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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.

A new method of measuring the integrated gradient of quadrupoles using a Hall probe is described. The proposed method is developed to measure a Halbach-type permanent magnet quadrupole (PMQ), whose roll angle is difficult to determine for alignment in the measurement. In the proposed method, we establish a roll angle alignment for the magnet, Hall probe, and motor stage based on the field distribution. The proposed method is analyzed and experimentally verified, and a setup is built. Measurement results have confirmed its feasibility for measuring the integrated gradient of PMQs with favorable reproducibility. This method is also an optional method for measuring the integrated gradient of electromagnets.

A modified Kombinierte Null Grad Struktur (KONUS) beam dynamics for inter-digital H-mode (IH) drift tube linac (DTL) is proposed in this study. This modified KONUS beam dynamics, which is aperiodic and is suitable for a short ( injector of a synchrotron-based proton therapy system, is divided into three sections, namely, rebunching ( − 80°), 0°acceleration, and debunching (10°). It is slightly different from the conventional KONUS cavity, and no focusing magnets exist in the cavity to reduce the cost of fabrication. The presence of 21 gaps in the cavity can accelerate a 15 mA proton beam from 3 MeV to 7 MeV. The cavity is tapered to adjust the gap voltage distribution, and the inner diameter varies from 196.8 mm to 232.6 mm. A cavity length of 1 m has a theoretical power consumption of 151 kW, whereas the maximum surface electric field is 40 MV/m (2.3 Kp). A new design process, which is similar to alternating-phase focusing method, is proposed. Details of the design method and results are presented in this study.

Bunch length and momentum spread are two crucial parameters in proton and ion linacs. A non-destructive model-independent rms bunch length and momentum spread measurement method that uses button BPMs is presented in this paper. By using the exponential relation between BPM signal amplitude and rms bunch length, as well as the linear relation between rms bunch length and drift time, rms bunch length and rms momentum spread are derived from several BPMs (three at least) located along the beam line. The effects of beam displacement, beam size, and BPM quantity on the measurement are analyzed. The rms bunch length and the rms momentum spread of the 3-MeV proton beam at the Compact Pulsed Hadron Source (CPHS) of Tsinghua University were successfully measured using this method. The measured rms momentum spread 0.54% is consistent with the simulation value. In addition, the resolution for the rms bunch length and the rms momentum spread measurements were ± 0.017 ns and ± 0.016%, respectively.

This paper describes the physical design of a 7 MeV proton linac injector for a synchrotron-based proton-therapy system, sponsored by the Chinese government under the support of the National Key Research and Development Program. The proton linac injector consists of an electron cyclotron resonance proton source, a low-energy beam transport section, a radio-frequency quadrupole (RFQ) accelerator, and an Alvarez-type drift tube linac (DTL). The peak current of the proton beam at the exit of the DTL accelerator is required to exceed 12 mA, with a normalized emittance of ⩽ 1 . 2 π mm mrad (90% particles), a repetition rate of 0.5 Hz, and a beam pulse width of 40–100 μ s . For the beam current whose momentum spread falls within ± 0.3%, the intensity must exceed 8 mA. The design of the linac injector is optimized in the principle of adopting sophisticated domestic technologies and cost control. Only one tetrode-based RF power amplifier is required after minimizing the total peak-power consumption of the RFQ and DTL accelerator, which is 378 kW. The idea of the RF power lines feeding two cavities with one amplifier is presented. To facilitate the manufacture of permanent-magnet quadrupoles and reduce the cost, the focusing strength at the end of the RFQ accelerator is optimized. Consequently, the focusing strength of all permanent-magnet quadrupoles is the same, and the DTL accelerator can be directly connected to match the RFQ accelerator without a medium-energy beam transport section between them. After construction, the 7 MeV proton linac injector is hoped to be the first homemade linac injector for a synchrotron-based proton-therapy facility in China.

This paper describes the physical design of a radio-frequency quadrupole (RFQ) accelerator for a compact accelerator-driven neutron source . The 325-MHz RFQ accelerates a 30-mA deuteron beam from 50 keV to 2 MeV, which is directly matched to the drift-tube linac (DTL) without a separate beam matching section. The objective of this design is to obtain a short RFQ with a tolerable transmission and power consumption. Using a modified four-section procedure, the inter-vane voltage is ramped with RFQ longitudinal position to obtain the required short length, and the focusing strength is varied to suppress beam loss. The RFQ has a length of 193.2 cm, with a peak power consumption of 186 kW, and a beam transmission of 94.1%.

This paper describes a real-time beam tuning method with an improved asynchronous advantage actor–critic (A3C) algorithm for accelerator systems. The operating parameters of devices are usually inconsistent with the predictions of physical designs because of errors in mechanical matching and installation. Therefore, parameter optimization methods such as pointwise scanning, evolutionary algorithms (EAs), and robust conjugate direction search are widely used in beam tuning to compensate for this inconsistency. However, it is difficult for them to deal with a large number of discrete local optima. The A3C algorithm, which has been applied in the automated control field, provides an approach for improving multi-dimensional optimization. The A3C algorithm is introduced and improved for the real-time beam tuning code for accelerators. Experiments in which optimization is achieved by using pointwise scanning, the genetic algorithm (one kind of EAs), and the A3C-algorithm are conducted and compared to optimize the currents of four steering magnets and two solenoids in the low-energy beam transport section (LEBT) of the Xi’an Proton Application Facility. Optimal currents are determined when the highest transmission of a radio frequency quadrupole (RFQ) accelerator downstream of the LEBT is achieved. The optimal work points of the tuned accelerator were obtained with currents of 0 A, 0 A, 0 A, and 0.1 A, for the four steering magnets, and 107 A and 96 A for the two solenoids. Furthermore, the highest transmission of the RFQ was 91.2%. Meanwhile, the lower time required for the optimization with the A3C algorithm was successfully verified. Optimization with the A3C algorithm consumed 42% and 78% less time than pointwise scanning with random initialization and pre-trained initialization of weights, respectively.

This paper describes the physical design, rf measurement, and tuning result of a 325 MHz four-vane radio frequency quadrupole (RFQ) with a ramped inter-vane voltage. The 3-meter-long RFQ will accelerate the negative hydrogen ion beam from 50 keV to 3 MeV with a peak current of 5 mA, a pulse width of $10--40\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, and a maximum repetition rate of 0.5 Hz. During tuning we reveal that the coupling loop of the coaxial coupler adopted for this RFQ enhances the interference between the dipole field distribution and the coupling coefficient. Therefore, an iteration method is adopted to simultaneously achieve the desired field distribution and coupling coefficient. After tuning, the relative error between the measured field distribution and the designed curve for the operating quadrupole mode is within $\ifmmode\pm\else\textpm\fi{}2.7%$, and the dipole components are about $\ifmmode\pm\else\textpm\fi{}1.9%$ of the quadrupole mode. The measured coupling coefficient of the rf power coupler equals 1.03, with the desired value of 1.04.

The 13-MeV proton linac of the Compact Pulsed Hadron Source (CPHS) at Tsinghua University, China, is composed of a 50-keV electron cyclotron resonance proton source, a 3-MeV four-vane-type radio-frequency quadrupole (RFQ) accelerator, and a drift tube linac (DTL). Precise measurement of the beam energies at the exit of the RFQ and the DTL is critical for DTL commissioning. Two button-type beam position monitors (BPMs) installed downstream of the RFQ are used to perform the measurement using a time-of-flight method. The effects of several factors on phase measurement accuracy are analyzed. The phase measurement accuracy of the BPMs at CPHS is better than $$\pm 1.03^\circ$$ at 325 MHz after corrections, corresponding to an energy measurement error of ± 0.07%. The beam energy measured at the exit of the RFQ is $$2.994\pm 0.0022$$ MeV, which is consistent with the design value.

A fiducialization method based on vibrating wire is described dedicated to the problem of locating the magnetic center relative to external fiducials for the small-aperture quadrupoles. The advantage of this method is that the measurement of the wire position, which may be the main error source, is no longer needed. The position of the magnetic center can be directly obtained by measuring the position shift of the magnet fiducials. This method has been validated on small Permanent Magnet Quadrupoles (PMQs). Experiments have confirmed its feasibility of measuring PMQs with good repeatability of about 10 μm, and shown its high sensitivity as well as convenience.

This work describes the design and realization of a radio frequency (RF) system with a single klystron to power two conjunct independent cavities, namely, an RF quadrupole (RFQ) and a drift tube linac (DTL). In contrast with a two-klystron scheme, an RF system that adopts a single-klystron scheme for two accelerating cavities exhibits simpler configuration, smaller occupation area, and lower cost. However, the low intrinsic flexibility of the single-klystron scheme needs to be compensated by independent adjustment of RF field phases and amplitudes inside the two cavities. A proposed RF system design based on the single-klystron scheme includes a 4.77 dB power divider, an adjustable power attenuator, and a phase shifter. The power attenuator and phase shifter are set in different branches of the RF power transmission line. This design is used to power an RFQ and a downstream DTL in the Compact Pulsed Hadron Source (CPHS) at Tsinghua University. During commissioning, the amplitudes inside the RFQ and DTL and the relative phase difference between them can be adjusted independently. The 13 MeV proton beam commissioning result verifies the design parameters and effectiveness of the single-klystron-based RF system. A proton beam energy of 12.92 MeV and a beam peak current of 41.2 mA at the exit of DTL were achieved, and the beam transmission from the entrance of RFQ to the exit of DTL reached 88.6%.

Transverse projected emittances of injected beams are strictly limited for synchrotrons with multi-loop injection. Horizontal and vertical emittances easily grow up when beam phase spaces in the x x ′ and y y ′ planes are coupled. Coupling in beam motion can be induced by tilted quadrupoles and solenoids. Various formalisms have been investigated to describe the beam coupling, which include the matrix and Hamiltonian formalism. In the matrix formalism, the transverse beam motion is parametrized by 4 × 4 beam matrices and the transfer matrices of the elements. Several methods or theories are used to obtain the decoupling matrix, including the equations solving, eigensystem analysis, geometrical method, Courant–Snyder theory and Hamiltonian perturbation theory. We propose an approach to obtain the decoupling matrix based on linear symplectic transformation theory. By adopting several consecutive tilted quadrupoles, the philosophy, as well as the numerical code for solving the decoupling and matching problem, are developed and checked for the general case. The transversely-coupled beam due to a 45 ∘ -rotated radio frequency quadrupole (RFQ) can be decoupled in the downstream drift tube linac with only five tilted fixed-gradient permanent-magnet quadrupoles (PMQs), which is suggested to be the minimal number of PMQs for decoupling and matching. The transverse emittance growth is verified to be suppressed below 5% by multi-particle simulation. The error study shows that the transmission rate can reach 98% with a probability of 99%.

This paper presents the tuning result of a tapered ridge-loaded waveguide coupler for the drift tube linac (DTL) of the compact pulsed hadron source (CPHS) at Tsinghua University. The coupler has been designed, manufactured, and mounted on the DTL cavity for the cold measurement and tuning. The iris diameter of the coupler which is related to the coupling coefficient needs to be determined in the tuning experiment, due to the difference between the designed and measured quality factors. Meanwhile, we found that the relationship between the coupling coefficient and iris diameter from the traditional analytical design method is not applicable when the iris diameter is relatively large. In this paper, the target coupling coefficient is analyzed, and the limit of the original analytical design is presented. The measurement method is introduced to improve the measurement efficiency and the tuning process of the coupling coefficient to the target value is described. After several iterations, the coupling coefficient is tuned to 1.54 which is close to the desired value of 1.56.

This paper describes the measurement of the transverse, two-dimensional profile of the high current proton beam in the linac at the Tsinghua University, by tomography mapping using a rotatable multi-wire scanner. The experiment is conducted at a beam energy of 3 MeV, a peak current of up to 32 mA, a repetition frequency of 20 Hz, and a beam pulse width of $50\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. The transverse density distribution is reconstructed utilizing the algebraic reconstruction technique (ART), by using the beam profiles measured at different rotation angles. The beam profile, measured at a fixed rotation angle, is obtained by 19 parallel carbon wires mounted on one board of the wire scanner monitor (WSM) moving along a straight line with a step increment of 0.1 mm. The results gathered from the computer tomography (CT) method could be verified by an independent profile measurement downstream of a multiple-slit aperture. This paper presents first measurement results of the high current proton beam during normal operation of the linac at the Tsinghua University. A dynamic range of $1{0}^{2}$ for the two-dimensional distribution was achieved.

This article is devoted to the study of a fast tuning method for the four-vane RFQ (Radio Frequency Quadrupole) accelerator with ramped-voltage. In the four-vane RFQ accelerator, the field distribution is formed mainly by the field of quadrupole modes and dipole modes. The purpose of tuning is acquiring the operating frequency and adjusting the inter-vane voltage distribution to the designed curve by changing the proportion of these modes. In this article, the transmission-line model of the RFQ is presented to describe the field distribution of all these modes. Then the perturbation theory is applied to calculate the effect of the tuners on the field distributions. On the basis of these analyses, the tuning method has been concluded and a computer program THURT (TsingHua University RFQ Tune) has been composed to perform all the calculations based on matrix operations. The tuning experiment carried out on the RFQ accelerator of the CPHS (Compact Pulsed Hadron Source) project at Tsinghua University is described. The merits and drawbacks of this method are also discussed.

As part of the design and machining of the RFQ accelerator in the Compact Pulsed Hadron Source (CPHS) project at Tsinghua University, the design process of the undercuts and dipole stabilizer rods is presented in this paper. In particular, the relationship between the inter-vane voltage slope and the local frequency of the undercut section is described quantitatively. With the identification of modes existing in the cavity, the specific parameters are optimized by the SUPERFISH and MAFIA codes. In addition, the water-cooling requirement of the dipole stabilizer rods is briefly discussed.

The design and construction status of the Radio Frequency Quadrupole (RFQ) accelerator for the Compact Pulsed Hadron Source (CPHS) at Tsinghua University is presented. The RFQ will accelerate protons from 50 keV to 3 MeV, with the RF frequency of 325 MHz. The total length of the RFQ cavity is about 3 m with the transmission rate of 97.2% predicted by PARMTEQM. Fine machining of the RFQ cavity was completed in July, 2011. The brazing of the vanes, the flanges and the cooling pipes has been finished in September, 2011.

The Compact Pulsed Hadron Source (CPHS) project is a university-based proton accelerator platform (13 MeV, 16 kW, 50 mA peak current, 0.5 ms pulse width at 50 Hz) for multi-disciplinary neutron and proton applications. The CPHS linac consists of a 3 MeV radio-frequency quadrupole (RFQ) linac and a 13 MeV drift tube linac (DTL). Both the RFQ and DTL share a 325 MHz, 2.1 MW klystron source. A single iris-type radio-frequency (RF) coupler is used to feed 537 kW of RF power to the RFQ cavity. Three-dimensional electromagnetic models of the ridge-loaded tapered waveguide (RLWG) and the coupler-cavity system are presented, and the design process and results of the RLWG and iris plate are described in detail.

A scheme, spontaneous emission high-gain harmonic generation (SEHG) free-electron laser (FEL), is proposed and analyzed for generating the X-ray FEL. The SEHG scheme works in a similar mechanism as high-gain harmonic generation (HGHG), but without the need for a seed laser. The scheme requires two undulators. The 1st undulator must be sufficiently long so that the energies of electrons are modulated within the bunch, but still away from saturation. A dispersion section is followed to transfer energy modulation into density modulation. The 2nd undulator simply serves as a radiator. A simple, one-dimensional, analytical estimation of SEHG is given to show the process of energy modulation and optimize the system parameters. The three-dimensional FEL simulation code, GENESIS, has been used to simulate, verify, and optimize the SEHG scheme for the soft X-ray free-electron laser (SXFEL) project in China. The simulation results are presented in comparison with the self-amplified spontaneous emission (SASE) and HGHG schemes. At 9 nm radiation wavelength, up to 120 MW of output power can be achieved by the SEHG scheme, with a total length of 47.3 m long undulators. Though the undulator length is comparable with the SASE scheme, the output bandwidth of the SEHG scheme is smaller. Moreover, it is tunable and does not require a seed laser. The SEHG scheme offers an attractive alternative option for the X-ray FEL.

During the past. decades, large-scale national neutron sources have been developed in Asia, Europe, and North America. Complementing such efforts, compact hadron beam complexes and neutron sources intended to serve primarily universities and industrial institutes have been proposed, and some have recently been established. Responding to the demand in China for pulsed neutron/proton-beam platforms that are dedicated to fundamental and applied research for users in multiple disciplines from materials characterization to hadron therapy and radiography to accelerator-driven sub-critical reactor systems (ADS) for nuclear waste transmutation, we have initiated the construction of a compact, yet expandable, accelerator complex-the Compact Pulsed Hadron Source (CPHS). It consists of an accelerator front-end (a high-intensity ion source, a 3-MeV radio-frequency quadrupole linac (RFQ), and a 13-MeV drift-tube linac (DTL)), a neutron target station (a beryllium target with solid methane and room-temperature water moderators/reflector), and experimental stations for neutron imaging/radiography, small-angle scattering, and proton irradiation. In the future, the CPHS may also serve as an injector to a ring for proton therapy and radiography or as the front end to an ADS test facility. In this paper, we describe the design of the CPHS technical systems and its intended operation.

A mode analysis is carried out with the MAFIA TS3 module to study the high-power microwave generation in the inward-emitting coaxial vircator with two opposed nonaxisymmetric emitting surfaces. The central angle thetas of the emittors, and the axial distance D between the cathode and anode reflector, are verified to have a large influence on the mode component. For a fixed diode voltage of 250 kV, the coaxial diode current is 13.7 kA when the beam injection is axisymmetric (thetas = 180deg). In this situation, the simulation gives the highest efficiency of 8.3% for D = 5.0 cm and 10.9% for D = 10.0 cm , comparing with the nonaxisymmetric beam injection. For the case of two opposed nonaxisymmetric emitting surfaces with central angle thetas, there exists a range for thetas to acquire a rather pure output of the TE11 mode (80% of the total power), which is 80deg < thetas < 160deg. However, the efficiency decreases with smaller thetas. The polarized direction of the main TE11 mode is consistent with the directional layout of the two opposed emitters. FFT transformation shows that the dominant frequency for thetas < 80deg can be estimated by the equations derived from the 2-D theoretical analysis of the symmetric coaxial vircator.

Two-dimensional steady-state analysis for the electron flow in an inward-emitting coaxial virtual cathode oscillator is presented. Based on the relativistic-fluid-Maxwell equations, the relativistic plasma frequency characteristics and its dependence on the geometric parameters are derived. The resulting plasma frequency solution scales linearly with the diode current. The exact dominant frequency is solved numerically, which gives the theoretical support for the simulation result that the microwave frequency in a coaxial vircator is similar to that produced by a planar vircator with the same small anode-cathode gap distance. Numerical result shows that higher frequency can be achieved in the coaxial vircator for higher applied voltage and smaller anode-cathode gap distance. The numerical solutions of the dominant frequencies predict the measured and simulated values within 10%

The phenomenon of longitudinal "trapping" in the simultaneous acceleration of intense H+ and H- beams in an RFQ accelerator is described in this paper. A new code named PARMTEQVC is compiled based on an old version of t-code PARMTEQ for the computer simulation. In this code, the 2D PIC method is applied to calculate the space charge forces. Multi-bunch effects are also taken into account. In our simulation the RFQ structure parameters which were designed only for single beam (H+ or H-) acceleration are adopted. The simulation result shows that more ions near the ends of buckets are "trapped" into the bunches with opposite charge state. These ions leave their own buckets gradually and become lost in either longitudinal or transverse direction, leading to a descent of the total transmission rate. Further studies reveal that the depth of longitudinal potential well decreases in the case of the simultaneous acceleration, which is attributed to the weaker bunching forces because of the longitudinal space charge interaction between the positive and negative ion bunches. It seems that a new RFQ structure should be specially designed for the simultaneous acceleration in order to neutralize the transverse space charge effect and increase the beam transmission rate at the same time. The design ideas are then presented. (C) 2004 Elsevier B.V. All rights reserved.

In this paper, the Child–Langmuir law and Langmuir–Blodgett law are generalized to the relativistic regime by a simple method. Two classical laws suitable for the nonrelativistic regime are modified to simple approximate expressions applicable for calculating the space-charge-limited currents of one-dimensional steady-state planar diodes and coaxial diodes under the relativistic regime. The simple approximate expressions, extending the Child–Langmuir law and Langmuir–Blodgett law to fit the full range of voltage, have small relative errors less than 1% for one-dimensional planar diodes and less than 5% for coaxial diodes.

The Compact Pulsed Hadron Source (CPHS) project that was launched in September 2009 at Tsinghua University has reached a first commissioning stage in conjunction with on-going activities to fulfill the eventual design goal of a ∼10 13 n/s epithermal-to-cold neutron yield for education, instrumentation development, and industrial applications. Here, we report the latest progress on the design, fabrication, engineering of the 13 MeV/16 kW proton accelerator system, the neutron target station and neutron beamlines, especially regarding the successful delivery of the first 3-MeV proton beam and the realization of the first neutron generation in 2013.