DTL cavity design and beam dynamics for a TAC linear proton accelerator

A 30 mA drift tube linac （DTL） accelerator has been designed using SUPERFISH code in the energy range of 3-55 MeV in the framework of the Turkish Accelerator Center （TAC） project. Optimization criteria in cavity design are effective shunt impedance （ZTT）, transit-time factor and electrical breakdown limit. In geometrical optimization we have aimed to increase the energy gain in each RF gap of the DTL cells by maximizing the effective shunt impedance （ZTT） and the transit-time factor. Beam dynamics studies of the DTL accelerator have been performed using beam dynamics simulation codes of PATH and PARMILA. The results of both codes have been compared. In the beam dynamical studies, the rms values of beam emittance have been taken into account and a low emittance growth in both x and y directions has been attempted.

Spatial and spectral measurement of laser-driven protons through radioactivation

The simultaneous measurement of the spatial profile and spectrum of laser-accelerated protons is important for further optimization of the beam qualities and applications.We report a detailed study regarding the underlying physics and regular procedure of such a measurement through the radioactivation of a stack composed of aluminum,copper,and CR-39 plates as well as radiochromic films(RCFs).After being radioactivated,the copper plates are placed on imaging plates(IPs)to detect the positrons emitted by the reaction products through contact imaging.The spectrum and energy-dependent spatial profile of the protons are then obtained from the IPs and confirmed by the measured ones from the RCFs and CR-39 plates.We also discuss the detection range,influence of electrons,radiation safety,and spatial resolution of this measurement.Finally,insights regarding the extension of the current method to online measurements and dynamic proton imaging are also provided.

Proton acceleration from picosecond-laser interaction with a hydrocarbon target

As an intense picosecond laser pulse irradiates a hydrocarbon target,the protons therein can be accelerated by the radiation pressure as well as the sheath field behind the target.We investigate the effect of the laser and hydrocarbon target parameters on proton acceleration with two/threedimensional particle-in-cell simulations.It is found that the resulting two-ion species plasma can generate a multiple peaked charge-separation field that accelerates the protons.In particular,a smaller carbon-to-hydrogen ratio,as well as the thinner and/or lower density of the target,leads to a larger sheath field and thus proton beams with a larger cutoff energy and smoother energy spectrum.These results may be useful in achieving high-flux quasi-monoenergetic proton beams by properly designing the hydrocarbon target.

Laser radiation pressure proton acceleration in gaseous target

An analytical model for hole boring proton acceleration by a circularly-polarized CO_(2) laser pulse in a gas jet is developed.The plasma density profile near the density peak is taken to be rectangular,with inner region thickness l around a laser wavelength and density 10%above the critical,while the outside density is 10%below the critical.On the rear side,plasma density falls off rapidly to a small value.The laser suffers strong reflection from the central region and,at normalized amplitude a _(0)≥1,creates a double layer.The space charge field of the double layer,moving with velocity v_(f)z,reflects up-stream protons to 2v_(f) velocity,incurring momentum loss at a rate comparable to radiation pressure.Reflection occurs for v_(f)≤ω_(p)√z_(f)lm/m_(p),where m and m_(p) are the electron and proton masses,z_(f) is the distance traveled by the compressed electron layer and u p is the plasma frequency.For Gaussian temporal profile of the laser and parabolic density profile of the upstream plasma,the proton energy distribution is narrowly peaked.

Generation of fast protons in moderate-intensity laser-plasma interaction from rear sheath

Forward fast protons are generated by the moderate-intensity laser-foil interaction. Protons with maximum energy 190 keV are measured by using magnetic spectrometer and CR-39 solid state track detectors along the direction normal to the rear surface. The experimental results are also modeled by the paxticle-in-cell method, investigating the timevarying electron temperature and the rear sheath field. The temporal and spatial structure of the sheath electrical field, revealed in the simulation, suggests that these protons are accelerated by target normal sheath acceleration （TNSA） mechanism.

Effect of Plasma Density on Proton Acceleration in a Rectangular Waveguide

Analytical studies are made for the proton acceleration during its motion inthe fields of the fundamental mode excited by a high-intensity microwave in a rectangular waveguide,when the proton is injected along the propagating direction of the mode. The trajectory of theproton is calculated and the expressions are obtained for the energy gain and acceleration gradienttogether with the effects of plasma density, microwave frequency and waveguide width. Energy gain of181 keV is attained by a 50 keV proton in a 0.015m x 0.020 m evacuated waveguide when 0.5 x 10^(10)W/m^2 microwave intensity is used. However, this gain increases to 1387 keV when the waveguide isfilled with a plasma having a density of 1.0 x 10^(19) m^(-3). Higher acceleration gradients areachieved when the proton is injected with a higher initial energy and also when the microwaveintensity increases. The effects of the microwave frequency and width of the waveguide are found todecrease the acceleration gradient.

Accelerated protons with energies up to 70 MeV based on the optimized SG-Ⅱ Peta-watt laser facility

The target backsheath field acceleration mechanism is one of the main mechanisms of laser-driven proton acceleration(LDPA)and strongly depends on the comprehensive performance of the ultrashort ultra-intense lasers used as the driving sources.The successful use of the SG-II Peta-watt(SG-II PW)laser facility for LDPA and its applications in radiographic diagnoses have been manifested by the good performance of the SG-II PW facility.Recently,the SG-II PW laser facility has undergone extensive maintenance and a comprehensive technical upgrade in terms of the seed source,laser contrast and terminal focus.LDPA experiments were performed using the maintained SG-II PW laser beam,and the highest cutoff energy of the proton beam was obviously increased.Accordingly,a double-film target structure was used,and the maximum cutoff energy of the proton beam was up to 70 MeV.These results demonstrate that the comprehensive performance of the SG-II PW laser facility was improved significantly.

Dynamic study of compressed electron layer driven by linearly polarized laser

The dynamics of the compressed electron layer（CEL） are investigated when a linearly polarized（LP） laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is verified by particlein-cell（PIC） simulations. It is found that the compressed layer disperses in a few cycles of the laser duration, because the CEL comes back with a large velocity in the opposite direction of the laser incident. A larger wavelength laser can be used to tailor the proton beam by reducing the turbulence of the CEL in the region of the LP laser acceleration.

OCTOPOD:single-bunch tomography for angular-spectral characterization of laser-driven protons

Laser-plasma accelerated(LPA)proton bunches are now applied for research fields ranging from ultra-high-dose-rate radiobiology to material science.Yet,the capabilities to characterize the spectrally and angularly broad LPA bunches lag behind the rapidly evolving applications.The OCTOPOD translates the angularly resolved spectral characterization of LPA proton bunches into the spatially resolved detection of the volumetric dose distribution deposited in a liquid scintillator.Up to 24 multi-pinhole arrays record projections of the scintillation light distribution and allow for tomographic reconstruction of the volumetric dose deposition pattern,from which proton spectra may be retrieved.Applying the OCTOPOD at a cyclotron,we show the reliable retrieval of various spatial dose deposition patterns and detector sensitivity over a broad dose range.Moreover,the OCTOPOD was installed at an LPA proton source,providing real-time data on proton acceleration performance and attesting the system optimal performance in the harsh laser-plasma environment.

Enhancement of the laser-driven proton source at PHELIX

We present a study of laser-driven ion acceleration with micrometre and sub-micrometre thick targets,which focuses on the enhancement of the maximum proton energy and the total number of accelerated particles at the PHELIX facility.Using laser pulses with a nanosecond temporal contrast of up to 10^-12 and an intensity of the order of 1020 W/cm^2,proton energies up to 93 MeV are achieved.Additionally,the conversion efficiency at 45°incidence angle was increased when changing the laser polarization to p,enabling similar proton energies and particle numbers as in the case of normal incidence and s-polarization,but reducing the debris on the last focusing optic.

Proton Beam Generated by Multi-Lasers Interaction with Rear-Holed Target

Multi-lasers are proposed to enhance the proton acceleration in laser plasma interaction. A rear-holed target is illuminated by three lasers from different directions. The scheme is demonstrated by two-dimensional particlein-cell simulations. The electron cloud shape is controlled well and the electron density is improved significantly. The electrons accelerated by the three lasers induce an enhanced target normal sheath acceleration(TNSA) which suppresses the proton beam divergence and improves the maximum proton energy. The maximum proton energy is 22.9 Me V, which increased significantly than that of a single-laser target interaction. Meanwhile, the average divergence angle(22.3?) is reduced. The dependence of the proton beam on the length of sidewall is investigated in detail and the optimal length is obtained.

Generation and application of high-contrast laser pulses using plasma mirror in the SULF-1PW beamline

The plasma mirror system was installed on the 1 PW laser beamline of Shanghai Superintense Ultrafast Laser Facility[SULF]for enhancing the temporal contrast of the laser pulse.About 2 orders of magnitude improvement on pulse contrast was measured on picosecond and nanosecond time scales.The experiments show that high-contrast laser pulses can significantly improve the cutoff energy and quantity of proton beams.Then different target distributions are assumed in particles in cell simulations,which can qualitatively assume the expansion of nanometer-scale foil.The high-contrast laser enables the SULF-1PW beamline to generally be of benefit for many potential applications.

Design and construction of the first prototype ionization chamber for CSNS and PA beam loss monitor (BLM) system

Design and construction of the first prototype ionization chamber for CSNS and Proton Accelerator （PA） beam loss monitor （BLM） system is reported. The low leakage current （〈0.1 pA）, good plateau （≈800 V） and linearity range up to 200 Roentgen/h are obtained in the first prototype. All of these give us good experience for further improving the ionization chamber construction.

Design of a 325 MHz half wave resonator prototype at IHEP

A 325 MHzβ=0.14 superconducting half-wave resonator prototype has been developed at the Institute of High Energy Physics, Beijing, which can be applied in the low energy section of continuous wave high current proton linear accelerators. The electromagnetic design, multipacting simulation, mechanical optimization and fabrication are introduced in detail. Test results at room temperature and 4.2 K, and a comparison between measured and simulated results, are analyzed in this paper.

Analysis and construction status of SG-Ⅱ 5PW laser facility

We present a recent progress of the SG-II 5 PW facility, which designed a multi-petawatt ultrashort pulse laser based on optical parametric chirped-pulse amplification(OPCPA). The prior two optical parametric amplifiers have been accomplished and chirped pulses with an energy of 49.7 J and a full-width-at-half-maximum(FWHM) spectrum bandwidth of 85 nm have been achieved. In the PW-scale optical parametric amplification(OPA), with the pump pulse that has an energy of 118 J from the second harmonic generation of the SG-II 7 th beam, the pump-to-signal conversion efficiency is up to 41.9%, which to the best of our knowledge is the highest among all of the reported values for OPCPA systems. The compressed pulse is higher than 37 J in 21 fs(1.76 PW), and the focal spot is ～10 μm after the closed-loop corrections by the adaptive optics. Limited by the repetition of the pump laser, the SG-II 5 PW facility operates one shot per hour. It has successfully been employed for high energy physics experiments.

New scheme to trigger fusion in a compact magnetic fusion device by combining muon catalysis and alpha heating effects

The application of laser pulses with psec or shorter duration enables nonthermal efficient ultrahigh acceleration of plasma blocks with homogeneous high ion energies exceeding ion current densities of 10^(12) A cm^(-2). The effects of ultrahigh acceleration of plasma blocks with high energy proton beams are proposed for muon production in a compact magnetic fusion device. The proposed new scheme consists of an ignition fusion spark by muon catalyzed fusion(μCF) in a small mirror-like configuration where low temperature D–T plasma is trapped for a duration of 1 μs. This initial fusion spark produces sufficient alpha heating in order to initiate the fusion process in the main device. The use of a multi-fluid global particle and energy balance code allows us to follow the temporal evolution of the reaction rate of the fusion process in the device. Recent progress on the ICAN and IZEST projects for high efficient high power and high repetition rate laser systems allows development of the proposed device for clean energy production. With the proposed approaches,experiments on fusion nuclear reactions and μCF process can be performed in magnetized plasmas in existing kJ/PW laser facilities as the GEKKO-LFEX, the PETAL and the ORION or in the near future laser facilities as the ELI-NP Romanian pillar.