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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.

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%.

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.

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.

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%