V. Nedorezov

Comparative study of amplified spontaneous emission and short pre-pulse impacts onto fast electron generation at sub-relativistic femtosecond laser-plasma interaction

This paper describes the study of hot electron generation under the action of intense (∼1018u2009W/cm2) femtosecond pulses onto the surface of a solid target, in the presence of a long pre-plasma, which varied with different spatial extents and densities. The corona was formed by pre-pulses with varied intensities and temporal profiles (amplified spontaneous emission (ASE) and short pre-pulses). The most efficient fast electron acceleration, to energies well beyond the ponderomotive potential, was observed if the ASE was able to form to the extent of ∼100u2009μm a slightly undercritical plasma. Energy of accelerated electrons underwent further growth if the laser pulse duration increased from ∼45 to ∼350 fs at constant energy fluence. The experimental results were supported by numerical simulations using 3D3V Mandor PIC code.

THE BGO-OD EXPERIMENT AT ELSA

Meson photoproduction is a key tool for the experimental investigation of the nucleon excitation spectrum. To disentangle the specific couplings of resonances, in addition to the rather well measured pion and eta photoproduction channels it is mandatory to obtain information on channels involving strange and vector mesons and higher mass pseudoscalar mesons, and the associated multi-particle final states with both charged and neutral particles. In this respect, the new BGO-OD experiment at the ELSA accelerator of the University of Bonns Physikalisches Institut provides unique instrumentation. We describe the experiment, present its status and the initial program of measurements.

Measurement of the Sigma beam asymmetry for the omega photoproduction off the proton and the neutron at the GRAAL experiment

We report on new measurements of the beam asymmetry for. photoproduction on protons and neutrons in hydrogen and deuterium targets from the GRAAL Collaboration. The beam asymmetry values are extracted from the reaction threshold (E-gamma = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement, leading to the worlds most precise measurements of the beam asymmetry for omega photoproduction off free protons. First experimental results on the deuteron allow the extraction of the Sigma beam asymmetry on quasifree nucleons. The beam asymmetry angular distributions obtained for the free and the quasifree kinematics show the same behavior, similar to the findings in pseudoscalar meson photoproduction reactions. First results of the beam asymmetry on the quasifree neutrons are presented, showing different strengths and angular distributions from the results on the proton target.

Investigation of the reaction D(γ, n)H near the threshold by means of powerful femtosecond laser radiation

The possibility of studying photonuclear reactions near the threshold by means of powerful femtosecond lasers is explored by considering the example of deuteron photodisintegration. The respective experiment was performed by employing the terawatt femtosecond laser facility of the International Laser Center at Moscow State University. The radiation from this facility is characterized by a pulse energy of up to 50 mJ, a duration of 50 fs, a repetition rate of 10 Hz, and a wavelength of 805 nm. This provides a power above 1018 W/cm2. Intense relativistic-electron and photon beams of energy up to 10 MeV were obtained after the optimization of relevant experimental parameters, including the focus of the laser beam, its time structure, and the choice of target. The use of these beams made it possible to study neutron generation in heavy water, to measure the time of neutron moderation, and to determine the detection efficiency. The experimental data obtained in this way are in qualitative agreement with the results of simulations based on the GEANT-4 and LOENТ code packages and indicate that it is possible to create a neutron source on the basis of the aforementioned laser. The cross section measured for deuteron photodisintegration complies with theoretical estimates available in the literature.

Experiments with Compton back scattered gamma beams (on GRAAL Collaboration results)

Experimental study of photonuclear reactions using Compton back scattered gamma beams at intermediate energies (from pion photoproduction threshold up to a few GeV) is reviewed. The last review of this subject was published about 10 years ago. Since then, new fundamental data on nucleon spin structure and photonuclear excitation mechanisms have appeared in the literature. Principal attention in the present review is paid to the GRAAL (Grenoble Accelerateur Anneau Laser) experiment conducted at ESRF (European Synchrotron Radiation Facility). It is shown that, in addition to the principal polarization experimental program based on high degree of gamma beam polarization, new results on different applications have been obtained. For example, total photoabsorption and partial meson photoproduction cross sections were measured with high accuracy, and a new method for studying an interaction of unstable short-lived mesons with nuclear media (tagged mesons) was proposed. New results on anisotropy of the speed of light with respect to the dipole of cosmic microwave background radiation were evaluated. It is noted that new opportunities arise due to unique conditions appearing while conducting the experiments with the Compton back scattering technique, which provides the hard photon spectrum, high degree of beam polarization, and low backgrounds.

An electron magnetic spectrometer for experiments on a terawatt femtosecond laser

Photonuclear methods used earlier in experiments at electron accelerators have been adapted for femtosecond pulsed lasers. In particular, the problem of measuring wide electron spectra under conditions of a high counting rate and, hence, a high probability of pulse pileup has been solved. To provide long-term stability of electron beams from plasma, a magnetic spectrometer combined with a magnetic-induction sensor has been developed. This spectrometer is capable of measuring the electron-beam characteristics in each ultrashort laser pulse. The results of the experiments carried out with the femtosecond laser system at the International Laser Center of the Moscow State University are presented.

Strangeness Photoproduction at the BGO-OD Experiment

The BGO-OD experiment at the ELSA accelerator facility uses an energy tagged bremsstrahlung photon beam to investigate the excitation structure of the nucleon. The setup consists of a highly segmented BGO calorimeter surrounding the target, with a particle tracking magnetic spectrometer at forward angles. nBGO-OD is ideal for investigating low momentum transfer processes due to the acceptance and high momentum resolution at forward angles. In particular, this enables the investigation of strangeness photoproduction where t-channel exchange mechanisms play an important role. This also allows access to low momentum exchange kinematics where extended, molecular structure may manifest in reaction mechanisms. nFirst key results at low

Results from polarized experiments at LEGS and GRAAL

t

Strangeness photoproduction at the BGO-OD experiment

indicate a cusp-like structure in

Measurement of Femtosecond Laser Plasma X-ray Spectra Using a Medipix Detector

K^+Sigma^0