Yuanrong Lu

The deuteron injector progress of the Peking University Neutron Imaging Facility project.

A deuteron radio frequency quadrupoles injector h has been developed at Peking University. A permanent magnetic electron cyclotron resonance (ECR) ion source is used in the injector system. A 50 keV 100 mA proton beam has been extracted from the ECR ion source and the measured normalized rms emittance is 0.11-0.14pi mm mrad. A deuteron beam has also been extracted at 50 kV with 83 mA total current and its emittance is less than 0.18pi mm mrad. The proton beam transmission has been investigated on a low energy beam transport test bench, and up to 93% transmission can be reached. The new injector with two solenoids has been designed and is being constructed. All the development results will be presented in this paper.

High-quality proton bunch from laser interaction with a gas-filled cone target

Generation of high-energy proton bunch from interaction of an intense short circularly polarized (CP) laser pulse with a gas-filled cone target (GCT) is investigated using two-dimensional particle-in-cell simulation. The GCT target consists of a hollow cone filled with near-critical gas-plasma and a thin foil attached to the tip of the cone. It is observed that as the laser pulse propagates in the gas-plasma, the nonlinear focusing will result in an enhancement of the laser pulse intensity. It is shown that a large number of energetic electrons are generated from the gas-plasma and accelerated by the self-focused laser pulse. The energetic electrons then transports through the foil, forming a backside sheath field which is stronger than that produced by a simple planar target. A quasi-monoenergetic proton beam with maximum energy of 181 MeV is produced from this GCT target irradiated by a CP laser pulse at an intensity of 2.6 × 1020 W/cm2, which is nearly three times higher compared to simple planar targe...

Upgrade of the extraction system of permanent magnet electron cyclotron resonance ion source

A set of new ion extraction electrodes have been designed for the permanent magnetic electron cyclotron resonance ion source at Peking University to improve beam quality and transmission. PBGUNS has been used to optimize the extraction electrodes and simulate the beam behavior at the extraction region. The experiments showed that with the new system, the beam half divergence angle can be less than 40 mrad and the normalized rms emittance is about 0.13pi mm mrad when the extracted current is 100 mA at 50 keV in pulse mode. The voltage of the suppression electrode has great effect on beam divergence. The effect of the microwave power and gas flow is also studied.

Frequency tunable x-ray/γ-ray source via Thomson backscattering on flying mirror from laser foil interaction

A scheme to generate a frequency tunable x-ray/γ-ray source via Thomson backscattering is proposed. In this model, a few-cycle drive pulse with relativistic intensity interacts with a target (combined with a thin and a thick foil) to produce a flying mirror, and a counter propagating probe pulse is applied to generate a high frequency pulse on it. By adjusting the separation between these two foils, the frequency of the Thomson backscattering light generated from the flying mirror can be tuned in a range from ω L to > 10 6 ω L , i.e., x-ray or γ-ray with tunable frequency is obtained. The energy dispersion of the flying mirror, as well as the spectrum width of the Thomson backscattering are studied.

Numerical simulation study on fluid dynamics of plasma window using argon

In this paper, a numerical 2D FLUENT-based magneto-hydrodynamic model has been developed to investigate the arc and flow field of plasma window, which is used as a windowless vacuum sealing device. The gas inlet, arc creation-developing and plasma expansion segments are all incorporated together in the integral model. An axis-symmetry cathode structure (hollow cathode) is used in the model. Current distribution of the arc is presented and discussed. The temperature, velocity, and pressure field are presented to show the physical mechanisms for the high pressure gap within the plasma window. Flow acceleration and viscosity effect are concluded as the main reasons for the pressure drop. The result for the pressure distribution in the cylindrical tube section has a good agreement with the analytical model. The validation for the sealing ability of plasma window is verified.

Determination of carrier-envelope phase of relativistic few-cycle laser pulses by Thomson backscattering spectroscopy.

A method is proposed to determine the carrier-envelope phase (CEP) of a relativistic few-cycle laser pulse via the frequency of the Thomson backscattering (TBS) light. We theoretically investigate the generation of a flying mirror when a few-cycle drive pulse with relativistic intensity interacts with a target combined with a thin and a thick foil. The frequency of the TBS light generated from the flying mirror shows a sensitive dependence on the CEP of the drive pulse. The obtained results are verified by one-dimensional particle-in-cell simulations and are explained by an analytical model.

Autofocused, enhanced proton acceleration from a nanometer-scale bulged foil

We report an autofocused, enhanced proton acceleration by the interaction of an intense laser pulse with a bulged target. These results are obtained from two-dimensional particle-in-cell simulations using a real Gaussian laser pulse, normally incident on a bulged/planar, 60 nm thick foil (C:H=1:1). When the laser pulse hits the precurved target, energetic protons are converged on the axis automatically. For the bulged foil, due to oblique incidence at the wing region, the efficient vacuum heating at larger incidence angles will result in more energetic hot electrons than from the flat foil. The enhancement of hot electron temperature and density will result in a larger longitudinal field, which contributes to an enhancement of proton energy. The maximum proton energy of 124 MeV is attained from a bulged target irradiated by a linear polarized laser pulse at an intensity of 1.3×1020 W/cm2, which is two times higher than from the planar target (61 MeV).

The influence of magnetic field configuration on an electron cyclotron resonance ion source

In an electron cyclotron resonance (ECR) ion source, the magnetic field along the axis of the plasma chamber and extraction system is a key parameter. At Peking University, a new 2.45 GHz ECR ion source (PMECR III), dedicated to proton production, has been developed to investigate the influence of the magnetic field on the gas discharge and beam characteristics. The magnetic configuration is provided by two permanent magnet rings independently tunable along the source axis. Moreover, the beam extraction position changes by moving the whole magnetic system along the source axis and by using different lengths of plasma electrode. A brief description of the source is reported and the magnetic field influence results are presented.

Distribution uniformity of laser-accelerated proton beams

Compared with conventional accelerators, laser plasma accelerators can generate high energy ions at a greatly reduced scale, due to their TV/m acceleration gradient. A compact laser plasma accelerator (CLAPA) has been built at the Institute of Heavy Ion Physics at Peking University. It will be used for applied research like biological irradiation, astrophysics simulations, etc. A beamline system with multiple quadrupoles and an analyzing magnet for laser-accelerated ions is proposed here. Since laser-accelerated ion beams have broad energy spectra and large angular divergence, the parameters (beam waist position in the Y direction, beam line layout, drift distance, magnet angles etc.) of the beamline system are carefully designed and optimised to obtain a radially symmetric proton distribution at the irradiation platform. Requirements of energy selection and differences in focusing or defocusing in application systems greatly influence the evolution of proton distributions. With optimal parameters, radially symmetric proton distributions can be achieved and protons with different energy spread within 5% have similar transverse areas at the experiment target.

A particle-in-cell mode beam dynamics simulation of medium energy beam transport for the SSC-Linac

A new linear accelerator system, called the SSC-Linac injector, is being designed at HIRFL (the heavy ion research facility of Lanzhou). As part of the SSC-Linac, the medium energy beam transport (MEBT) consists of seven magnetic quadrupoles, a re-buncher and a diagnose box. The total length of this segment is about 1.75 m. The beam dynamics simulation in MEBT has been studied using the TRACK 3D particle-in-cell code, and the simulation result shows that the beam accelerated from the radio frequency quadrupole (RFQ) matches well with the acceptance of the following drift tube linac (DTL) in both the transverse and longitudinal phase spaces, and that most of the particles can be captured by the final sector focusing cyclotron for further acceleration. The longitudinal emittance of the RFQ and the longitudinal acceptance of the DTL was calculated in detail, and a multi-particle beam dynamics simulation from the ion source to the end of the DTL was done to verify the original design.