Energy Linac Research Articles

Experiments in nonperturbative electron beam characterization with the MIT microwiggler at the accelerator test facility at BNL

We report first measurements of a single-shot method through which the properties of an electron beam at linac energies may be studied using the spontaneous emission of a micro-wiggler. The setup is simple and the measurement efficient. A simple set of scaling laws describe the natural line width, energy spread and divergence broadening of spontaneous emission in a narrow bandwidth radiation cone. A systematic series of experiments were performed with the MIT Microwiggler at the Accelerator Test Facility at BNL which demonstrated the response of the cone to changes in the beam quality. Estimates of divergence were obtained from the measurements of the radiation cone.

CP violation in heavy neutrino mediated e−e− → W−W−

We consider the reaction e − e − → W − W − mediated by possible heavy neutrino exchange at future LINAC energies of s ⪢ 2 m w . This reaction is sensitive to CP phases of the neutrino mixing matrices, even at the level of Born amplitudes. Certain integrated cross-sections are shown to have the power to resolve the CP phases when the experimental configurations are varied. Asymmetries sensitive to CP violation (involving initial QED phases) for e − e − and e + e + reactions are constructed and their consequences considered.

Oxygen desorption during niobium sputtering for superconducting RF accelerators

Superconducting niobium RF resonators are an integral part of the various moderm accelerators — from low energy LINACs for heavy ions to electron-positron colliders with energy up to hundreds of GeV. Traditionally, the resonators have been made from high purity sheet niobium, but recently this design started to give way to copper substructures onto which a thin niobium film is deposited. Resonators with sputtered niobium are less expensive and higher acceleration fields can be achieved because the high thermoconductivity of copper eliminates thermal quenches. Unfortunately, that advantage is counterbalanced by degradation of the quality factor, Q, with increasing accelerating field. It is generally believed that Q degradation is caused by non-metallic impurities embedded into the niobium film during discharge sputtering. However, neither the amount and composition nor sources of the impurities are known with any certainty. This paper examines the hypothesis that the mechanism of film contamination is the decomposition of the surface oxides on the copper substrate and the niobium cathode when they are exposed to the sputtering discharge. Quantitative connections between the impurity concentrations, the pressure of active gases during sputtering and the residual resistivity ratio of niobium are established and applied to the experimental data published by CERN. Calculations show that, with ignition of the sputtering discharge, the residual pressure of oxygen-containing gases increases by two orders of magnitude. To reduce the contamination of niobium films during sputtering, outgassing of the sputtering system in helium discharge is proposed.

Beam position correction in the Fermilab high energy linac

The Fermilab Linac has been upgraded by the replacement of four 201.25 MHz drift tube linac tanks with seven 805 MHz side-coupled cavity modules. The reduction in physical aperture of the side-coupled modules has made control of the transverse beam position a more important consideration. An algorithm has been implemented to achieve a global least-squares minimization of beam position errors. Under typical operating conditions the RMS error in the horizontal plane is reduced by a factor of 8 (from 0.54 mm to 0.07 mm) and in the vertical plane by a factor of 4 (from 0.59 mm to 0.14 mm).

Low power-high energy linacs for irradiation in polymeric systems

Electron beam applications in the chemistry of polymers are given, such as the production of acrylamide polymers which are of great interest in the treatment of waste industrial waters. These products have unique properties: the required dose is smaller by a factor of 10 to 100 as compared to the dose requirement for other polymeric materials and they are used in aqueous solutions in a concentration of one or two magnitude orders smaller than conventional anorganic flocculants. The acrylamide polymer technologies were first developed on a semi-industrial scale with γ-ray sources and then transferred to the linacs built in the Accelerator Laboratory in the Institute of Atomic Physics. An original feature was included in order to make our linear accelerators using an electron gun of the diode type to render them suitable to obtain programmed beam single shots and pulse trains. It is particularly useful for automatic control of irradiation processing in order to provide the maximum efficiency of the electron beam application. The preliminary results obtained using our electron beam sources make the assumption that the low output power linacs are economically attractive for commercial production of acrylamide type polymers: about 2000 kg per beam kWh.

The Elettra beam line control system

Elettra is a third generation Synchrotron Light Source located in Trieste (Italy). It consists of a full energy linac injector and a storage ring with beam energies between 1.5 and 2 GeV. The facility is scheduled to be operational by end 1993. For the whole project 22 beam lines from insertion devices are foreseen, each of them is composed of a large number of measurement and controls instruments, most of them embedded in intelligent devices; in addition each beam line can be considered unique compared to the others, having been designed to provide a different kind of synchrotron radiation. This results in a large not homogenous environment where more than 200,000 physical points have to be controlled. A joint team composed of Softeco Sismat and Digital Equipment has developed a fully automated beam line control system able to give full remote controls, with different kind of access rights, to beam line users and beam line specialists as well as a full integration with experiment control systems.

Dosimetric Accuracy of a Dual Photon Energy Linac at Low Monitor Setting for Various Pulse Repetition Frequencies

Accuracy of dose delivery at low monitor unit setting is studied for a dual photon energy linear accelerator. Dose delivered per MU is found to be constant for both the photon beams for MU settings above 30. For lower MUs there is definite deviation from the calibrated value and the error is found to be increasing as fewer MUs are set for dose delivery. This dose/MU ratio at low MU setting is found to be dose-rate dependent, showing an increasing trend with pulse repetition frequency (PRF). Also, the dosimetric ratio is observed to be mode dependent; its value for an 18MV beam is almost double that observed in the case of a 6MV beam at very low MU setting. The magnitude of this error should be determined for each energy so that appropriate corrections can be applied if very low MUs are to be used.

Characteristics of neutron beam generated by 500 MeV proton beam

A neutron irradiation facility was constructed at PARMS, University of Tsukuba to produce an ultrahigh energy neutron beam with a depth dose distribution superior to an x-ray beam generated by a modern linac. This neutron beam was produced from the reaction on a thick uranium target struck by a 500 MeV proton beam from the booster synchrotron of the High Energy Physics Laboratory. The percentage depth dose of this neutron beam was nearly equivalent to that of x-rays at around 20 MV and the dose rate was 15 cGy per minute. The relative biological effectiveness (RBE) of this neutron beam has been estimated using the cell inactivation effect and the HMV-I cell line. The survival curve of cells after neutron irradiation has a shoulder with n and Dq of 8 and 2.3 Gy, respectively. The RBE value at the 10 −2 survival level for the present neutron beam as compared with 137Cs gamma rays was 1.24. The results suggest that the biological effects of ultrahigh energy neutrons are not large enough to be useful, although the depth dose distribution of neutrons can be superior to that of high energy linac x-rays.

A multi-rod collimator for a high energy linac

A multi-rod collimator for a high energy linac

A beam profile monitor for small electron beams

Measurement of beam properties at the foci of high energy linacs is difficult due to the small size of the waists in proposed and existing accelerators (1 nm −2 μm). This article considers the use of bremsstrahlung radiation from thin foils to measure the size and phase space density these beams using nonimaging optics. The components of the system are described, and the ultimate resolution, evaluated theoretically for the case of the Final Focus Test Beam (FFTB) at Stanford Linear Accelerator Center, is a few nm.

Increase of emittance in high energy e+e− colliders by inverse multiple Touschek effect in single bunches

Coulomb scattering of particles in an accelerated bunch, due to the longitudinal momentum distribution, leads to the creation of a contribution to the transverse emittance which adds to the natural emittance. This process has been calculated for high energy linacs used in high energy e+e− linear colliders. At beam energies of 500 GeV to 1 TeV, it has been found that the effect is weak.

Effect of Collimator Setting on the Output of Rectangular Fields from Linear Accelerators

The output (cGy/mu) of a rectangular field from a linear accelerator is not always the same as that of its equivalent square field. We have summarized output variations with upper and lower collimator setting for 4, 6, and 24 MV X-rays. It is concluded that an error in output on the order of a few percent is introduced for elongated fields if lower set of collimator jaws is used for setting the longer dimension of the field, and computing the output using equivalent square method. It is recommended that specific guidelines be developed regarding rectangular field setting on high energy linacs.

Magnets for the German Spallation Source SNQ

For the SNQ high energy linac 320 quadrupoles with a gradient of up to 6 T/m and a bore of 80 mm will be used. The design and mechanical layout of a cheap prototype are presented. The coils with a low resistivity and a high filling factor are produced by simply sawing from a forged hollow copper block. Due to radiation exposure no organic materials are used and insulation is established by spacers and distance. Design criteria and layout of the beam line and the tolerances are discussed.

On the Performance of Computer Automatic Tuning at the Unilac

The Unilac is a multi-particle, variable energy linac. Automatic tuning procedures have been used in routine operation for many years. The algorithms were developed for transverse beam optimization at the linac and the transport lines in the experimental area. A successful application demands a thorough understanding of the machine, which results in a good computer modeling. It will be shown that linear treatment of the beam dynamics is normally sufficient, limitations for special sections of the accelerator will be discussed. Complementary, limitations in hardware, e.g. misalignment of components, tolerances in magnetic strengths and in beam diagnostics, affect the effectiveness of the procedures.

Advances in linear accelerator design for radiotherapy.

The microwave-powered electron linear accelerator, or linac, is becoming the dominant radiotherapy treatment unit. Several technical advances, combined with attention to how patients are most effectively set up and treated, have led to continuing improvements in linac radiotherapy. This review describes: improvements in accelerator structures, widely variable energy linacs , microtrons , beam transport systems, and treatment head design.

Improving the buildup and depth-dose characteristics of high energy photon beams by using electron filters.

The attributes of high energy photon beams, i.e., low surface dose, large dmax and improved %DD, are compromised with increase in field size. This is due to the relative increase with field size of the electron component in the beam, as shown by recent experiments done here using a sweeeping magnet. The present study shows that the advantages can be partially regained with the use of foils to remove electrons. Various thicknesses of Al, Cu, Sn, and Pb were placed in a 25-MV linac x-ray beam at several sites in the treatment head. Buildup curves were measured with a "pancake" chamber for various SSD and field sizes. The magnitude of improvement achieved is dependent upon field size, SSD, the atomic number of the foil material, and foil thickness. Pb foil (0.55 gcm-2) provided the best overall improvement. Surface dose reduction of 10%-20% can be achieved along with significant increase in dmax and %DD. These findings suggest a new design feature for the next generation of high energy linacs.

High Power, Low Energy Linac for Radiation Processing

An efficient low-energy electron linac which could produce low cost radiation for large volume bulk processing is described. In particular, the energy in a beam of 12 MeV electrons impinging on a thin-walled wastewater duct is used effectively to disinfect the water without the harmful side effects of chlorination. The main feature of the design is that about one half of the electrical power consumed is delivered as electron beam power. A standing-wave electron linac, operated with 100% duty factor, based on the proven performance of the Chalk River Electron Test Accelerator and producing 600 kW of electron beam power at 12 MeV would deliver 105 rads (1000 grays) to 3.8 × 107 l/day) (107 U.S. gallons/day). A water processing linac would consist of an electron gun, a buncher, a graded-s accelerator and several s=1 modules. Redundancy in accelerator and rf modules would be provided for reliability and to allow servicing of pre-accelerator and rf modules concurrently with accelerator operation.