RFQ Accelerators Research Articles

Realizing long radio-frequency quadrupole accelerators with multiple shorter and independent cavities

It is well known that the tuning of a long radio-frequency quadrupole accelerator is demanding. This study investigated how to realize efficient long RFQ accelerators with multiple shorter and independent cavities. From the RF point of view, the use of shorter cavities has many advantages e.g. enlarged mode separation and simplified RF tuning, but the beam matching between cavities will become an issue, especially at high current and low energy. Taking a more than 9 m long, high current machine as an example, this paper presents the design concepts and methods leading to good beam quality and high beam transmission throughout a multi-stage RFQ accelerator.

A new method to compensate the longitudinal component of the end-fields in 4-rod RFQ accelerators

In 4-rod RFQ accelerators, typically there is a non-zero on-axis longitudinal electric field component between the electrodes and the end-plates at both the beam entrance and exit. The longitudinal end-field component is caused by the violation of the quadrupole potential symmetry due to an uneven charging of the electrode overhang ends. This is a direct consequence of the design inherent geometric asymmetry of the conventional 4-rod structure. Because the unwanted end-fields will change the nominal beam properties e.g. energy, emittance and transmission, they can cause problems for the beam matching, especially to the downstream accelerating structures. In this paper, a novel method is being proposed to even out the potentials on all four electrode ends to ±|Uvane∕2|, like in the case of 4-vane RFQs. Benefitting from this new method, a full compensation of the longitudinal component of the end-fields in the studied 4-rod RFQ model has been achieved.

Performance of cryocoolers in a High Temperature Superconducting ECR ion source (HTS-ECR) and its application for the High Current Injector Programme at IUAC, New Delhi

At the Inter University Accelerator Centre, a high current injector programme (HCI) is being developed as an alternate injector to the superconducting linear accelerator (SC-LINAC). For this purpose, a high temperature superconducting ECR ion source, PKDELIS, based on Gifford McMahon cryocoolers was designed, installed and commissioned in the Low Energy Beam Transport section of the high current injector. The ion source will inject multiply charged ions having A/q ∼ 6 for further acceleration into the downstream RFQ and DTL accelerators before final injection into the superconducting linear accelerator. The details of the design, and experimental results of the ion source together with performance of the cryocoolers are presented in this paper.

ON THE BEAM DYNAMICS OPTIMIZATION PROBLEM

The problem of dynamics optimization for charged particle beams in RFQ accelerators is discussed in this article. The problems of the optimization of particle capture into the acceleration mode are analyzed. Accelerators with low injection energies are considered.

Particle-in-cell method for numerical simulation of beam and plasma dynamics

The finite particle methods are widely applied for the numerical simulation of a continuous medium including continuum electrodynamics and simulation of charged particle beams. This method allows studying the detailed beam and plasma characteristics, the distribution function of particles, the nonlinear self and external fields. The so-called “particle-in-cell” method is the most effective for simulations in the physics of plasma and charged particle beams. Two examples illustrate the application of the particle-in-cell method in beam dynamics and plasma physics. The first is a program library for numerical simulation and optimization of multi-component ion beam transportation from ion sources where RFQ accelerators are presented. The latest version of the library was applied for simulations of transverse dynamics of an argon ion beam in the beam line from the 18 GHz ECR Ion Source to the linear RFQ accelerator of the RIKEN Beam Factory. A partial neutralization of ion beam charge due to secondary electrons was assumed to realize the real experimental conditions in the beam line and was used in simulation. The second is a summary of recent development of physical and mathematical basements and the first version of a computer code library aimed for the three-dimensional (3D) simulation of the ECR plasma and ion production in the ECR ion source.

Simulation and optimization of ion beam lines from ion sources

A program library based on the macro particle method for numerical simulation and optimization of multi-component ion beam transportation from ion sources to RFQ accelerators is presented. The latest version of library was applied for simulations of transverse dynamics of an argon ion beam in the beam line from the 18 GHz ECR Ion Source to the linear RFQ accelerator of the RIKEN Beam Factory.

Investigation of 26 MHz RFQ accelerators

Integral Split Ring (ISR) RFQ accelerators with four mini-vanes have been developed at Peking University. A 26 MHz RFQ prototype with cavity diameter of 50 cm was constructed. N +, O + and O − ions have been accelerated to 300 keV at inter-vane rf voltage of 80 kV. It showed that the RFQ suits to accelerate heavy ions at low rf frequency stably and effectively. Now a 26 MHz ISR RFQ has been designed and is being constructed, which cavity diameter and length of 75 and 260 cm, respectively. O + and O − ions will be accelerated to 1 MeV at rf voltage of 70 kV. All the results and discussions are presented in this paper.

Design of low frequency RFQ accelerators for ion implantation

The development of the 4-Rod RFQ as an injector for low charged heavy ions has demonstrated that structures with spiral-shaped or straight stems can be made highly efficient, very stable, and compact. The design of the RFQ structures and results of optimizations with respect to impedance and field quality will be discussed.

Injection of high brightness H − beams into RFQ accelerators

The state of the art in low-energy beam transport (LEBT) of high brightness H − beams from the source to the RFQ accelerator using gas focusing and solenoid or quadrupole magnets is reviewed. Proposed advanced LEBT schemes with electric focusing elements presently under investigation avoid charge neutralization and hence the beam loss and emittance degradation inherent in gas focusing systems. Results of conceptual design studies for LEBT systems with electrostatic quadrupoles (ESQ) and einzel lenses are discussed.

Operating principle of the plasma beam ion source

The plasma beam ion source is a high current ion source of the plasmatron family. It was originally designed for the injection of low energy proton beams into RFQ accelerators. The source consists of a duoplasmatron and one or two adjacent chambers, where the plasma from the primary DP-discharge is guided and compressed by an inhomogeneous solenoidal field. A high density plasma column of 200 mm length and 1–4 mm diameter is formed. In this second stage the plasma can be manipulated with a great degree of independence from the primary discharge, thus allowing, for instance, the adjustment of the electron energy to enhance a certain ionization process (e.g., a proton fraction >90% can be obtained). With argon as an auxiliary gas in the primary discharge the production of oxygen ions in the second stage is possible. By the insertion of a probe into the fringe of the plasma beam and the subsequent evaporation, ions from various solids including carbon, iron, copper, or tungsten have been produced.

Optimising a High-Current Injector to Incorporate a Fast Beam Kicker

In addition to the requirement that the beam current limit imposed by space charge forces be as high as possible, provision should also be made in the injector for fast switching of the high-power beam. The intolerableness of beam losses in a high-current linear accelerator, such as proposed for the German spallation neutron source (SNQ) project, requires that the beam switching occurs cleanly between the beam bunches. The method of accomplishing this technically demanding beam switching is limited by the constraints on the injector chain such as the final energy and operating frequency. These boundary conditions, plus the choice of ion source voltage also come to bear on the current limit in the injector. In the SNQ the injector is asked to supply 200 mA of protons at 200 MHz at an energy of 2 MeV, while the ion source voltage has been chosen to be 50 keV. In order to satisfy all the constraints on the injector and at the same time deliver the maximum current, a frequency jump becomes necessary. The requirements for rf power efficiency in the high-energy linac also demand that all rf buckets be filled. A funneling scheme has been proposed for the SNQ wheremore » this can be achieved by doubling the current and bunch frequency from a pair of injectors operating at half the frequency. A new technique is also put forward in this paper for debunching a beam at an intermediate energy between two RFQ accelerators allowing an arbitrarily large frequency transition to be made between the two RFQ's and raising the current limit even further.« less