Short Bunches Research Articles

Spontaneous Cyclotron Radiation of a Dense Electron Bunch

Short dense electron bunches generated by today’s photoinjectors [1–5] are advantageous for creating powerful sources of spontaneous coherent radiation in the terahertz frequency range. The regime of spontaneous radiation is achieved when the bunch size in the phase space is well below 2π. Coulomb interaction between particles in the dense bunches increases the axial size, and thus can limit the power and duration of radiation. It is shown this problem can be solved for cyclotron radiation by compensating for the Coulomb repulsion in the phase space.

A cryogenic linear ion trap beamline for providing keV ion bunches.

A new cryogenic linear ion trap beamline has been constructed and commissioned, which serves to inject cold molecular and cluster ions into the RIKEN cryogenic electrostatic ring (RICE). Ions are created with an electrospray ion source, and a quadrupole mass filter is used for mass-selection prior to trap injection. The radio frequency octupole ion trap can be continuously loaded with ions and features a fast ion extraction mode to create short ion bunches with tens of μs duration. We report here on the simulations and development of the ion trap beamline and validate performance with the moderately heavy molecular cation methylene blue. Characterization of the novel trap design with additional wedge-shaped electrodes was carried out, which includes the determination of the temporal and spatial shape of the ion bunch and the total number of ions after extraction. Finally, these ion bunches are synchronized with the switching of a pulsed high-voltage acceleration device downstream of the trap, where the ions obtain a kinetic energy of up to 20 keV. The preparation and control of the keV ion beam are demonstrated for the ion injection into RICE.

Undulator radiation of premodulated and nonmodulated electron bunches in the negative mass instability regime

The use of both spatial premodulation and negative mass instability is promising for the energy enhancement and spectrum narrowing of the undulator radiation from dense electron bunches formed in modern laser-driven photoinjectors. Combining these two methods will permit the formation of long-lived trains of short and dense bunches with very large charges, which are able to efficiently emit a much more powerful and narrow-band terahertz radiation than single bunches with the same total charge, or premodulated bunches in the positive mass oscillation regime. The development of radiation instability in the negative mass regime can enable significant quasiperiodic self-modulation of relatively long nonmodulated bunches resulting in their fairly efficient and selective terahertz superradiance.

Electron beamline of a Linac-based injector applied to a compact free electron laser-terahertz radiation source

Required by high-power, continuously tunable terahertz (THz) radiation from a room-scale source, a Linac-based beam injector was developed, which aimed at achieving a tradeoff between performance and cost efficiency. Under the overall consideration of compactness, design choices for the injector involving short bunch generation, energy boosting, as well as performance improvement were described in detail. A testing bench with online diagnostic elements was refitted from a prototype of such a Linac-driven THz radiation source, and measurement results indicated that such an injector showed an outstanding advantage in terms of system miniaturization without negatively affecting beam performance.

Beam dynamics studies for beam focusing and solenoid alignment at SINBAD

SINBAD facility (Short INnovative Bunches and Accelerators at DESY) under construction at DESY plans to host several experiments for the production of ultra-short bunches and will be a test facility for high-gradient compact novel acceleration techniques. The ARES (Accelerator Research Experiment at SINBAD) linac is foreseen to produce ultra-short bunches to be injected e.g. into Novel Dielectric Laser Acceleration structures or Laser Wake-Field Acceleration experiments. The work presented in this paper is based on optimization of the focusing system consisting of solenoids for the ARES, which have been studied earlier in detail but is revisited for updated beamline. Moreover tolerances for the possible misalignment of solenoids are presented investigating the effect on the beam properties during the gun commissioning.

Transport and analysis of electron beams from a laser wakefield accelerator in the 100 MeV energy range with a dedicated magnetic line

Electron bunches generated by laser driven wakefield acceleration are transported and analyzed using a magnetic line composed of a triplet of quadrupoles and a dipole. Short pulse bunches with a total charge of ≈130pC, and broad band energy spectra in the range 45 to 150MeV are generated by ionization assisted injection in a gas cell. The electron source is imaged about one meter away from the exit of the gas cell by the magnetic line, delivering electron bunches at a stable position in the image plane where a charge density of ≈2.9pC∕mm2 at an energy of 69.4±0.6MeV is achieved. This magnetic line improves dramatically the accuracy of energy determination of this electron source, leading to an energy error as low as 8.6‰ in the 70MeV range for 5mrad divergence electron bunch and considering the resolution of the entire detection system. The transport of bunches with improved stability and energy selection paves the way to various applications including multi-stage laser plasma acceleration.

Coherent transition radiation from femtosecond electron bunches at the accelerator-based THz light source in Thailand

A THz radiation source based on electron linear accelerator (linac) has been built and commissioned at the Plasma and Beam Physics (PBP) Research Facility, Chiang Mai University (CMU) in Thailand since 2005. The accelerator system consists of a thermionic cathode RF electron gun, a magnetic bunch compressor in a form of an alpha-magnet, an S-band travelling-wave linac structure, beam steering and focusing magnets, beam and radiation diagnostic instruments, control system and other support components. Electron bunches with an average kinetic energy of about 8 MeV, a bunch charge of 100 pC, and a bunch length of 300 fs can be produced from the PBP-CMU electron linac system. Coherent THz radiation generated via transition radiation technique is used as a source of the THz spectroscopy and THz imaging applications. The measured power spectrum indicates that the radiation obtained from short electron bunches covers the frequency range of 0.15–1.80 THz or the wavenumber range of 5–60 cm−1. Experimental results reveal that a peak radiation power as high as 0.2 MW per micropulse and an average radiation power of 2.5 mW can be obtained. Generation and characterization of the coherent THz transition radiation as well as some examples of experiments to achieve electron bunch length and radiation power spectrum are reported and discussed in this paper.

Wake measurements of a dechirper jaw with nonzero tilt angle

The RadiaBeam/SLAC dechirper at the Linac Coherent Light Source (LCLS) is being used as a fast kicker, by inducing transverse wakefields, to e.g. facilitate Fresh-slice, two-color laser operation. The dechirper jaws are independently adjustable at both ends, and it is difficult to avoid leaving residual (longitudinal) tilt in them during set-up. In this report we develop a model independent method of removing unknown tilt in a jaw. In addition, for a short uniform bunch passing by a single dechirper plate, we derive an explicit analytical formula for the transverse wake kick as function of average plate offset and tilt angle. We perform wake kick measurements for the different dechirper jaws of the RadiaBeam/SLAC dechirper, and find that the agreement between measurement and theory is excellent.

A THz Spectroscopy System Based on Coherent Radiation from Ultrashort Electron Bunches

A spectroscopy system will be discussed for coherent THz transition radiation emitted from short electron bunches, which are generated from a system consisting of an RF gun with a thermionic cathode, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator for post-acceleration. The THz radiation is generated as backward transition radiation when electron bunches pass through an aluminum foil. The emitted THz transition radiation, which is coherent at wavelengths equal to and longer than the electron bunch length, is coupled to a Michelson interferometer. The performance of the spectroscopy system employing a Michelson interferometer is discussed. The radiation power spectra under different conditions are presented. As an example, the optical constant of a silicon wafer can be obtained using the dispersive Fourier transform spectroscopy (DFTS) technique.

Design and optimization of a compact laser-driven proton beamline

Laser-accelerated protons, generated by irradiating a solid target with a short, energetic laser pulse at high intensity (I > 1018 W·cm−2), represent a complementary if not outperforming source compared to conventional accelerators, due to their intrinsic features, such as high beam charge and short bunch duration. However, the broadband energy spectrum of these proton sources is a bottleneck that precludes their use in applications requiring a more reduced energy spread. Consequently, in recent times strong effort has been put to overcome these limits and to develop laser-driven proton beamlines with low energy spread. In this paper, we report on beam dynamics simulations aiming at optimizing a laser-driven beamline - i.e. a laser-based proton source coupled to conventional magnetic beam manipulation devices - producing protons with a reduced energy spread, usable for applications. The energy range of investigation goes from 2 to 20 MeV, i.e. the typical proton energies that can be routinely obtained using commercial TW-power class laser systems. Our beamline design is capable of reducing the energy spread below 20%, still keeping the overall transmission efficiency around 1% and producing a proton spot-size in the range of 10 mm2. We briefly discuss the results in the context of applications in the domain of Cultural Heritage.

Heating of a Thin Crystal Target at the Passage of High-Energy Short Electron Bunches

A theoretical model is developed for the heating and deformation of a thin target at the passage of short electron bunches through it at the energies of modern free electron lasers, and the corresponding simulation is carried out. It is demonstrated that under these conditions, the target can undergo overheating or be irreversibly deformed, which makes it difficult to use such targets for the diagnostics of free electron beams used in modern lasers.

Evolution of dense spatially modulated electron bunches

An analytical theory describing the dynamics of relativistic moving 1D electron pulses (layers) with the density modulation affected by a space charge has been revised and generalized for its application to the formation of dense picosecond bunches from linear accelerators with laser-driven photo injectors, and its good agreement with General Particle Tracer simulations has been demonstrated. Evolution of quasi-one-dimensional bunches (disks), for which the derived formulas predict longitudinal expansion, is compared with that for thin and long electron cylinders (threads), for which the excitation of non-linear waves with density spikes was found earlier by Musumeci et al. [Phys. Rev. Lett. 106(18), 184801 (2011)] and Musumeci et al. [Phys. Rev. Spec. Top. --Accel. Beams 16(10), 100701 (2013)]. Both types of bunches can be used for efficiency enhancement of THz sources based on the Doppler frequency up-shifted coherent spontaneous radiation of electrons. Despite the strong Coulomb repulsion, the periodicity of a preliminary modulation in dense 1D layers persists during their expansion in the most interesting case of a relatively small change in particle energy. However, the period of modulation increases and its amplitude decreases in time. In the case of a large change in electron energy, the uniformity of periodicity is broken due to different relativistic changes in longitudinal scales along the bunch: the “period” of modulation decreases and its amplitude increases from the rear to the front boundary. Nevertheless, the use of relatively long electron bunches with a proper preliminary spatial modulation of density can provide a significantly higher power and a narrower spectrum of coherent spontaneous radiation of dense bunches than in the case of initially short single bunches with the same charge.

Seeding of the self-modulation in a long proton bunch by charge cancellation with a short electron bunch

In plasma wakefield accelerators (e.g. AWAKE) the proton bunch self-modulation is seeded by the ionization front of a high-power laser pulse ionizing a vapour and by the resulting steep edge of the driving bunch profile inside the created plasma.In this paper, we present calculations in 2D linear theory for a concept of a different self-modulation seeding mechanism based on electron injection. The whole proton bunch propagates through a preformed plasma and the effective beam current is modulated by the external injection of a short electron bunch at the centre of the proton beam. The resulting sharp edge in the effective beam current in the trailing part of the proton bunch is driving large wakefields that can lead to a growth of the seeded self-modulation (SSM).Furthermore, we discuss the feasibility for applications in AWAKE Run 2.

Tracking code simulation for passive harmonic cavity in the SSRF storage ring

A passive third harmonic cavity is planned to install in the Shanghai Synchrotron Radiation Facility (SSRF) storage ring to increase the beam lifetime. Transient beam loading effect generated by the gap between the bunch trains can suppress the bunch lengthening effect of the third harmonic cavity. Besides, the field excited in the passive operation of the harmonic cavity depends on the bunch distribution. To estimate the effect of transient beam loading, the tracking codes have been developed. The tracking simulation has been performed for three uniform filling cases and a nonuniform filling case. The lengthened bunch distribution has also been included into tracking code simulation. The effect of the lengthened bunch has also been investigated. The uniform filling pattern with shorter bunch trains shows lower transient beam loading effect and can give much higher bunch lengthening factors compared to those of non-uniform filling case. The effect caused by the reduction of the harmonic voltage generated by lengthened bunch distribution causes a great reduction on bunch lengthening effect in uniform filling case, while the lengthening factor of non-uniform filling case does not affect much in this issue. The tracking codes have been developed to estimate the effect of transient beam loading for three uniform filling cases and a non-uniform filling case. The larger gap in the filling pattern can cause the higher transient beam loading effect and lower bunch lengthening effect. The lengthened bunch distribution has also been included into tracking code simulation. The effect of the lengthened bunch has also been investigated. The more the bunch is lengthened, the greater reduction on bunch lengthening effect.

Analytical model and error analysis of arbitrary phasing technique for bunch length measurement

An analytical model of an RF phasing method using arbitrary phase scanning for bunch length measurement is reported. We set up a statistical model instead of a linear chirp approximation to analyze the energy modulation process. It is found that, assuming a short bunch (σφ∕2π→0) and small relative energy spread (σγ∕γr→0), the energy spread (Y=σγ2) at the exit of the traveling wave linac has a parabolic relationship with the cosine value of the injection phase (X=cosφr|z=0), i.e., Y=AX2+BX+C. Analogous to quadrupole strength scanning for emittance measurement, this phase scanning method can be used to obtain the bunch length by measuring the energy spread at different injection phases. The injection phases can be randomly chosen, which is significantly different from the commonly used zero-phasing method. Further, the systematic error of the reported method, such as the influence of the space charge effect, is analyzed. This technique will be especially useful at low energies when the beam quality is dramatically degraded and is hard to measure using the zero-phasing method.

Full PIC simulation of a first ACHIP experiment @ SINBAD

In laser illuminated dielectric accelerators (DLA) high acceleration gradients can be achieved due to high damage thresholds of the materials at optical frequencies. This is a necessity for developing more compact particle accelerator technologies. The Accelerator on a CHip International Program (ACHIP) funded by the Gordon and Betty Moore Foundation is researching such devices. DESY Hamburg is part of the collaboration. The dedicated accelerator research facility SINBAD is particularly well suited for DLA experiments at relativistic electron energies. High quality beams and short bunch lengths are anticipated from the ARES linac which is currently under construction at SINBAD. The aim of the experiment is the injection of a short electron bunch from the ARES linac into a DLA. In this study the results of one of the first possible experiments at the facility are estimated via a combination of particle-in-cell (PIC) and tracking simulations. ASTRA is used to simulate an electron bunch from the ARES linac at a suitable working point. The dielectric part of the setup will be simulated using the PIC code from CST Particle Studio incorporating the retrieved bunch from the ASTRA simulation. The energy spectra of the electron bunches are calculated as would be measured from a spectrometer dipole with and without the laser fields.

Lattice design and start-to-end simulations for the ARES linac

The SINBAD (Short INnovative Bunches and Accelerators at DESY) project is a dedicated, long-term accelerator research and development facility currently under construction at the DESY Hamburg campus, aiming to provide an infrastructure for developing ultrafast science and several types of novel high-gradient accelerators. We present the design of the magnetic lattice as well as the start-to-end simulation results for ultrashort electron bunch generation at the ARES (Accelerator Research Experiment at SINBAD) linac, which is one of the two experiments at the SINBAD facility. In order to meet the requirements for external injection into high-gradient accelerators, the ARES linac was optimized to provide ∼100 MeV, low charge (0.5 to 30 picocoulombs) and ultrashort electron bunches (sub-femtosecond to dozens of femtoseconds) with ultrasmall spot sizes (less than a few micrometers) and excellent timing stability (rms bunch arrival-time jitter <10 femtoseconds).

Simulation of deflecting structures for dielectric laser driven accelerators

In laser illuminated dielectric accelerators (DLA) high acceleration gradients can be achieved, due to high damage thresholds of the materials at optical frequencies. This is a necessity in developing more compact particle accelerator technologies. The Accelerator on a CHip International Program funded by the Gordon and Betty Moore Foundation is researching such devices. Means to manipulate the beam, i.e. focusing and deflection, are needed for the proper operation of such devices. These means should rely on the same technologies for manufacturing and powering like the accelerating structures. In this study different concepts for dielectric laser driven deflecting structures are investigated via particle-in-cell (PIC) simulations and compared afterwards. The comparison is conducted with respect to the suitability for beam manipulation. Another interesting application will be investigated as a diagnostic device for ultra short electron bunches from conventional accelerators functioning like a radio frequency transverse deflecting cavity (TDS).

Simulation study of an LWFA-based electron injector for AWAKE Run 2

The AWAKE experiment aims to demonstrate preservation of injected electron beam quality during acceleration in proton-driven plasma waves. The short bunch duration required to correctly load the wakefield is challenging to meet with the current electron injector system, given the space available to the beamline. An LWFA readily provides short-duration electron beams with sufficient charge from a compact design, and provides a scalable option for future electron acceleration experiments at AWAKE. Simulations of a shock-front injected LWFA demonstrate a 43 TW laser system would be sufficient to produce the required charge over a range of energies beyond 100 MeV. LWFA beams typically have high peak current and large divergence on exiting their native plasmas, and optimisation of bunch parameters before injection into the proton-driven wakefields is required. Compact beam transport solutions are discussed.

Coherent backward transition radiation from sub-fs “pancake-like” bunches as a tool for beam diagnostics

Coherent transition radiation (CTR), produced by short electron bunches, is a well-known radiation mechanism which is applied for bunch length monitoring. By analyzing the CTR spectrum it is possible to reconstruct the longitudinal bunch shape. Such a technique is widely used for accelerators with electron beams possessing bunch lengths σz much larger than their transverse beam sizes σx,y (with σx,y,z the rms transverse and longitudinal beam sizes). New developments in the field of accelerator technology allow to produce and accelerate bunches with a duration of less than 1 fs with a “pancake-like” bunch shape, i.e. with σz ≪ σx,y. Diagnostic schemes of this kind of bunches give rise to interesting effects, for example connected with a transverse asymmetry (σx ≠ σy), with a tilting of the bunches (i.e. the bunch axis does not coincide with the velocity direction), and even more. Spectral-angular distributions of coherent backward transition radiation (BTR) were simulated taking into account a possible bunch tilt. It is shown that spectral intensity measurements of coherent BTR emitted in backward direction allow to obtain information about the bunch length.