Drive Laser System Research Articles

Advanced drive laser system for a high-brightness continuous-wave photocathode electron gun.

In order to enhance the performance of a continuous-wave photocathode electron gun at Peking University, and to achieve electron beams with higher current and brightness, a multifunctional drive laser system named PULSE (Peking University drive Laser System for high-brightness Electron source) has been developed. This innovative system is capable of delivering an average output power of 120 W infrared laser pulse at 81.25 MHz, as well as approximately 13.8 W of green power with reliable stability. The utilization of two stages of photonic crystal fibers plays a crucial role in achieving this output. Additionally, the incorporation of two acousto-optic modulators enables the selection of macro-pulses with varying repetition frequencies and duty cycles, which is essential for effective electron beam diagnosis. Furthermore, the system employs a series of birefringent crystals for temporal pulse shaping, allowing for stacking Gaussian pulses into multiple types of distribution. Overall, the optical schematic and operating performance of PULSE are detailed in this paper.

Partition of Omega-like facility into two configurations of 24 and 36 laser beams to improve implosion performance

An Omega-like beam configuration is considered where the 60-beam layout can be separated into two independent sub-configurations with 24 and 36 laser beams, each minimizing direct drive illumination non-uniformity. Two different laser focal spot profiles, one associated with each configuration, are proposed to apply the zooming technique in order to increase the laser-target coupling efficiency. This approach is used by 1D hydrodynamics simulations of the implosion of a direct-drive capsule characterized by a relatively large aspect ratio A = 7 and an optimized laser pulse shape delivering a maximum of 30 TW and 30 kJ, with different temporal pulse shapes in each of the two sets of beams. It is shown that zooming allows for an optimistic 1D thermonuclear energy gain greater than one while without zooming the thermonuclear gain remains largely below one. While this is incompatible with the as-built Omega laser, it provides a promising option for a future intermediate-energy direct drive laser system.

Toward using collective x-ray Thomson scattering to study C–H demixing and hydrogen metallization in warm dense matter conditions

The insulator–metal transition in liquid hydrogen is an important phenomenon to understand the interiors of gas giants, such as Jupiter and Saturn, as well as the physical and chemical behavior of materials at high pressures and temperatures. Here, the path toward an experimental approach is detailed based on spectrally resolved x-ray scattering, tailored to observe and characterize hydrogen metallization in dynamically compressed hydrocarbons in the regime of carbon–hydrogen phase separation. With the help of time-dependent density functional theory calculations and scattering spectra from undriven carbon samples collected at the European x-ray Free-Electron Laser Facility (EuXFEL), we demonstrate sufficient data quality for observing C–H demixing and investigating the presence of liquid metallic hydrogen in future experiments using the reprated drive laser systems at EuXFEL.

High Beam Quality and High Power Mode-locked Solid-state Laser for Photocathode Drive Laser System

High Beam Quality and High Power Mode-locked Solid-state Laser for Photocathode Drive Laser System

Theoretical modeling and analyzing structural characteristics of AlGaAs/GaAs negative electron affinity array cathode with optically and electrically injected variable bandgap

In order to obtain high emission current efficiency of the AlGaAs/GaAs NEA array cathode, this array cathode has two ways to form electron emission, i.e. optical injection and electrical injection. The two-dimensional continuity equation of electronic transport in the variable bandgap emission array is solved numerically by using the finite volume method thereby obtaining the emission current and emission current efficiency. Simulation obtains the optimal parameter range for each layer of the AlGaAs/GaAs NEA array cathode under both optical injection and electrical injection. The results show that the optimal angle of incident light for the array cathode under light injection is 10°–30° for selecting an array micro-nano column with a duty cycle of 2/3. Under the condition of light injection, the P-type variable bandgap AlGaAs layer array micro-nano column height ranges from 0.3–0.6 μm. Under the condition of electrical injection, the height of the micro-nano column of the P-type variable bandgap AlGaAs layer array is 0.1–0.3 μm. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under light injection are 0.5–2.5 μm, 0.5–1.0 μm and 0.2–0.5 μm, respectively. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under electrical injection conditions are 2–3 μm, 0.8–1.2 μm and 0.1–0.3 μm, respectively. The optimal doping concentration of P-type AlGaAs layer and N-type AlGaAs layer under light injection are range from 5×10<sup>18</sup> to 1×10<sup>19</sup> cm<sup>–3</sup> and from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup>, respectively. The optimal doping concentration of the P-type AlGaAs layer and the N-type AlGaAs layer under electrical injection range from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup> and from 5×10<sup>17</sup> to 1×10<sup>18</sup> cm<sup>–3</sup>, respectively. The maximum efficiency of the emission current under the light injection is 35.04%, and the maximum emission current per unit length is 10.3 nA/μm. The maximum efficiency of the emission current under electrical injection is 31.23%, and the maximum emission current per unit length is 105.5 μA/μm. Electric injection cathode does not need expensive and complex drive laser system, and the light injection control mode is simple, so light injection or electrical injection control mode can be chosen as needed. The research of array cathode, which integrates the advantages of many advanced technologies, is of great significance for enriching the cathode array cathode emission theory and expanding its application field.

High power drive laser system for photocathode at IHEP.

The photocathode drive laser system in the Institute of High Energy Physics (IHEP) has been upgraded. An all-fiber drive laser system has been developed using photonic crystal fibers and photonic crystal rods as the main gain medium. This system has been operated stably. The output infrared (IR) power reaches 116.2 W. The pulse width and maximum output power of the green laser generated by the second harmonic generation (SHG) are less than 2 ps and about 39.4 W, respectively. The SHG efficiency exceeds 60%. This paper introduces the development of the drive laser system and reports the measurement results of the performance test.

JuSPARC - The Jülich Short-Pulsed Particle and Radiation Center

JuSPARC, the Jülich Short-Pulsed Particle and Radiation Center, is a laser-driven facility to enable research with short-pulsed photon and particle beams to be performed at the Forschungszentrum Jülich. The conceptual design of JuSPARC is determined by a set of state-of-the-art time-resolved instruments, which are designed to address the electronic, spin, and structural states of matter and their dynamic behaviour. From these instruments and experiments JuSPARC derives the need of operating several dedicated high pulse-power laser systems at highest possible repetition rates. They serve as core units for optimized photon up-conversion techniques generating the light pulses for the respective experiments. The applications also include experiments with spin polarized particle beams, which require the use of laser-based polarized gas targets. Thus, in its rst stage JuSPARC comprises four driving laser systems, called JuSPARC_VEGA, JuSPARC_DENEB, JuSPARC_SIRIUS and JuSPARC_MIRA, which are outlined in this article.

Commissioning the photoinjector of a gamma-ray light source

The Xian gamma-ray light source (XGLS) is a proposed novel high brightness gamma-ray source based on inverse Compton scattering (ICS) between the high brightness electron and laser beams. The photoinjector of the accelerator is one of the main components that determines the electron beam quality and related gamma-ray parameters [Phys. Rev. Accel. Beams 21, 030701 (2018); 20, 080701 (2017); 8, 100702 (2005)]. In order to reduce the risk of this project, a photoinjector including a laser-driven photocathode rf gun, two S-band tubes, and a diagnostic section was constructed and installed from 2016, serving as a pilot plant for the development of the key technologies for the projects. The injector is designed to deliver high quality electron bunches of typically 500 pC charge and $\ensuremath{\sim}120\text{ }\text{ }\mathrm{MeV}$ energy with low emittance and jitter at 10 Hz repetition. Initial system commissioning with an electron beam was finished in the end of 2017, with a goal of a stable 0.6--0.7 mm mrad emittance in a 500-pC bunch clearly demonstrated. In this paper, we report the experimental results and experience obtained in the commissioning, including rf gun, drive laser system, timing and synchronization system, and beam diagnostics.

Amplification of flat laser pulse train.

We present modeling and measurements of flattop amplification of a laser pulse train in a diode pumped Nd:YLF system. We establish a theoretical model, accounting for the transverse Gaussian shape of an amplified laser beam, in order to explain remaining slopes in the pulse train energy. The influence of the transverse Gaussian shape on the train's flatness has been experimentally verified. Based on the model we are able to increase the total amplification of a long train of infrared seed beam in the drive laser system at the Fermilab Accelerator Science and Technology facility. The single-pass amplifier improvements resulted in a gain of ∼7 with flat output pulse train for up to 1000 seed pulses.

Spatio-spectral structures in high harmonic generation driven by tightly focused high repetition rate lasers

We investigate the spatio-spectral properties of extreme ultraviolet (XUV) high harmonic radiation driven by high repetition rate femtosecond laser systems. In the spatio-spectral domain, ring-shaped structures at each harmonic order associated with long-trajectory electrons are found to form arrow-shaped structures at the cutoff. These structures are observed with two different laser systems: an optical parametric chirped-pulse amplifier system at a central wavelength of 1.55 μm and 125 kHz repetition rate, and a temporally compressed femtosecond ytterbium fiber amplifier at 1.03 μm wavelength and 100 kHz repetition rate. As recently pointed out, the observed structures are well explained by considering the space–time atomic dipole-induced phase for short and long electron trajectories in the generation plane. The tighter focusing geometry and longer wavelength associated with these emerging driving laser systems increase the ring-like divergence and spectral broadening for high harmonics. Cutoff energies and photon fluxes obtained in argon and neon are also reported. Overall, these results shed new light on the properties of XUV radiation driven by these recently developed high average power laser systems, paving the way to high photon-flux XUV beamlines.

Drive laser system for the DC-SRF photoinjector at Peking University**Supported by National Basic Research Project (973) (2011CB808302, 2011CB808304)

Photoinjectors are widely used for linear accelerators as electron sources to generate high-brightness electron beams. The drive laser, which determines the timing structure and quality of the electron beam, is a crucial component of a photoinjector. A new drive laser system has been designed and constructed for the upgraded 3.5-cell DC-SRF photoinjector at Peking University. The drive laser system consists of a 1064 nm laser oscillator, a four-stage amplifier, second and fourth harmonic generators, an optical system to transfer the UV pulses to the photocathode, and a synchronization system. The drive laser system has been successfully applied during stable operation of the DC-SRF photoinjector and its performance meets requirements. A 266 nm laser with an average power close to 1 W can be delivered to illuminate the Cs2Te photocathode and the instability is less than 5% for long time operation. The design considerations for improving the UV laser quality, a detailed description of the laser system, and its performance are presented in this paper.

Development of fiber laser system for a photocathode gun

It is difficult to find commercial laser products meeting the requirements of ERL photocathode gun. Therefore, a drive laser system is being developed at Institute of High Energy Physics (IHEP), which comprises two kinds of laser seeds, 1.3 GHz and 100 MHz at repetition rate. Fiber amplifiers were being constructed and a test system was set up. The output power of the main amplifier can reach up to 27 W. With a 5-mm-long LBO crystal, the green second harmonic laser of 6 W was achieved in the preliminary validation experiment. ©, 2015, Editorial Office of High Power Laser and Particle Beams. All right reserved.

Photocathodes for High Brightness Photo Injectors

The development of the photo injector has become a key technology for accelerator-based light sources and for the electron collider. There are a lot of opportunities to improve the injector quality, as well as photocathode function. A better photocathode can reduce the specification requirements of the drive laser system. The identification of better photocathodes is always a principal technical challenge, especially for the high brightness injectors. In this contribution, we will briefly review the status of photocathode research and the developing trends that are foreseen for the future.

Impact of the spatial laser distribution on photocathode gun operation

It is widely believed that a drive laser with uniform temporal and spatial laser profiles is required to generate the lowest emittance beam at the photoinjector. However, for a given 3 ps smooth-Gaussian laser temporal profile, our recent simulations indicate that a truncated-Gaussian laser spatial profile produces an electron beam with smaller emittance. The simulation results are qualitatively confirmed by later analytical calculation, and also confirmed by measurements: emittance reduction of $\ensuremath{\sim}25%$ was observed at the linac coherent light source (LCLS) injector with a truncated-Gaussian laser spatial profile at the nominal operating bunch charge of 150 pC. There was a significant secondary benefit---laser transmission through the iris for the truncated-Gaussian profile was about twice that compared to the nearly uniform distribution, which significantly loosens the laser power and quantum efficiency requirements for drive laser system and photocathode. Since February 9, 2012, the drive laser with the truncated-Gaussian spatial distribution has been used for LCLS routine user operations and the corresponding free electron laser power is at least the same as the one when using the nearly uniform spatial profile.

Status and perspectives of superconducting radio-frequency gun development for BERLinPro

As part of the BERLinPro study, HZB is developing an SRF photoelectron injector. The R&D will be carried out in three stages, the first of which is currently being installed at HZB's HoBiCaT facility. It consists of an SRF cavity with SC solenoid, electron beam diagnosics and drivelaser systems.

Photo-injector study for the ELETTRA linac FEL

A new injector is one of the necessary upgrades to be made in the existing ELETTRA linac to improve its performance to the level required for FEL operation. To this end, the design of an RF photo-injector is in progress at ELETTRA. Several types of RF guns are currently under consideration. The drive laser system, beam optics, diagnostics, and other relevant issues are also being addressed.

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.

Mode locking in coherently driven laser systems

A new concept of mode locking for a three-level gain medium with two coherent fields is proposed. The strong field applied on the coupling transition produces broadening of the lasing transition. The broadened emission spectrum allows one to generate pulses with durations far beyond limits inherent to a conventional mode-locking technique.

Output energy fluctuations of the Nd:YAG amplifier chain of a photoinjector drive laser system

In the Etude d'un LaSer Accordable, electron bunches consist of trains of picosecond pulses extracted from a photocathode by a drive laser system: This system consists of a mode-locked Nd:YAG oscillator followed by a pulse compressor, an amplifier chain, and a second-harmonic-generation stage. The performance of the linac critically depends on the energy stability of this system. It is demonstrated theoretically and experimentally that the energy fluctuations of the Nd:YAG oscillator are significantly reduced by the amplifier chain, that the remaining energy fluctuations of the whole system are mainly due to amplifier pump fluctuations, and that the amplifier chain can be optimized to reduce the global energy fluctuations from 1.5% rms to <1% rms.

The Neptune photoinjector

The RF photoinjector in the Neptune advanced accelerator laboratory, along with associated beam diagnostics, transport and phase-space manipulation techniques are described. This versatile injector has been designed to produce short-pulse electron beams for a variety of uses: ultra-short bunches for injection into a next-generation plasma beatwave acceleration experiment, 2 C. Clayton et al., these proceedings. 2 space-charge dominated beam physics studies, plasma wake-field acceleration driver, plasma lensing, and free-electron laser microbunching techniques. The component parts of the photoinjector, the RF gun, photocathode drive laser systems, booster linac, RF system, chicane compressor, beam diagnostic systems, and control system, are discussed. The present status of photoinjector commissioning at Neptune is reviewed, and proposed experiments are detailed.