[en] Photocathode electron guns are key to the generation of high-quality electron bunches, which are currently the primary source of electrons for linear electron accelerators. The photogun test bench built at the Joint Institute for Nuclear Research (JINR) is currently being used to further develop the hollow (backside irradiated) photocathode concept. A major achievement was the replacement of the hollow photocathode by a technologically more feasible transmission photocathode made from a metal mesh that serves as a substrate for films of various photomaterials. A number of thin-film cathodes on quartz glass substrates are fabricated by photolithography. The vectorial photoeffect (related to the surface-normal component of the wave electric field) is observed and found to significantly affect the quantum efficiency. The dependence of the quantum efficiency of diamond-like carbon photocathodes on the manufacturing technology is investigated. The Rutherford backscattering and elastic recoil detection techniques are combined to carry out an elemental analysis of the films. An estimate of the emittance of a 400 pC electron beam is obtained using the cross-section method. (conferences and symposia)

[en] We propose a method for laser pulse temporal shaping applying just an opaque solid 2D mask as a shaping element. The research is motivated by recent experimental data on temporal distortions produced in chirped picosecond pulses by spatial light modulators (SLMs) commonly used in shaping schemes and consequent demand for alternative solutions.

In the presented scheme, the effect of grey-level masking in the spectral domain is achieved by dimensional reduction from 2D shaping on air to 1D obtained modulation at the fiber output. The mask is placed in the Fourier plane of a compressor with zero frequency dispersion. Instead of a spherical telescope in the compressor, a cylindrical one is used. Due to large linear chirp in the input pulse, spectral profile corresponds to the temporal distribution.

We demonstrated in the experiment conversion of a 40 ps bell-shaped pulse into a rectangular pulse, demanded, for example, in photoinjector technology. Our scheme showed itself as a cheap and easy alternative to SLM shaping, therefore we believe it can be interesting for some applications. (paper)

[en] The use of high brightness electron beams in Free Electron Laser (FEL) applications is of increasing importance. One of the most promising methods to generate such beams is the usage of shaped photocathode laser pulses. It has already demonstrated that temporal and transverse flat-top laser pulses can produce very low emittance beams [1]. Nevertheless, based on beam simulations further improvements can be achieved using quasi-ellipsoidal laser pulses, e.g. 30% reduction in transverse projected emittance at 1 nC bunch charge. In a collaboration between DESY, the Institute of Applied Physics of the Russian Academy of Science (IAP RAS) in Nizhny Novgorod and the Joint Institute of Nuclear Research (JINR) in Dubna such a laser system capable of producing trains of laser pulses with a quasi-ellipsoidal distribution, has been developed. The prototype of the system was installed at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and is currently in the commissioning phase. In the following, the laser system will be introduced, the procedure of pulse shaping will be described and the last experimental results will be shown.

[en] The linear characteristics of polyethylene terephthalate, such as index of refraction, spectral transmittance and transverse distribution of phase and polarization distortions, have been measured. Spectrum broadening of laser pulses (intensity over 1.3 TW cm⁻²) after propagation through a 0.7 mm thick sample has been demonstrated in experiment. Taking into consideration almost unlimited aperture, submillimeter thickness and the low cost of polyethylene terephthalate, the obtained results demonstrate that it is a prospective material to be used for spectrum broadening and subsequent time compression of petawatt laser pulses to single cycle regime (Mourou et al 2014 Single cycle thin film compressor opening the door to zeptosecond-exawatt physics Eur. Phys. J. Spec. Top. 223 1181–8). (letter)

[en] Methods for the spatio-temporal shaping of photocathode laser pulses for generating high brightness electron beams in modern linear accelerators are discussed. The possibility of forming triangular laser pulses and quasi-ellipsoidal structures is analyzed. The proposed setup for generating shaped laser pulses was realised at the Institute of Applied Physics (IAP) of the Russian Academy of Sciences (RAS). Currently, a prototype of the pulse-shaping laser system is installed at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). Preliminary experiments on electron beam generation using ultraviolet laser pulses from this system were carried out at PITZ, in which electron bunches with a 0.5-nC charge and a transverse normalized emittance of 1.1 mm mrad were obtained. A new scheme for the three-dimensional shaping of laser beams using a volume Bragg profiled grating is proposed at IAP RAS and is currently being tested for further electron beam generation experiments at the PITZ photoinjector. (conferences and symposia)

[en] Beam driven wakefield acceleration is one of the most advanced novel accelerator concepts. This process occurs in a plasma or a slow-wave structure in which the drive beam deposits energy in the form of a wake and the main beam is accelerated by this wake. For efficient acceleration, it is important to shape the drive beam current and to increase the so-called transformer ratio: the energy gain by the main beam to the energy loss of the drive beam. Triangular bunch shapes are among the most promising distributions considered for increasing the transformer ratio. In most cases, the drive beam is shaped with the help of electron beam phase space manipulation along with removal of portions of the beam with a scraper. This type of shaping is complicated by itself, causing charge loss and thermal heating, but also leads to beam instabilities due to the altered phase space of the beam. In this paper, we demonstrate experimentally the shaping of picosecond laser pulses in the form of a triangle with the help of a spatial light modulator. Such laser pulses can be used to generate a triangular electron bunch from a photoinjector. Considerations on the required pulse profile, comparison to other shaping methods, experimental results, and numerical analysis of the impact of pulse shaper parameters on the reproducibility of the triangular laser pulse shape are presented. (letter)

[en] A method for shaping photocathode laser driver pulses into 3D ellipsoidal form has been proposed and implemented. The key idea of the method is to use a chirped Bragg grating recorded within the ellipsoid volume and absent outside it. If a beam with a constant (within the grating reflection band) spectral density and uniform (within the grating aperture) cross-section is incident on such a grating, the reflected beam will be a 3D ellipsoid in space and time. 3D ellipsoidal beams were obtained in experiment for the first time. It is expected that such laser beams will allow the electron bunch emittance to be reduced when applied at R"± photo injectors. (letter)

[en] The specific features of experimental implementation of a cross-correlator with a scan rate above 1600 cm s^-1 and a spatial delay amplitude of more than 15 mm are considered. The possibility of measuring the width of femtosecond pulses propagating in a train 300 μs in duration with a repetition rate of 1 MHz is demonstrated. A time resolution of 300 fs for the maximum time window of 50 ps is attained. The cross-correlator is aimed at testing 3D pulses of a laser driver of an electron photo-injector. (laser applications and other topics in quantum electronics)

[en] A laser system is designed for operation with a photocathode electron gun for a linear accelerator with the following parameters of radiation at a wavelength of 262 nm (the fourth harmonic of a Nd:YLF laser). The pulse trains (macropulses) with a repetition rate of 5 Hz and a duration of 900 μs consist of 8-ps micropulses with an energy of 1.4 μJ and a repetition rate of 2.708 MHz. This repetition rate is variable within ±32 kHz and is stabilised by an external signal with an accuracy of 10 Hz. Due to the use of a feedback-controlled acousto-optic modulator, the root-mean-square deviation of the micropulse energy in the first and second harmonics is 2.5% and 3.6%, respectively. Using the decaying branch of the dependence of the second-to-fourth harmonic conversion efficiency on the second harmonic intensity, we decreased the root-mean-square deviation of the energy of the fourth-harmonic micropulses to 2.3% at the first-to-fourth harmonic conversion efficiency of 27%. (lasers and amplifiers)