DC Accelerators Research Articles

Shielding Effectiveness of HV-HF Transformer for Accelerator Applications

High-voltage high-frequency transformers are employed in DC accelerators. The high frequency produced by the inverter at lower voltage is stepped up to several tens of kilovolts using step-up transformers. These transformers are subjected to high-frequency transients from high-voltage multiplier columns. Electrostatic shielding is very effective in common mode noise reduction. However, in accelerator applications, shielding effectiveness obtained using conductive shields is found compromised in practice. This paper investigates the reasons behind this phenomenon and explores remedial measures.

Characterising the Pelletron beam at the University of Melbourne

The Pelletron at the University of Melbourne is a DC particle accelerator that has been used for materials analysis and ion implantation since its installation in the 1970s. Today, it is primarily used to produce proton and helium beams up to 3.5 MeV and 1.5 MeV respectively, for three beamlines with a range of different experiments. The beam stability and fundamental beam parameters such as the transverse phase space distribution and emittance have not been previously investigated in detail. We have performed systematic measurements to determine the beam current variation, finding persistent fluctuations: our study determined that internal wire scanners used for monitoring the beam profile contribute to a periodic current drop. We also found that large terminal voltage oscillations reduce the average current. In addition, the phase space has been measured using a custom-built slit-grid apparatus. We found that the Courant–Snyder (Twiss) parameters are a function of the operational characteristics of the Pelletron, with the emittance more than doubling between measurements. To parametrise the beam despite the fluctuations, we introduce a ‘minimum bounding ellipse’ described by a set of Courant–Snyder parameters that is representative of the beam characteristics for all the measurements: for our data, this has β= 8.2(5)m, α=-5.2(3), and an emittance of 0.34(3)mmmrad. These parameters will enable future beamline design in Melbourne, and give an indication for the requirements at other Pelletron facilities. Our characterisation techniques can provide a low-cost method to probe beam parameters for improvements in the operation and efficiency of similar DC accelerators, allowing for more accurate measurements and reduced data acquisition times.

Direct conversion of methane to heavier gaseous alkanes using an electron beam

The circulation treatment of methane with 0.5 MeV electron beam in a cyclone-type reactor, which protects the synthesized products from back degradation, has been studied. As a result of the conversion, the content of methane in the circulating gas can be reduced to 30–43 wt%. The main products are C2C5 alkanes, and the ethane content in the mixture can reach almost 40 wt%. By-products are hydrogen and C6C13 high-octane gasoline alkanes. Electron beam treatment can also be used to concentrate helium in natural hydrocarbon mixtures. The key control factors for conversion are the composition of the feedstock, the rate of beam energy deposition in the gas, temperature, and the residence time of the gas components in the beam penetration area. The features of the use of powerful technological DC accelerators are noted.

Design and simulation of a 1 MeV, 100 mA parallel-fed capacitive coupled cascade industrial accelerator

The construction of indigenous parallel-fed capacitive coupled cascade accelerators for industrial electron beam processing has been taken up at accelerator division of NSTRI at Iran atomic energy organization. This machine has been previously developed by Radiation Dynamics Inc. (RDI), IBA Industrial and High Voltage Engineering Europa B.V. (HVE) companies. Parallel-fed capacitive coupled cascade generators are reliable and high current sources of DC accelerators with many scientific, medical and industrial applications. We have selected this mechine because of some superiorities of that in comparison with many existing accelerators. These superiorities are as follows: compactness, simplicity, cheapness, having typically higher amount of beam current, wide range of beam energy and addressing many areas of applications. In this article, the main parts of a 1MeV, 100mA parallel-fed cascade industrial accelerator have been analyzed, designed and simulated. These parts are the electron gun, accelerating tube, voltage multiplier column (VMC) and some of the other components of the VMC. The electron gun is of a diode type with the Pierce configuration. Two electron guns with different cathode shapes have been designed and compared with each other. The intended cathode is a Dispenser type with a tungsten heater and porous tungsten matrix wich is impregnated with LaB6 and works in ambient vacuum of 10–7 Torr. The required current in our accelerator is about 100mA. For the next step, the accelerating tubes with different electrode geometries have been designed, simulated and compared with each other and the best geometry has been selected. For the VMC, we have tried to achieve a low ripple output DC voltage. The VMC has been analyzed and simulated with PSpice simulation software and optimum values of different parameters were achieved. We have tried to get the optimum values of the PSpice simulations by a mechanical design in CST Particle studio. At the end, the parameters of the mechanically designed VMC had a good accordance with the results from PSpice.

Development of Electron Guns for Linacs and DC Accelerator

Electrons guns for RF linacs and DC Accelerators are designed and developed at Electron Beam Centre (EBC)/APPD/BARC. Planar geometry grid and Pierce geometry grid configuration diode and triode guns with LaB6 cathode are developed. The cathode assembly consists of cups and heat shields made out of Tantalum and Rhenium sheets. The cathode assembly and the electron guns are tested on a test bench for beam characterization. The paper presents the development of the electron guns.

LEADS-DC: A computer code for intense dc beam nonlinear transport simulation

An intense dc beam nonlinear transport code has been developed. The code is written in Visual FORTRAN 6.6 and has ∼13000 lines. The particle distribution in the transverse cross section is uniform or Gaussian. The space charge forces are calculated by the PIC (particle in cell) scheme, and the effects of the applied fields on the particle motion are calculated with the Lie algebraic method through the third order approximation. Obviously, the solutions to the equations of particle motion are self-consistent. The results obtained from the theoretical analysis have been put in the computer code. Many optical beam elements are contained in the code. So, the code can simulate the intense dc particle motions in the beam transport lines, high voltage dc accelerators and ion implanters.

Conceptual design of a helicon ion source for high-current dc accelerators

Characteristics of helicon plasmas are examined as a candidate for accelerator ion sources. Helicon plasmas have much higher density with very high power efficiency in a relatively lower magnetic field than electron cyclotron resonance plasmas do. High electron density and high electron temperature are beneficial to extract high ion current density. Low magnetic field favors for low-emittance accelerators. Taking advantage of placing antenna outside of the plasma chamber, source contamination can be kept low enough to maintain a stable operation for longer periods. Based on these characteristics, new designs of high current ion sources using helicon waves are proposed for accelerator applications for the first time.

Conceptual design of a helicon ion source for high-current dc accelerators

Characteristics of helicon plasmas are examined as a candidate for accelerator ion sources. Helicon plasmas have much higher density with very high power efficiency in a relatively lower magnetic field than electron cyclotron resonance plasmas do. High electron density and high electron temperature are beneficial to extract high ion current density. Low magnetic field favors for low-emittance accelerators. Taking advantage of placing antenna outside of the plasma chamber, source contamination can be kept low enough to maintain a stable operation for longer periods. Based on these characteristics, new designs of high current ion sources using helicon waves are proposed for accelerator applications for the first time.

Application of the pulse-shape technique to proton-alpha discrimination in Si-detector arrays

The capability of the pulse-shape technique with reversed n-type Si detectors for discrimination of protons and alphas produced in fusion-evaporation reactions was tested at the VICKSI cyclotron in Berlin. We applied a zero-crossing technique which does not need any external time reference, and which can therefore be exploited at DC accelerators. Excellent proton-alpha discrimination in the full energy range of the evaporation spectra, but also charge and even isotope resolution for heavier ions produced in projectile fragmentation, was obtained with detectors of an existing Si ball. There is no doubt that the pulse-shape discrimination works well with detectors from serial production and under experimental conditions which are typical for nuclear structure studies. An application of this technique in Si detector arrays is obvious, but some special features must be considered in the design of the electronics. The particle discrimination depends strongly on the electric field distribution inside the detector. Stabilization of the bias voltage at the detector is therefore recommended. A consequence of the rear-side injection mode is a strong variation of the charge-collection time with energy, charge, and mass number of the detected ion. To obtain a precise energy signal it is indispensable to correct for the ballistic deficit.

A high current tandem accelerator

We highlight in this paper the main features of beam transport and losses which hitherto have limited MeV dc accelerators to low beam current. High cur

A model to describe the bremsstrahlung spectrum produced by a positive-ion Van de Graaff accelerator

A model that simulates the measured X-radiation spectra produced by low-energy positive-ion dc accelerators has been developed as a linear superpositio

Neutral beam injectors for the International Thermonuclear Experimental Reactor

The International Thermonuclear Experimental Reactor (ITER) has been proposed as the next major step in the development of fusion power [International Thermonuclear Experimental Reactor (ITER), Establishment of ITER: Relevant Documents, International Atomic Energy Agency, Vienna (1988)]. During the Conceptual Design Activity, deuterium neutral beams were chosen for heating, driving current, and controlling the current profile in the central region of the ITER plasma. In order to penetrate to the center of the ITER plasma, an energy of 1.3 MeV is required, an order of magnitude higher than in existing neutral beam systems. The neutral beam system must deliver 75 MW of D0 to the plasma, with a pulse length that ultimately will reach two weeks. The neutral injection system will consist of nine modules, and will be based on D− ion sources and novel high-current dc accelerators optimized for flexible operation and control of breakdowns. The negative ions can be converted to neutrals in gas or plasma targets, with a resulting system efficiency of 40% to 50%. The successful development and operation of neutral beam injectors for ITER will be a major step toward the application of neutral beams on power reactors.

High power pulsed accelerators with potential industrial applications

Radiation processing sources fall into two categories, electron beams and nuclear decay products (gamma rays). Applications requiring high average power ( > 100 kW), high dose rate, or limited deposition depth are performed with electrons. Typically, the electron sources that are used for radiation processing are either DC accelerators or linacs with peak powers in the 50 to 200 kW regime. Recent advances have made the development of pulsed electron sources in this parameter range attractive as well. Pulsed sources operate with space charge limited “cold cathode” emission on the microsecond or submicrosecond time scale. As a result, the dose rates are much higher than currently available. This paper will describe a design for a 10 MV, 100 kW or greater accelerator for large area electron beam irradiation or for bremsstrahlung conversion. The design is based on pulse transformers to generate megavolt, microsecond pulses, saturable magnetic core switching to compress to shorter pulse lengths, pulse-forming line adder techniques to transform to 10 MV, and a magnetically insulated cold cathode diode for electron beam formation. The status of these various technologies is reviewed.

Optimum sample utilization in secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is capable of detecting all elements, most of them with high sensitivity. Since the sample volume is eroded during SIMS analysis, particular attention must be paid to the optimum utilization of the consumed sample material. Post-ionization of the sputtered neutrals by different methods has had only limited success; there are basic problems in post-ionizing the neutrals and then effectively collecting the ions for mass spectrometric analysis. The better alternative is to maximize the ionized fraction of the sputtered particles by a suitable choice of the sputtering conditions. The next step is then effective collection and transmission of the secondary ions through the mass analyzer. Various types of mass analyzer are suitable for SIMS: quadrupoe mass filters, sector field mass spectrometers, dc accelerators, time-of-flight mass spectrometers, and sector field mass spectrographs. The choice depends on the analytical requirements such as trace sensitivity, spatial resolution, mass-resolving power, number of elements of interest and duration of analysis. For optimum performance regarding these parameters a large double-focusing mass spectrograph with on-line simulataneous multi-element detection is required.

Secondary Ion Mass Spectrometry at Close to Single-Atom Concentration Using DC Accelerators

Experiments have been conducted which show that secondary ion mass spectrometry (SIMS), together with the addition of a rather low voltage (2 MV) DC accelerator, can become an extremely powerful new technique in many fields of research related to the characterization of surface and bulk solids and with high spatial resolution. The technology is available today to build an instrument that would have high lateral resolution (∼ 1 μm) coupled with a secondary ion transmission coefficient from target to detector >10% with no mass interferences from molecular ions. Molecular ions can be fragmented and analyzed separately. We believe that this combination of secondary ion mass spectrometers and accelerators represents a most significant advance in analytical techniques.

Secondary ion mass spectrometry at close to single‐atom concentration using DC accelerators

AbstractExperiments have been conducted which show that secondary ion mass spectrometry (SIMS), together with the addition of a rather low voltage (2 MV) DC accelerator, can become an extremely powerful new technique in many fields of research related to the characterization of surface and bulk solids and with high spatial resolution. The technology is available today to build an instrument that would have high lateral resolution (∼ 1 μm) coupled with a secondary ion transmission coefficient from target to detector >10% with no mass interferences from molecular ions. Molecular ions can be fragmented and analyzed separately. We believe that this combination of secondary ion mass spectrometers and accelerators represents a most significant advance in analytical techniques.

Transients in Large Tandem Accelerators

The stored energy in dc accelerators capable of reaching terminal potentials of 30 or 40 MV is more than an order of magnitude greater than in existing machines. To analyze the concentration of electrical fields under surge conditions the accelerator has been modeled by lumped constant networks. The analysis shows that in structures similar to the present ones the voltage distribution during a surge is very uneven and micro-discharges in the tube can initiate complete accelerator collapse.

Recent Advances in DC Accelerators

Recent Advances in DC Accelerators

Use of an Isochronous Cyclotron for Neutron Tie-of-Flight Experimentation

The design of the U. S. Naval Radiological Defense Laboratory's (USNRDL) cyclotron for the production of subnanosecond, high intensity, charged particle beam pulses for neutron time-of-flight experiments, is described. Klystron bunching of the extracted cyclotron beam is possible for short bunching path lengths between cyclotron and target but it demands a beam of large energy spread -- greater than 1%. A method of linearizing the effect of the sinusoidally varying, accelerating potential is described, and it is shown that klystron beam bunching is possible with reduced beam energy spread. It is seen that the cyclotron competes with single stage, dc accelerators for neutron time-of-flight experimentation and will furnish large beam currents at higher energies in excess of the beam currents contemplated for the dc tandem machines.

Energy Doubling in dc Accelerators

It is generally believed that charged particles cannot be accelerated from ground potential to ground potential unless they pass through a system which has associated iwth it a time varying magnetic field. D.C. electric fields must satisfy the equation {contour_integral} Eds = 0, while the time varying fields used in radio-frequency accelerators and betatrons are freed from this restriction of scalar potential theory. In 1932, AJ Dempster produced protons with an energy of 45 Kev, by passing them from an electrode at +22.5 kv dc to ground. The protons were first accelerated to ground potential, with an energy gain of 22.5 kev. A small fraction of the protons then picked up an electron from a residual gas molecule, and ''coasted'' to a second electrode at +22.5 kv. Then a small fraction of these neutral hydrogen atoms lost their electrons, and were accelerated to ground with a second gain in energy equal to 22.5 kev. An accelerator of this type is obviously impractical for several reasons. The probability of neutralizing a proton varies inversely with a high power of the particle velocity, so the scheme would not work at energies of interest to nuclear physicists. Even at the low energies wheremore » neutralization is not negligible, the energy spread of the beam would be wide because charge exchange could take place at all points along the beam trajectory.« less