahlung photons exiting from the accelerator treatment head. These photons were then transported through a stereotactic collimator system and into a water phantom placed at isocenter. The energy spectrum was also used as input for the convolution method of photon dose calculation. Monte Carlo and convolution results were compared with the measured data obtained using an ionization chamber, a diode, and film.

Corkscrew modes in linear accelelators

Abstract: The corkscrew mode in linear accelerators, in which a solenoidal beam-transport system is used, is investigated both analytically and numerically. By assuming that both the acceleration gaps and the drift regions between the adjacent magnets are infinitesimal, it is possible to treat the corkscrew mode analytically and to obtain an expression for the corkscrew mode amplitude and the effective emittance growth as a function of the energy variation within the beam pulse and of the tilts of the injector and the solenoid magnets. Reduction of the corkscrew mode amplitude is possible by an energy compensation scheme. This energy compensation scheme is studied numerically for the Experimental Test Accelerator II (ETA-II) configuration. We find that applying the energy compensation scheme at the beginning of the accelerator can reduce the corkscrew mode amplitude by a factor of two.

Plasma wake-field accelerator experiments at KEK

Abstract: We report on a plasma wake-field accelerator experiment using a high-intensity 250 MeV electron beam of the linac at KEK (National Laboratory for High Energy Physics, Japan). The experiment provedthe plasma wake-fields excited by a train of several bunches accelerate a trailing bunch. We observed an approximately 4 MeV shift of beam energy in a 1 m long plasma with a density of 4 × 1011 cm−3. We describe the present status and the future plan of experimets based on a new scheme of a plasma wake-field accelerator driven by a train of multiple bunches.

Principles of non-Liouvillean pulse compression by photoionization for heavy-ion fusion drivers

Abstract: Photoionization of single-charged heavy ions has been proposed recently by Rubbia (1989) as a non-Liouvillean injection scheme from the linac into the storage rings of a driver accelerator for inertial confinement fusion. The main idea of this scheme is the accumulation of high currents of heavy ions without the usually inevitable increase of phase space. Here we suggest the use of the photoionization idea in an alternative scheme: if it is applied at the final stage of pulse compression (replacing the conventional bunch compression by an rf voltage, which always increases the momentum spread), there is a significant advantage in the performance of the accelerator. We show, in particular, that this new compression scheme can potentially relax the tough stability limitations, which were identified in the heavy-ion fusion reactor study HIBALL (Badger et al. 1984). Moreover, it is promising for achieving higher beam power, which is suitable for indirectly driven fusion targets (1016 W/g, in contrast with 1014 W/g for the directly driven targets in HIBALL).The idea of non-Liouvillean bunch compression is to stack a large number of bunches (typically 50–100) in the same phase-space volume during a change of charge state of the ion. A particular feature of this scheme with regard to beam dynamics is its transient nature, since the time required is one revolution per bunch. After the stacking the intense bunch is ejected and directly guided to the target. The present study is a first step in exploring the possibly limiting effect of space charge under the parameter conditions of a full-size driver accelerator. Preliminary results indicate that there is a limit to the effective stacking number (non-Liouvillean “compression factor”), which is, however, not prohibitive. Requirements on the power of the photon beam from a free-electron laser are also discussed. It is seen that resonant cross sections of the order of 10−15 cm2 lead to photon beam powers of a few megawatts.

Characteristics of photon beams from Philips SL25 linear accelerators

Abstract: The Philips SL25 accelerator is a multimodality machine offering asymmetric collimator jaws and a new type of beam bending and transport system. It produces photon beams, nominally at 6 and 25 MV, and a scattered electron beam with nine selectable energies between 4 and 22 MeV. Dosimetric characteristics for the 6- and 25-MV photon beams are presented with respect to field flatness, surface and depth dose characteristics, isodose distribution, field size factors for both open and wedged fields, and narrow beam transmission data in different materials.

Levels of leakage radiation from electron collimators of a linear accelerator.

Abstract: The leakage radiation from electron applicators used with our linear accelerator has been measured. For the applicators 6 X 6 to 25 X 25 cm size, the leakage was measured in the plane of the patient and on the sides of the applicators with the available electron energies of 6, 9, 12, 15 and 18 MeV. The levels were significant. The highest leakage on the side was for the combination of 6 X 6-cm applicator and 9-MeV electrons (32%) and in the plane of the patient for 25 X 25-cm applicator with 18 MeV (10%) relative to the peak dose. Adding lead 1-2 mm, at appropriate locations inside the applicators has reduced the leakages to acceptable levels without affecting the beam parameters.

Beam-beam deflection as a beam tuning tool at the SLAC linear collider

Abstract: To achieve maximum integrated luminosity at the SLAC Linear Collider, a method of noninvasive beam tuning is required. Traditional luminosity monitors based on Bhabha scattering are inadequate because of low instantaneous counting rates. Coherent deflections of one beam by the electromagnetic field of the other are sensitive not only to the relative steering of the two bunches but also to their spot sizes. A brief description of beam-beam deflection theory forms the basis for a discussion of this phenomenon as a tool for single-beam tuning and for luminosity optimization at the interaction point of the SLC.

Linear accelerator for production of tritium: physics design challenges*

Abstract: In the summer of 1989, a collaboration between Los Alamos National Laboratory and Brookhaven National Laboratory conducted a study to establish a reference design of a facility for accelerator production of tritium (APT) The APT concept is that of a neutron-spallation source, which is based on the use of high-energy protons to bombard lead nuclei, resulting in the production of large quantities of neutrons Neutrons from the lead are captured by lithium to produce tritium This paper describes the design of a 16-GeV, 250-mA proton cw linear accelerator for APT

Binary rf pulse compression experiment at SLAC

Abstract: Using rf pulse compression it will be possible to boost the 50- to 100-MW output expected from high-power microwave tubes operating in the 10- to 20-GHz frequency range, to the 300- to 1000-MW level required by the next generation of high-gradient linacs for linear for linear colliders. A high-power X-band three-stage binary rf pulse compressor has been implemented and operated at the Stanford Linear Accelerator Center (SLAC). In each of three successive stages, the rf pulse-length is compressed by half, and the peak power is approximately doubled. The experimental results presented here have been obtained at low-power (1-kW) and high-power (15-MW) input levels in initial testing with a TWT and a klystron. Rf pulses initially 770 nsec long have been compressed to 60 nsec. Peak power gains of 1.8 per stage, and 5.5 for three stages, have been measured. This corresponds to a peak power compression efficiency of about 90% per stage, or about 70% for three stages, consistent with the individual component losses. The principle of operation of a binary pulse compressor (BPC) is described in detail elsewhere. We recently have implemented and operated at SLAC a high-power (high-vacuum) three-stage X-band BPC. First results from the high-power three-stage BPCmore » experiment are reported here.« less

Progress report on high-gradient RF studies in copper accelerator structures

Abstract: This paper is a progress report on studies carried out at the Stanford Linear Accelerator Center to understand the behavior of copper accelerator structures under extremely high RF fields. Such structures are being designed for future electron-positron linear colliders. Recent studies include field emission and breakdown experiments with an S-band slotted-disk structure, a single demountable S-band cavity and a short X-band structure which has not yet been tested. The demountable cavity was built specifically to examine the effects of copper quality, surface conditioning, gaseous exposures, and surface damage. Results to date and recent theoretical conjectures are discussed.

Super conducting linear accelerator loaded with a sapphire crystal

Abstract: A dielectric loaded superconducting linear accelerator (linac) (10) is disclosed which includes an accelerating structure formed of a cylindrical sapphire crystal (14) having a centrally disposed passage (16) for reception of a particle beam (18) to be accelerated. A superconductive material layer (12), such as niobium, surrounds the exterior surface of the sapphire crystal (14). When the linac (10) is operated at a superconductive temperature of less than 2 °K, the loss tangents of the sapphire and niobium are very low so that the linac operates very efficiently. The uniform shape of the sapphire crystal (14) insures that wakefields generated by the charged particles as they pass through the linac will be minimized. The linac has a very high Q which enables it to store energy over a long period of time and reduces peak power requirements.

Calculating Beam Breakup in Superconducting Linear Accelerators

Abstract: As the intensity of a particle beam passing through a linear accelerator is raised, interactions between particles play an increasingly prominent role in determining the overall dynamics of the beam. These many body effects, known collectively as beam breakup, tend to degrade the quality of the transported beam, and hence they must be calculated to accurately predict the evolution of the beam as it traverses the accelerator. Several codes which compute various collective effects have been developed and used to simulate the dynamics of beams passing through superconducting accelerator structures. All the codes use the same basic algorithm: the beam is tracked through elements giving the focusing forces on the particles, and at the appropriate locations in the linac, localized forces are impressed on the particles which model the electromagnetic interactions. Here, a difficulty is that the usual ''Coulomb'' interaction between particles is changed by the electromagnetic environment of the accelerator. By such calculations it has been shown that recirculating linear accelerators such as the one being built at the Continuous Electron Beam Accelerator Facility (CEBAF) should remain stable against multipass beam breakup instability as long as the average current does not exceed about 20 mA, that the beam qualitymore » at CEBAF will be degraded when the single bunch charge approaches 10{sup 9} electrons, and that the beam quality of superconducting linacs that are optimized for high current transport begins to decrease at around 10{sup 10} electrons per bunch. The latter result is of interest to individuals who would use superconducting linacs as beam sources for free electron lasers or for superconducting colliders for high energy physics research.« less

Progress in eta-ii magnetic field alignment using stretched wire and low energy electron beam techniques·

Abstract: Flux line alignment of the solenoidal focus magnets used on the ETA-II linear induction accelerator is a key element leading to a reduction of beam corkscrew motion. Two techniques have been used on the ETA-II accelerator to measure and establish magnet alignment. A low energy electron beam has been used to directly map magnetic field lines, and recent work has utilized a pulsed stretched wire technique to measure magnet tilts and offsets with respect to a reference axis. This paper reports on the techniques used in the ETA-II accelerator alignment, and presents results from those measurements which show that accelerator is magnetically aligned to within {approximately}{plus minus}200 microns. 3 refs., 8 figs.

The Delta-t tuneup procedure for the Fermilab Linac

Abstract: The delta-t procedure provides a means of monitoring and setting to design values the phase and amplitude of fields within a linear accelerator. Deviations from design values of energy entering an accelerator module can also be determined. The procedure will be used to tune the upgraded linac under construction at Fermilab. Results of preliminary tests of the technique on the existing 200 MeV linac show qualitative trends in agreement with analysis. Quantitative comparisons show some differences. 4 refs., 6 figs.

Free electron laser

Abstract: A high gain, single-pass free electron laser formed of a high brilliance electron injector source, a linear accelerator which imparts high energy to the electron beam, and an undulator capable of extremely high magnetic fields, yet with a very short period. The electron injector source is the first stage (gap) of the linear accelerator or a radial line transformer driven by fast circular switch. The linear accelerator is formed of a plurality of accelerating gaps arranged in series. These gaps are energized in sequence by releasing a single pulse of energy which propagates simultaneously along a plurality of transmission lines, each of which feeds the gaps. The transmission lines are graduated in length so that pulse power is present at each gap as the accelerated electrons pass therethrough. The transmission lines for each gap are open circuited at their ends. The undualtor has a structure similar to the accelerator, except that the transmission lines for each gap are substantially short circuited at their ends, thus converting the electric field into magnetic field. A small amount of resistance is retained in order to generate a small electric field for replenishing the electron bunch with the energy lost as it traverses through the undulator structure.

The ETA-II induction linac as a high-average-power FEL driver☆

Abstract: The Experimental Test Accelerator II (ETA-II) is the first induction linac designed specifically to FEL requirements. It is primarily intended to demonstrate induction accelerator technology for high-average-power, high-brightness electron beams, and will be used to drive a 140 and 250 GHz microwave FEL for plasma heating experiments in the Microwave Tokamak Experiment (MTX) at LLNL. Its features include high-vacuum design which allows the use of an intrinsically bright dispenser cathode, induction cells designed to minimize BBU growth rate, and careful attention to magnetic alignment to minimize radial sweep due to beam corkscrew. The use of magnetic switches allows high-average-power operation. At present ETA-II is being used to drive 140 GHz plasma heating experiments. These experiments require nominal beam parameters of 6 MeV energy, 2 kA current, 20 ns pulse width and a brightness of 1 × 108 A/(m rad)2 at the wiggler with a pulse repetition frequency (prf) of 0.5 Hz. Future 250 GHz experiments require beam parameters of 10 MeV energy, 3 kA current, 50 ns pulse width and a brightness of 1 × 108 A/(m rad)2 with a 5 kHz prf for 0.5 s. In this paper we discuss the present status of ETA-II parameters and the phased development program necessary to satisfy these future requirements.

Initial observations of high-charge, low-emittance electron beams at HIBAF (High Brightness Accelerator FEL)

Abstract: We report our initial measurements of bright (high-charge, low-emittance) electron beams generated at the Los Alamos High Brightness Accelerator FEL (HIBAF) Facility. Normalized emittance values of less than 50 {pi} mm-mrad for charges ranging from 0.7 to 8.7 nC were obtained for single micropulses at a y-waist and at an energy of 14.7 MeV. These measurements were part of the commissioning campaign on the HIBAF photoelectric injector. Macropulse measurements have also been performed and are compared with PARMELA simulations. 5 refs., 8 figs., 3 tabs.

Rf power sources for linear colliders

Abstract: The next generation of linear colliders requires peak power sources of over 200 MW per meter at frequencies above 10 GHz at pulse widths of less than 100 nsec. Several power sources are under active development, including a conventional klystron with rf pulse compression, a relativistic klystron (RK) and a crossed-field amplifier. Power from one of these has energized a 0.5 meter two- section High Gradient Accelerator (HGA) and accelerated a beam at over 80 MeV meter. Results of tests with these experimental devices are presented here.

Beam test of the SLD silicon-tungsten luminosity monitor

Abstract: A set of calorimeters using silicon diode sampling has been constructed to provide the small angle electromagnetic calorimetry for SLD (Stanford Linear Collider Large Detector) at SLAC (Stanford Linear Accelerator Center). The luminosity monitor and small angle tagger (LMSAT) and the medium angle silicon calorimeter (MASC) measure the direction and energy of electrons and photons in the region of 23-190 mrd from the SLD beam axis. The calorimeter uses nearly two square meters of silicon diodes segmented into cells approximately one square centimeter in area. One LMSAT module has been tested with electrons at the Brookhaven National Laboratory. The stack consists of 23 layers of silicon diodes separated by about one-radiation-length (3.5-mm) layers of tungsten alloy. The results of this beam test, including energy and spatial resolution, are presented. >

INEX simulations of the Boeing FEL system

Abstract: The INEX (integrated numerical experiment) numerical model is applied to the 0.6 μm FEL oscillator at the Boeing laboratory in Seattle, WA. This sytem consists of a 110 MeV L-band rf linac, a beam transport line from the accelerator to the entrance of the wiggler, the 5.0 m THUNDER variable-taper wiggler, and a near-concentric two-mirror optical oscillator. Many aspects of the model for the electron beam accelerator and transport line agree with experimental measurements. Predictions for lasing performance are compared with data obtained in May and June 1989, using a mild-tapered wiggler. We obtain good agreement with the achieved extraction efficiency, while ID pulse simulations reproduce the observed sideband instability.

Energy sweep compensation of induction accelerators

Abstract: The ETA-II linear induction accelerator (LIA) is designed to drive a microwave free electron laser (FEL). Beam energy sweep must be limited to {plus minus}1% for 50 ns to limit beam corkscrew motion and ensure high power FEL output over the full duration of the beam flattop. To achieve this energy sweep requirement, we have implemented a pulse distribution system and are planning implementation of a tapered pulse forming line (PFL) in the pulse generators driving acceleration gaps. The pulse distribution system assures proper phasing of the high voltage pulse to the electron beam. Additionally, cell-to-cell coupling of beam induced transients is reduced. The tapered PFL compensates for accelerator cell and loading nonlinearities. Circuit simulations show good agreement with preliminary data and predict the required energy sweep requirement can be met.

A Linear Acceleration in Space--The Beam Experiment Aboard Rocket

Abstract: On July 13, 1989 the BEAM experiment Aboard Rocket (BEAR) linear accelerator was successfully launched and operated in space. The flight demonstrated that a neutral hydrogen beam could be successfully propagated in an exoatmospheric environment. The accelerator, which was the result of an extensive collaboration between Los Alamos National Laboratory and industrial partners, was designed to produce a 10 mA (equivalent), 1 MeV neutral hydrogen beam in 50 {mu}s pulses at 5 Hz. The major components were a 30 keV H{sup {minus}} injector a 1 MeV radio frequency quadrupole, two 425 Mhz RF amplifiers, a gas cell neutralizer, beam optics, vacuum system and controls. The design was strongly constrained by the need for a lightweight rugged system that would survive the rigors of launch and operate autonomously. Following the flight the accelerator was recovered and operated again on the laboratory. 6 figs., 2 tabs.

Design of a wire imaging synchrotron radiation detector

Abstract: The design of a detector invented to measure the positions of synchrotron radiation beams for the precision energy spectrometers of the Stanford Linear Collider (SLC) is described. The energy measurements involve the determination, on a pulse-by-pulse basis, of the separation of pairs of intense beams of synchrotron photons in the megaelectronvolt energy range. The detector intercepts the beams with arrays of fine wires. The ejection of Compton recoil electrons results in charges being developed in the wires, thus enabling a determination of beam positions. >

A new trajectory correction technique for linacs

Abstract: In this paper, we describe a new trajectory correction technique for high energy linear accelerators. Current correction techniques force the beam trajectory to follow misalignments of the Beam Position Monitors. Since the particle bunch has a finite energy spread and particles with different energies are deflected differently, this causes chromatic'' dilution of the transverse beam emittance. The algorithm, which we describe in this paper, reduces the chromatic error by minimizing the energy dependence of the trajectory. To test the method we compare the effectiveness of our algorithm with a standard correction technique in simulations on a design linac for a Next Linear Collider. The simulations indicate that chromatic dilution would be debilitating in a future linear collider because of the very small beam sizes required to achieve the necessary luminosity. Thus, we feel that this technique will prove essential for future linear colliders. 3 refs., 6 figs., 2 tabs.

The measurement of channeling radiation from a linear-accelerator electron beam at high currents

Abstract: In this paper, we describe the instrumentation used in the production and detection of channeling radiation from a linear-accelerator electron beam at high currents. Channeling-radiation spectra were measured for 30 meV electrons channeled along the (100) and (110) planes of a 15 μm thick silicon crystal. Spectra were taken for photon energies from 10 to 38 keV with a 0.2 keV resolution in 5 minutes. Using an average electron current of 3 μA, over 109 channeling photons/s were incident on the spectrometer, eliminating single-pulse counting as a means of spectral measurement, since collimation to 10−11 sr would have been necessary to avoid pileup. The bremsstrahlung radiation emitted from the channeling target crystal produced a high-energy noise background. A Bragg spectrometer, using a mosaic-spread Bragg crystal, was designed to measure the intense channeling-radiation X-rays in the noisy environment. This spectrometer provides a means to measure intense channeling-radiation spectra from any achievable electron current.

Beam position measurement in the CEBAF recirculating linacs by use of pseudorandom pulse sequences

Abstract: The recirculating linear accelerator at CEBAF presents unique problems in beam position measurement. As many as five beams with different energies may be simultaneously in the linac. Modulation of the beam intensity by pseudorandom pulse sequences offers a simple, effective method for distinguishing between the individual beamlets.

New compensation of transverse wakefield effects in a linac by displacing accelerating structures

Abstract: Beams accelerated in the linac of a linear collider experience transverse wakefield effects due to small residual misalignments of the accelerating structure. These wakefields lead to emittance growth. The traditional correction method is to add induced betatron oscillations to the trajectory of the beam to counteract the effects of the unknown actual errors and, thereby, reduce the emittance enlargement. However, practical considerations make this solution operationally difficult. In this note a second correction method is proposed where the positions of the accelerating structures are remotely controlled. By adding position offsets of the RF structure at the spatial frequency of betatron oscillations, direct wakefield reduction can be made. A hardware solution suitable for the SLC is presented which does not move the quadrupoles or position monitors.

Critical system issues and modeling requirements - the problem of beam energy sweep in an electron linear induction accelerator

Abstract: In this paper we discuss system issues and modeling requirements within the context of energy sweep in an electron linear induction accelerator. When needed, particular parameter values are taken from the ETA-II linear induction accelerator at Lawrence Livermore National Laboratory. For this paper, the most important parameter is energy sweep during a pulse. It is important to have low energy sweep to satisfy the FEL resonance condition and to limit the beam corkscrew motion. It is desired to achieve {Delta}E/E = {plus minus}1% for a 50-ns flattop whereas the present level of performance is {Delta}E/E = {plus minus}1% in 10 ns. To improve this situation we will identify a number of areas in which modeling could help increase understanding and improve our ability to design linear induction accelerators.