High Current Proton Linac Research Articles

Development of a radio frequency quadrupole linac implemented with the equipartitioning beam dynamics scheme

Radio frequency quadrupole linac (RFQ) is the key component which realized modern high-current proton linacs, however, many RFQs are designed based on very conventional design schemes. We developed 3-MeV 50-mA ${\mathrm{H}}^{\ensuremath{-}}$ (negative hydrogen) RFQ based on a beam space-charge physics concept. The equipartitioning scheme, which is widely used in the high intensity linac design, is implemented into the RFQ design. Design performances of 99.1% transmission, $0.24\ensuremath{\pi}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ transverse normalized rms emittance, and $0.11\ensuremath{\pi}\text{ }\text{ }\mathrm{MeV}\text{ }\mathrm{deg}$ for longitudinal direction are achieved with a vane length of 3 m and the final energy of 3 MeV. An RFQ employing this design concept was fabricated, and the design performance was confirmed by beam commissioning. In this paper, the design and result of the beam commissioning of this fully equipartitioned RFQ are described.

Design of a low-energy proton facility for space radiation effect research based on a compact neutron source

Abstract This paper provides the physical design of a low-energy proton irradiation facility for space radiation effect research, based on an existing linear accelerator (linac)-based compact neutron source . The beam requirement of the low-energy proton irradiation consists of a proton flux range of 1 0 8 – 1 0 9 p ∕ cm 2 ∕ s with a nonuniformity of less than 10% in the target diameter area of 2 cm, and optional proton energies of 1 MeV to 13 MeV. Based on the existing high-current proton linac, the flux range and nonuniformity of the proton beam at the target can be achieved by inserting beam apertures, adjusting quadrupole fields to expand the beam size, and decreasing the repetition rate of the beam pulse. To measure the energy distribution of the protons, one CdZnTe detector can be employed. The above approach is performed on the physical design of the low-energy proton facility for the Compact Pulsed Hadron Source at Tsinghua University. The proton flux at the target can be reduced from 6 . 2 × 1 0 14 p ∕ cm 2 ∕ s to 1 . 2 × 1 0 8 − 3 . 0 × 1 0 9 p ∕ cm 2 ∕ s , and proton energies of 0.56, 5.0, 8.2, and 10.5 MeV can be obtained separately with the energy degrader (aluminum) thicknesses of 75, 800, 550, and 300 μ m. Irradiation nonuniformity is less than 10%. Energy spread is less than 10% for the proton energy of 3.0, 5.0, 8.2, and 10.5 MeV, except for the low energy of 0.56 MeV.

Radio frequency measurement and tuning of a 13 MeV Alvarez-type drift tube linac for a compact pulsed hadron source.

This paper describes the radio frequency (RF) measurement and tuning result of a 13 MeV Alvarez-type drift tube linac (DTL) for a compact pulsed hadron source (CPHS) at Tsinghua University. The design, machining, assembly, and alignment of the DTL are presented for integrity. The CPHS project consists of a high-current proton linac (13 MeV, 16 kW, peak current of 50 mA, 0.5 ms pulse width at 50 Hz), a neutron target station, a small-angle neutron scattering instrument, and a neutron imaging/radiology station. The linac contains an electron cyclotron resonance ion source, a low energy beam transport line, a four-vane radio frequency quadrupole (RFQ) accelerator, an Alvarez-type DTL, a high energy beam transport line, and a RF power supply and distributor. Construction on the CPHS started in June 2009, and the CPHS has provided 2000 h since 2013 to users with the neutrons produced by the 3 MeV proton beam from the radio frequency quadrupole bombarding on the beryllium target as an achievement of its mid-term objective. Presently, the tuning of the assembled DTL cavity has been completed successfully. The 4.3-m-long DTL consists of 40 accelerating cells, among which 39 full-length drift tubes (DTs) are suspended inside the cavity, and two half-length DTs are mounted inside the two end flanges of the cavity. Each DT contains a permanent magnet quadrupole. Thirteen post couplers and nine tuners are available for the tuning of the field. The relative error of the field after tuning is within ±1.6%, with a tilt sensitivity within ±33%/MHz in all cells. The beam energy will reach its designed value of 13 MeV after the DTL is installed in the beam line downstream the 3 MeV RFQ accelerator.

Design of a 10 MeV normal conducting CW proton linac based on equidistant multi-gap CH cavities**Supported by National Natural Science Foundation of China (11375122, 91126003)

Continuous wave (CW) high current proton linacs have wide applications as the front end of high power proton machines. The low energy part of such a linac is the most difficult and there is currently no widely accepted solution. Based on the analysis of the focusing properties of the CW low energy proton linac, a 10 MeV low energy normal conducting proton linac based on equidistant seven-gap Cross-bar H-type (CH) cavities is proposed. The linac is composed of ten 7-gap CH cavities and the transverse focusing is maintained by quadrupole doublets located between the cavities. The total length of the linac is less than 6 meters and the average acceleration gradient is about 1.2 MeV/m. The electromagnetic properties of the cavities are investigated by Microwave Studio. At the nominal acceleration gradient the maximum surface electric field in the cavities is less than 1.3 times the Kilpatrick limit, and the Ohmic loss of each cavity is less than 35 kW. Multi-particle beam dynamics simulations are performed with Tracewin code, and the results show that the beam dynamics of the linac are quite stable, the linac has the capability to accelerate up to 30 mA beam with acceptable dynamics behavior.

Superconducting Solenoid Lens for a High Energy Part of a Proton Linac Front End

Development of a high-current proton linac at FNAL went through many stages, starting from a pulsed 8 GeV linac, then to the HINS linac front end R&D, and now toward the ProjectX CW linac. For different parts of the accelerator front end in each of these linacs, the design requires solenoid-based focusing lenses that can provide the needed transverse focusing on a very tight real estate environment. The multiple, often contradictory, design requirements of focusing lenses include the need for high focusing strength, small footprint, very low fringe field, and embedded steering coils. To meet these requirements, a series of prototype lenses were built and tested. Performances of the lenses designed for low energy parts of the linac front end have been reported earlier. This report presents lens design and test data for the high energy part of a proton linac front end, up to an energy of ~ 100 MeV. For these lenses, reliable protection from high voltages or temperatures during a quench becomes important, and a new protection scheme was developed, which allows more flexibility and reliability. Details of the magnetic axis position have also been investigated.

Beam simulations with initial bunch noise in superconducting rf proton linacs

Circular machines are plagued by coupled bunch instabilities (CBI), driven by impedance peaks, where then all cavity higher order modes (HOMs) are possible drivers. Limiting the CBI growth rate is the fundamental reason that all superconducting rf cavities in circular machines are equipped with HOM dampers. The question arises if for similar reasons HOM damping would not be imperative also in high current superconducting rf proton linacs. Therefore we have simulated the longitudinal bunched beam dynamics in such machines, also including charge and position noise on the injected bunches. Simulations were executed for a generic linac with properties close to the planned SPL at CERN, SNS, or Project X at FNAL. It was found that with strong bunch noise and monopole HOMs with high ${Q}_{\mathrm{ext}}$ large beam scatter, possibly exceeding the admittance of a receiving machine, cannot be excluded. A transverse simulation shows similar requirements. Therefore including initial bunch noise in any beam dynamic study on superconducting rf cavities in high current proton linacs is advisable before envisaging to omit all HOM dampers.

Design and simulation of a beam position monitor for the high current proton linac

In this paper, the 2-D electrostatic field software, POISSON, is used to calculate the characteristic impedance of a BPM (beam position monitor) for a high current proton linac. Furthermore, the time-domain 3-D module of MAFIA with a beam microbunch at a varying offset from the axis is used to compute the induced voltage on the electrodes as a function of time. Finally, the effect of low β beams on the induced voltage, the sensitivity and the signal dynamic range of the BPM are discussed.

Development of an all-permanent-magnet microwave ion source equipped with multicusp magnetic fields for high current proton beam production

An all-permanent-magnet (APM) microwave hydrogen ion source was developed to reduce the size and to simplify structure of a conventional solenoid coil microwave ion source developed for reliability improvement of high current proton linac application systems. The difficulty in developing the APM source was sensitive dependence of the source performance on axial magnetic field in the microwave discharge chamber. It was difficult to produce high current proton beam stably without precise tuning of the magnetic field using solenoid coils. We lowered the sensitivity using multicusp magnetic fields for plasma confinement at the discharge chamber sidewall of the source. This enabled stable high current proton beam production with the APM microwave ion source with no tuning coil. The water cooling and the power supply for the coils are not necessary for the APM source, which leads to better reliability and system simplification. The outer diameter of the APM source was around 300 mm, which was 20% lower than the coil source. The APM source produced a maximum hydrogen ion beam current of 65 mA (high current density of 330 mA/cm(2), proton ratio of 87%, and beam energy of 30 keV) with a 5 mm diameter extraction aperture, pulse width of 400 micros, and 20 Hz repetition rate at 1.3 kW microwave power. This performance is almost the same as the best performances of the conventional coil sources. The extracted ion beams were focused with electrostatic five-grid lens to match beam to acceptance of radio-frequency quadrupole linacs. The maximum focused beam current through the orifice (5 mm radius) and the lens was 36 mA and the 90% focused beam half-width was 1-2 mm.

Controlling Beam Halo-Chaos by Adaptive Control Exterior Magnetic Field**The project supported by National Natural Science Foundation of China under Grant Nos. 10247005 and 70071047, and the Natural Science Foundation of Guangxi Province of China under Grant No. 0135063

In this paper, the parametric adaptive method for controlling the beam halo-chaos in the periodic focusing channels of high-current proton linacs is presented. The study of proton beam halo-chaos based on controlled beam envelope equation and the results of particles-in-cell simulations for macro-particle beam show that the proton beam chaotic envelope as well as the beam rsm radius can be controlled to the beam matched radius using this method. For the Kapchinskij-Vladimirskij (K-V) distribution of initial proton beam, all statistical physical quantities of the beam halo-chaos are largely reduced. This control method has an advantage of the control halo-chaos since the exterior magnetic field as controlled parameter can be rather easily adjusted in the periodical magnetic focusing channels for the experiment.

Development of RF linac for high-current applications

High-current proton linacs are promising sources of neutrons for material processing and research applications. Recently, a linac design that makes use of a combination of normal-conducting (NC) and superconducting (SC) linac technologies has been proposed for the US Accelerator Production of Tritium (APT) Project. As a result, a multi-year engineering development and demonstration (ED&D) program is underway. In this paper, we will describe the design and merits of the NC/SC hybrid approach. The scope, technology issues, and present status of the ED&D Program, and the participation of industry will also be described.

Beam Dynamics in a High Current Proton Linac

The High Energy part of the SNQ-LinAc (HELA) is designed to accelerate protons from 102 MeV to 1100 MeV with peak current of 200 mA and average current of 5 mA. We show here that HELA has good beam dynamical properties. The design provides: i) good safety margines (small emittance growth), ii) flexibility (the final energy can be varied nearly instantaneously and operation with two or more different energies in parallel is possible), iii) satisfactory operation even if one accelerating cavity fails and iiii) tolerances of the effective gap voltage and phase of the cavities that are technically feasible.

Design, Construction, and Testing of RF Structures for a Proton Linear Accelerator

The factors influencing the choice of a RF accelerating structure for acceleration of protons to high energies in a high-current proton linac are reviewed. Characteristics of resonant periodic cavity chains are described, and the properties of two modes of operation, the π-mode and the π/2-mode, are discussed. The cloverleaf cavity chain, suitable for use in the π-mode, and four possible configurations suitable for use in the π/2-mode are described. A new accelerator coupling scheme usable in π/2-mode tank operation is also discussed. Studies of possible fabrication techniques, and the results of a fabricational program utilizing machined forgings to produce a cloverleaf chain are described. Sparking and electron acceleration tests are also briefly mentioned.

Resonantly Coupled Accelerating Structures for High-Current Proton Linacs

The results of model studies and an equivalent circuit analysis of the operation of a chain of cavities in the /g=P//2 mode is given. Various /g=p//2 mode structures are described and the performance of models of these structures is discussed. Results of numerical solutions to an equivalent circuit applicable to the problem are also given, which include frequency error tolerances and coupling tolerances, for typical cases of interest. Multiple drive of a long resonator chain is also discussed.