High-charge Beam Research Articles

Beam physics studies for a high charge and high beam quality laser-plasma accelerator

Beam physics studies for a high charge and high beam quality laser-plasma accelerator

PLATEAU FORMATION ON ACCELERATING WAKEFIELD FOR ELECTRON-WITNESS-BUNCH AND ON DECELERATING WAKEFIELD FOR DRIVER-BUNCHES IN A PLASMA

Plasma wakefield acceleration promises compact sources of high-brightness relativistic electron and positron beams. Applications (particle colliders and free-electron lasers) of plasma wakefield accelerators demand low ener-gy spread beams and high-efficiency operation. Achieving both requires plateau formation on both the accelerating field for witness-bunch and the decelerating fields for driver-bunches by controlled beam loading of the plasma wave with careful tailored current profiles. We demonstrate by numerical simulation by 2.5D PIC code LCODE such optimal beam loading in a linear and blowout electron-driven plasma accelerator with RF generated low and high beam charge and high beam quality.

Measurements and Simulations of High Charge Beam in the APS Booster

Measurements and Simulations of High Charge Beam in the APS Booster

Efficient algorithm for high fidelity collisional charged particle beam dynamics

High fidelity charged particle beam dynamics, especially if collisional, require specialized numerical methods. The challenges include the ability to deal with very large particle numbers, long-range electromagnetic forces, and vast spatial and timescales. We developed a novel algorithm to address these challenges. The main characteristics of the algorithm are Strang splitting to separate near and far forces, the fast multipole method to lower the computational cost of the far region, and the Sim\`o integrator to capture all close encounters efficiently in the near region. The algorithm is fully adaptive both in space and time, while maintaining symplecticity to machine precision. Its performance is illustrated with two challenging examples from nonlinear multiparticle beam dynamics, including the first electron cooling simulations based on first principles.

High energy implementation of coil-target scheme for guided re-acceleration of laser-driven protons

Developing compact ion accelerators using intense lasers is a very active area of research, motivated by a strong applicative potential in science, industry and healthcare. However, proposed applications in medical therapy, as well as in nuclear and particle physics demand a strict control of ion energy, as well as of the angular and spectral distribution of ion beam, beyond the intrinsic limitations of the several acceleration mechanisms explored so far. Here we report on the production of highly collimated (sim 0.2^{circ } half angle divergence), high-charge (10s of pC) and quasi-monoenergetic proton beams up to sim 50 MeV, using a recently developed method based on helical coil targetry. In this concept, ions accelerated from a laser-irradiated foil are post-accelerated and conditioned in a helical structure positioned at the rear of the foil. The pencil beam of protons was produced by guided post-acceleration at a rate of sim 2 GeV/m, without sacrificing the excellent beam emittance of the laser-driven proton beams. 3D particle tracing simulations indicate the possibility of sustaining high acceleration gradients over extended helical coil lengths, thus maximising the gain from such miniature accelerating modules.

Toward a plasma-based accelerator at high beam energy with high beam charge and high beam quality

From plasma-wakefield acceleration as a physics experiment toward a plasma-based accelerator as a user facility, the beam physics issues remaining to be solved are still numerous. Providing beams with high energy, charge, and quality simultaneously, not only within the plasma but also at the user doorstep itself, is the main concern. Despite its tremendous efficiency in particle acceleration, the wakefield displays a complex 3D profile which, associated to the beam-loading field induced by the accelerated beam itself, makes the acceleration of high charge to high energy often incompatible with high beam quality. Beam extraction from the plasma without quality degradation for a transfer either to the next plasma stage or to the user application is another difficulty to consider. This article presents the substantial studies carried out and the different innovative methods employed for tackling all these different issues. Efforts focused on achieving the challenging beam parameters targeted by the EuPRAXIA accelerator facility project. The lessons learned at the end of these in-depth simulations and optimizations are highlighted. The sensitivity to different error sources is also estimated to point out the critical components of such an accelerator. Finally, the needs in terms of laser and plasma parameters are provided.

High power conditioning and benchmarking of planar nitrogen-incorporated ultrananocrystalline diamond field emission electron source

Planar nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, has emerged as a unique field emission source attractive for accelerator applications because of its capability to generate high charge beam and handle moderate vacuum conditions. Most importantly, (N)UNCD sources are simple to produce: conventional high aspect ratio isolated emitters are not required to be formed on the surface, and the actual emitter surface roughness is on the order of only 100~nm. Careful reliability assessment of (N)UNCD is required before it may find routine application in accelerator systems. In the present study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode has been conditioned to $\sim$42~MV/m in a well-controlled manner. It reached a maximum output charge of 15~nC corresponding to an average current of 6~mA during an emission period of 2.5~$\mu$s. Imaging of emission current revealed a large number of isolated emitters (density over 100/cm$^{2}$) distributed on the cathode, which is consistent with previous tests in dc environments. The performance metrics, the emission imaging, and the systematic study of emission properties during rf conditioning in a wide gradient range assert (N)UNCD as an enabling electron source for rf injector designs serving industrial and scientific applications. These studies also improve the fundamental knowledge of the practical conditioning procedure via better understanding of emission mechanisms.

Design studies of a proof-of-principle experiment on THz SASE FEL at PITZ

A free-electron laser based THz source is undergoing design studies at the Photo Injector Test facility at DESY in Zeuthen (PITZ). It is considered as a prototype for pump-probe experiments at the European XFEL, benefiting from the fact that the electron beam from the PITZ facility has an identical pulse train structure as the XFEL pulses. In the proposed proof-of-principle experiment, the electron beam (up to 4 nC bunch charge and 200 A peak current) will be accelerated to 16-22 MeV/c to generate SASE radiations in an LCLS-I undulator in the THz range between 60 and 100 pm with an expected energy of up to ~1 mJ/pulse. In this paper, we report our simulations on the optimization of the photo-injector and the design of the transport and matching beamline. Experimental investigations on the generation, characterization and matching of the high charge beam in the existing 22-m-long beamline will also be presented.

Eupraxia, A Step Toward A Plasma-Wakefield Based Accelerator With High Beam Quality

The EuPRAXIA project aims at designing the world's first accelerator based on advanced plasma-wakefield techniques to deliver 5 GeV electron beams that simultaneously have high charge, low emittance and low energy spread, which are required for applications by future user communities. Meeting this challenging objective will only be possible through dedicated effort. Many injection/acceleration schemes and techniques have been explored by means of thorough simulations in more than ten European research institutes. This enables selection of the most appropriate methods for solving each particular problem. The specific challenge of generating, extracting and transporting high charge beams, while maintaining the high quality needed for user applications, are being tackled using innovative approaches. This article highlights preliminary results obtained by the EuPRAXIA collaboration, which also exhibit the required laser and plasma parameters.

Highly-collimated, high-charge and broadband MeV electron beams produced by magnetizing solids irradiated by high-intensity lasers

Laser irradiation of solid targets can drive short and high-charge relativistic electron bunches over micron-scale acceleration gradients. However, for a long time, this technique was not considered a viable means of electron acceleration due to the large intrinsic divergence (∼50° half-angle) of the electrons. Recently, a reduction in this divergence to 10°–20° half-angle has been obtained, using plasma-based magnetic fields or very high contrast laser pulses to extract the electrons into the vacuum. Here we show that we can further improve the electron beam collimation, down to ∼1.5° half-angle, of a high-charge (6 nC) beam, and in a highly reproducible manner, while using standard stand-alone 100 TW-class laser pulses. This is obtained by embedding the laser-target interaction in an external, large-scale (cm), homogeneous, extremely stable, and high-strength (20 T) magnetic field that is independent of the laser. With upcoming multi-PW, high repetition-rate lasers, this technique opens the door to achieving even higher charges (>100 nC).

Prospects for laser-driven ion acceleration through controlled displacement of electrons by standing waves

During the interaction of intense femtosecond laser pulses with various targets, the natural mechanisms of laser energy transformation inherently lack temporal control and thus commonly do not provide opportunities for a controlled generation of a well-collimated, high-charge beam of ions with a given energy of particular interest. In an effort to alleviate this problem, it was recently proposed that the ions can be dragged by an electron bunch trapped in a controllably moving potential well formed by laser radiation. Such standing-wave acceleration (SWA) can be achieved through reflection of a chirped laser pulse from a mirror, which has been formulated as the concept of chirped-standing-wave acceleration (CSWA). Here, we analyse general feasibility aspects of the SWA approach and demonstrate its reasonable robustness against field structure imperfections, such as those caused by misalignment, ellipticity, and limited contrast. Using this, we also identify prospects and limitations of the CSWA concept.

High-quality electron beam generation in a proton-driven hollow plasma wakefield accelerator

Simulations of proton-driven plasma wakefield accelerators have demonstrated substantially higher accelerating gradients compared to conventional accelerators and the viability of accelerating electrons to the energy frontier in a single plasma stage. However, due to the strong intrinsic transverse fields varying both radially and in time, the witness beam quality is still far from suitable for practical application in future colliders. Here we demonstrate efficient acceleration of electrons in proton-driven wakefields in a hollow plasma channel. In this regime, the witness bunch is positioned in the region with a strong accelerating field, free from plasma electrons and ions. We show that the electron beam carrying the charge of about 10% of 1 TeV proton driver charge can be accelerated to 0.6 TeV with preserved normalized emittance in a single channel of 700 m. This high quality and high charge beam may pave the way for the development of future plasma-based energy frontier colliders.

Physics of fully-loaded laser-plasma accelerators

While large efforts have been devoted to improving the quality of electron beams from laser plasma accelerators, often to the detriment of the charge, many applications do not require very high quality but high-charge beams. Despite this need, the acceleration of largely charged beams has been barely studied. Here we explore both experimentally and numerically the physics of highly loaded wakefield acceleration. We find that the shape of the electron spectra is strikingly independent of the laser energy, due to the emergence of a saturation effect induced by beamloading. A transition from quasi-Maxwellian spectra at high plasma densities to flatter spectra at lower densities is also found, which is shown to be produced by the wakefield driven by the electron bunch itself after the laser depletion.

Demonstration of cathode emittance dominated high bunch charge beams in a DC gun-based photoinjector

We present the results of transverse emittance and longitudinal current profile measurements of high bunch charge (≥100 pC) beams produced in the DC gun-based Cornell energy recovery linac photoinjector. In particular, we show that the cathode thermal and core beam emittances dominate the final 95% and core emittances measured at 9–9.5 MeV. Additionally, we demonstrate excellent agreement between optimized 3D space charge simulations and measurement, and show that the quality of the transverse laser distribution limits the optimal simulated and measured emittances. These results, previously thought achievable only with RF guns, demonstrate that DC gun based photoinjectors are capable of delivering beams with sufficient single bunch charge and beam quality suitable for many current and next generation accelerator projects such as Energy Recovery Linacs and Free Electron Lasers.

Fast electron generation using PW-class PEARL facility

We use a PW-class PEARL facility to study fast electron beam generation during high intensity laser pulse interaction with a supersonic gas jet. We show that electron beams with several hundreds of MeV and relatively large charges, of hundreds of pC and more, can be effectively produced without any guiding structures. PIC simulations also confirm the obtained experimental data and provide optimized conditions of laser–plasma interaction for high-charged beam production.

INJECTOR STUDY FOR COMPACT HARD X-RAY SOURCE VIA LASER COMPTON SCATTERING

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

RECENT STRUCTURE BASED HIGH GRADIENT WAKEFIELD EXPERIMENTS AT ARGONNE

Dielectric structures promise to support high field, especially for short wakefield pulses produced by a high charged electron beam traveling in a dielectric tube. To push the gradient higher, we have tested two structures using recent upgraded Argonne wakefield accelerator facility that capable of producing up to 100 nC charge and bunch length of < 13 ps (FWHM). Here we report on the experiment results that more than 80 nC beam passes through a 14 GHz dielectric loaded wakefield structure that produced an accelerating field of ~ 45 MV/m . The two structures consist of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively, and with length of 102 mm and 23 mm long. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step in these experiments, another structure, with an output coupler, has been designed and is presently being fabricated.

Space charge modeling of dense electron beams with large energy spreads

Theoretical and numerical studies of the transport in vacuum of multi-nC, multi-MeV electron beams are performed using several methods, including envelope models, a novel semianalytic approach using ellipsoidal shell decomposition, a modified electrostatic particle-in-cell method, and a point-to-point interaction model. The effects of space-charge forces on the longitudinal and transverse bunch properties are evaluated for various bunch lengths, energies, energy spreads, and charges. An evaluation of the various methods for studying space-charge effects in large energy spread, high charge beams is summarized. Examples are given for beam distributions typical of those generated by plasma-based accelerators. It is found that, for the highly correlated beams produced in the self-modulated regime, the high energy portion of the beam can gain significant energy while propagating in vacuum due to space-charge effects.

Performance study of high current photo-cathode RF-gun with new laser system

For a sub-picosecond pump-and-probe-type radiation chemistry work, a new synchronized electron linac and laser system was installed in Nuclear Engineering Research Laboratory (NERL) of University of Tokyo. The new 0.3 TW laser system with a Ti:Sapphire oscillator (795 nm) and amplifiers generate 300 ps pulses at 10 Hz at first. The laser is splitted into the two beams. The first is compressed to 4 ∼ 11 ps so as to drive the RF-Gun and generate a pump-electron-beam. The second is compressed to 100 fs and used for the probe-light. The drive laser is converted to the third harmonics (265 nm, < 250 µJ). Moreover, a new 15 MW klystron was installed for the linac RF system. The RF power is divided to the gun and the accelerating section. Finally the time jitter between the pump-electron-beam and the probe-laser is designed to be 320 fs (rms). By using the new drive laser system, the charge of 7 nC per bunch was observed at the exit of the gun. Improvement of the vacuum in the gun (< 10 −9 Torr) is most effective to obtain such a high charge beam. After two and a half years operation of the gun, no degradation of QE has been found for the Cu photo-cathode.

High-Charge S-Band Photocathode RF-Gun and Linac System for Radiation Research.

For sub-picosecond pump-and-probe-type radiation chemistry work, a new synchronized electron linac and laser system was installed in the Nuclear Engineering Research Laboratory (NERL) of University of Tokyo. The new laser system, with a Ti:Sapphire oscillator (795 nm) and amplifiers, generates 300 ps pulses at 10 Hz. The laser is transported through the vacuum chamber and then split into two beams. The first is compressed and converted to the third harmonics (265 nm, <250 μJ, 4–11 ps) so as to drive the photocathode RF-gun and generate a pump-electron beam. The second is compressed to 100 fs and used for the probe light. The high-power RF, which is provided by a new 15 MW klystron, is divided into the gun and the accelerating section. Finally, a time jitter of 330 fs (rms) was achieved between the pump-electron beam and the probe laser, which is equivalent to the design value of 320 fs. A charge of 7 nC/bunch was observed at the exit of the gun from this new laser system. Improvement of the vacuum in the gun (<10-9 Torr) is the most effective way to obtain such a high-charge beam. After about three years of operation, the Cu photocathode has shown no degradation of quantum efficiency.