Low Energy Spread Research Articles

Generating 10–40 MeV high quality monoenergetic electron beams using a 5 TW 60 fs laser at Tsinghua University

A unique facility for laser plasma physics and advanced accelerator research has recently been built at Tsinghua University. This system is based on a Tsinghua Thomson scattering X-ray source (TTX), which combines an ultrafast TW laser with a synchronized 45 MeV high brightness linac. In our recent laser wakefield acceleration experiments, we have obtained 10–40 MeV high quality monoenergetic electron beams by running the laser at 5 TW peak power. Under certain conditions a very low relative energy spread of a few percent can be achieved. Absolute charge calibration for three different scintillating screens has also been performed using the linac system.

Design principles for high quality electron beams via colliding pulses in laser plasma accelerators

Laser plasma based accelerators have the potential to reduce dramatically the size and cost of future particle colliders and light sources. Production of high quality beams along with reproducibility, tunability, and efficiency are required for many applications. We present design principles for two-pulse colliding laser pulse injection mechanisms, which can meet these requirements. Simulations are used to determine the best conditions for the production of high quality beams: high charge, low energy spread, and low emittance. Simulations also allow access to the internal dynamics of the interaction, providing insight regarding further improvement of the beam quality. We find that a 20 pC beam can be accelerated to 300 MeV in 4 mm with only a few percent energy spread and transverse normalized emittance close to 1 mm mrad, using a 10 TW laser. We demonstrate that this design scales according to linear theory. Control of the laser pulse mode content and subsequent evolution in the plasma channel are shown to be critical for achieving the highest beam quality.

Low energy spread electron beams from ionization injection in a weakly relativistic laser wakefield accelerator

We show via two-dimensional particle-in-cell simulations that low energy spread, relativistic electron beams (>120 MeV, <15%) can be produced in the weakly non-linear regime of a plasma wakefield, driven by a moderate power laser pulse (initial a0 < 1). Higher ionization states of a high-Z trace species, mixed in a background H plasma, provide the source of injected electrons. Injection occurs even though the laser intensity is initially well below the trapping threshold, as it is found that the laser pulse evolves until it fulfils the trapping requirements through self-compression. By careful control of intensity and density, the amount of evolution and hence of trapping can be controlled. Acceleration is terminated by depletion due to the extended evolution time, leading to narrow energy spread features even for long interaction lengths. Particle tracking shows that electrons ‘born’ at the periphery of the laser pulse are more likely to follow smoother trajectories inside the wakefield and subsequently to be trapped and accelerated.

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.

Quasi-monoenergetic proton beams by laser-plasma X-rays

We report the details of a technique for the production of proton beams with very low energy spread exploiting the short soft X-rays obtained by laser ablation. These beams have been generated by the dissociation and ionization of an hydrogen buffer gas induced by the laser-plasma X-rays and then accelerated by means of an electrostatic accelerator. Their properties have been analyzed through the time-of-flight method applying different accelerating voltages. The resulting energetic spread ranges between 6 and 11%, as a function of the applied voltage. Such a system could be extremely useful for producing quasi-monoenergetic proton beams.

Laser-driven three-stage heavy-ion acceleration from relativistic laser-plasma interaction

A three-stage heavy ion acceleration scheme for generation of high-energy quasimonoenergetic heavy ion beams is investigated using two-dimensional particle-in-cell simulation and analytical modeling. The scheme is based on the interaction of an intense linearly polarized laser pulse with a compound two-layer target (a front heavy ion layer + a second light ion layer). We identify that, under appropriate conditions, the heavy ions preaccelerated by a two-stage acceleration process in the front layer can be injected into the light ion shock wave in the second layer for a further third-stage acceleration. These injected heavy ions are not influenced by the screening effect from the light ions, and an isolated high-energy heavy ion beam with relatively low-energy spread is thus formed. Two-dimensional particle-in-cell simulations show that ∼100MeV/u quasimonoenergetic Fe24+ beams can be obtained by linearly polarized laser pulses at intensities of 1.1×1021W/cm2.

Physical design of FEL injector based on the performance-enhanced EC-ITC RF gun

To meet the requirements of high performance THz-FEL (Free Electron Laser), a compact scheme of FEL injector was proposed. A thermionic cathode was chosen to emit electrons instead of a photo-cathode with its complex structure and high cost. The effective bunch charge was improved to ∼200 pC by adopting an enhanced EC-ITC (External Cathode Independently Tunable Cells) RF gun to extract micro-bunches; back bombardment effects were almost eliminated as well. Constant gradient accelerator structures were designed to improve energy to ∼14 MeV, while the focusing system was applied for emittance suppressing and bunch state maintenance. The physical design and beam dynamics of the key components for the FEL injector were analyzed. Furthermore, start-to-end simulations with multi-pulses were performed using homemade MATLAB and Parmela. The results show that continual high brightness electron bunches with a low energy spread and emittance could be obtained stably.

Cold cathode rf guns based study on field emission

Recently cold cathodes based on field emission have drawn attention and been considered to drive accelerators and free electron lasers, due to the progress in field emitter arrays and planar emitters like diamond films. In this paper, we reviewed the characteristics of field emission in rf fields. Simulations of S-band rf guns consisting of a cathode cell and a full cell were done. We showed that a shorter cathode cell with a length of 0.25--0.3 of $\ensuremath{\lambda}/2$ is in favor of obtaining both low emittance and low energy spread bunches when the amplitude of electric field on the cathode surface ranges from 60 to $80\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. A single cell test cavity has been installed to study field emission of diamond films and the measured beam current showed a good agreement with theoretical calculations.

Off line ion source terminal

The off-line ion source (OLIS) terminal provides beams from stable isotopes to ISAC (see Fig. 1) experiments as well as for accelerator commissioning and for pilot beams for radioactive beam experiments. The OLIS terminal (see Fig. 2) is equipped with a microwave driven cusp source for single and double charge ions, a surface ion source for low energy spread alkali beams, and a multi-charge ion source.

Optical Transverse Injection in Laser-Plasma Acceleration

Laser-wakefield acceleration constitutes a promising technology for future electron accelerators. A crucial step in such an accelerator is the injection of electrons into the wakefield, which will largely determine the properties of the extracted beam. We present here a new paradigm of colliding-pulse injection, which allows us to generate high-quality electron bunches having both a very low emittance (0.17 mm·mrad) and a low energy spread (2%), while retaining a high charge (~100 pC) and a short duration (3 fs). In this paradigm, the pulse collision provokes a transient expansion of the accelerating bubble, which then leads to transverse electron injection. This mechanism contrasts with previously observed optical injection mechanisms, which were essentially longitudinal. We also specify the range of parameters in which this new type of injection occurs and show that it is within reach of existing high-intensity laser facilities.

Cold atomic beam ion source for focused ion beam applications

We report measurements and modeling of an ion source that is based on ionization of a laser-cooled atomic beam. We show a high brightness and a low energy spread, suitable for use in next-generation, high-resolution focused ion beam systems. Our measurements of total ion current as a function of ionization conditions support an analytical model that also predicts the cross-sectional current density and spatial distribution of ions created in the source. The model predicts a peak brightness of 2 × 107 A m−2 sr−1 eV−1 and an energy spread less than 0.34 eV. The model is also combined with Monte-Carlo simulations of the inter-ion Coulomb forces to show that the source can be operated at several picoamperes with a brightness above 1 × 107 A m−2 sr−1 eV−1. We estimate that when combined with a conventional ion focusing column, an ion source with these properties could focus a 1 pA beam into a spot smaller than 1 nm. A total current greater than 5 nA was measured in a lower-brightness configuration of the ion source, demonstrating the possibility of a high current mode of operation.

Modeling HIF relevant longitudinal dynamics in UMER

The foremost challenge for Heavy-Ion Fusion (HIF) is achieving sufficiently low emittances and small energy spreads in the presence of intense space-charge, to achieve the high deposition densities necessary for pellet ignition. The University of Maryland Electron Ring (UMER) uses intense low-energy electron beams to access the scaled physics of HIF drivers. In particular, the long path-length propagation in UMER presents an opportunity to study, at realistic scales, the longitudinal beam dynamics and manipulations required for such a driver. With the use of induction modules, as in the ion machines such as NDCX-II, the resulting bunch dynamics show evidence of space-charge waves excited by an initial mismatch between the detailed initial beam distribution at the bunch ends and the applied focusing waveforms, persisting with multiple damped reflections propagating along the bunch flat-top. Using the induction module we are able to suppress space-charge waves with great accuracy, at amplitudes that include wave steepening prior to the formation of solitary wave trains. The longitudinal dynamics largely dominates when no containment fields are applied, coupling through the natural chromaticity of the ring even within the first turn. After subsequent turns, the bunch elongates and wraps the circumference of the machine multiple times; eventually reaching a point of instability that has also been shown through simulation, obtaining excellent agreement when the detailed longitudinal dynamics of the experiment are carefully incorporated into the model.

Shock-Front Injector for High-Quality Laser-Plasma Acceleration

We report the generation of stable and tunable electron bunches with very low absolute energy spread (ΔE ≈ 5 MeV) accelerated in laser wakefields via injection and trapping at a sharp downward density jump produced by a shock front in a supersonic gas flow. The peak of the highly stable and reproducible electron energy spectrum was tuned over more than 1 order of magnitude, containing a charge of 1-100 pC and a charge per energy interval of more than 10 pC/MeV. Laser-plasma electron acceleration with Ti:sapphire lasers using this novel injection mechanism provides high-quality electron bunches tailored for applications.

Berlin Energy Recovery Linac Project BERLinPro

The “Helmholtz-Zentrum für Materialien und Energie,” HZB, in Berlin looks back on a long and successful history in the development and operation of storage ring (SR)-based synchrotron radiation sources, starting in the early 1980s with BESSY I, followed by BESSY II in the late 1990s and the Metrology Light Source set up for the PTB (Physikalisch-Technische Bundesanstalt) in 2008. Novel concepts to better tailor the radiation properties of storage rings to the user's demands (e.g., for short pulses such as low alpha optics and femto-slicing facility) have been pioneered or improved at the HZB. Despite these efforts and the recent progress made in the development towards diffraction-limited SR, it seems right to look into alternative accelerator technologies providing high average current and short-pulsed (sub-ps) beams of exceptional brilliance and low energy spread without the fundamental restrictions of a beam circulating in an equilibrium state of the phase-space distribution.

Stable, tunable, quasimonoenergetic electron beams produced in a laser wakefield near the threshold for self-injection

Stable operation of a laser-plasma accelerator near the threshold for electron self-injection in the blowout regime has been demonstrated with 25--60 TW, 30 fs laser pulses focused into a 3--4 millimeter length gas jet. Nearly Gaussian shape and high nanosecond contrast of the focused pulse appear to be critically important for controllable, tunable generation of 250--430 MeV electron bunches with a low-energy spread, $\ensuremath{\sim}10\text{ }\text{ }\mathrm{pC}$ charge, a few-mrad divergence and pointing stability, and a vanishingly small low-energy background. The physical nature of the near-threshold behavior is examined using three-dimensional particle-in-cell simulations. Simulations indicate that properly locating the nonlinear focus of the laser pulse within the plasma suppresses continuous injection, thus reducing the low-energy tail of the electron beam.

Monte Carlo charge transport and photoemission from negative electron affinity GaAs photocathodes

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. Here, photoemission from such photocathodes is modeled using detailed Monte Carlo electron transport simulations. Simulations show a quantitative agreement with the experimental results for quantum efficiency, energy distributions of emitted electrons, and response time without the assumption of any ad hoc parameters. This agreement between simulation and experiment sheds light on the mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance.

Semiconducted symphony: ultrafast field emitter array

Semiconducted symphony: ultrafast field emitter array

Photocathode device using diamondoid and cesium bromide films

A photocathode structure is presented that shows promise for use in high brightness electron sources. The structure consists of a metal substrate, a monolayer of a diamondoid derivative, and a thin film of cesium bromide. Diamondoid monolayers reduce the energy spread of electron emitters, while cesium bromide increases the yield and stability of cathodes. We demonstrate that the combined structure retains these properties, producing an emitter with lower energy spread than the corresponding cesium bromide emitter (1.06 eV versus 1.45 eV) and higher yield and stability than un-coated diamondoid emitters.

Visualization of beams from ionic liquid ion sources for focused ion beam applications

Ionic liquid ion sources (ILIS) share qualities with liquid metal ion sources necessary for focused ion beams (FIB) operation, such as pure ion emission and low energy spreads. These sources work at lower currents, room temperature, and are able to produce both positive and negative ions from a large number of ionic liquids. The influence of the applied voltage on the beam emitted from the ILIS based on the liquid EMI-BF4 (1-ethyl 3-methyl imidazolium tetrafluoroborate) is characterized by using a beam visualization system. As the applied voltage is increased, the source transitions from single to multiple beam emission. The visualization tool is also used to determine the spatial distribution of the neutral particle population contained within the beam emitted from ILIS. Cluster ions can break during flight, yielding a new ion and neutral particles. This neutral population can be highly energetic, and might be of interest for material treatment applications where charging of targets is undesirable. For FIB applications, these neutrals could lead to undesired effects in the sample, and so it is necessary to know the distribution of the neutrals within the beam in order to specify the filtering and optics required to implement an ILIS-based FIB. It is found that 20% of the ion beam results from fragmentation events, and that the neutral population is concentrated at the center of the beam.

Tuning Fifth-Order Aberrations in a Quadrupole-Octupole Corrector

The resolution of conventional electron microscopes is usually limited by spherical aberration. Microscopes equipped with aberration correctors are then primarily limited by higher order, chromatic, and misalignment aberrations. In particular the Nion third-order aberration correctors installed on machines with a low energy spread and possessing sophisticated alignment software were limited by the uncorrected fifth-order aberrations. Here we show how the Nion fifth-order aberration corrector can be used to adjust and reduce some of the fourth- and fifth-order aberrations in a probe-corrected scanning transmission electron microscope.