Multi MeV protons, deuterons and carbon ions produced by the PALS laser system

Multi MeV ions and fusion neutrons were generated by focused radiation of the 3 TW Prague Asterix Laser System (PALS). The use of 8 μm Al foil as XUV filter positioned in front of an ion collector allowed measuring currents of 4-MeV protons emitted behind a thin target in the forward direction. The proton energy of 4 MeV generated by a PALS laser irradiance Iλ2~5×1016 W cm-2 μm2 on target is nominally reachable for picosecond lasers when they deliver the intensity Iλ2~3×1018 W cm-2 μm2. The enhanced maximum proton energy is favoured by a non-linear interaction of the laser beam with the pre-generated plasma. Nonlinear processes also cause enhancement in the yield of fusion neutrons per focused laser energy from the CD2 plasma. The obtained results show that an equivalent neutron yield was reached by ps- and sub-ps laser beams for Iλ2~1019 W cm-2 μm2. The hampering influence of the electromagnetic pulse generated within the interaction chamber on diagnostics signals was eliminated

Semiconductor Detectors for Observation of Multi-MeV Protons and Ions Produced by Lasers

The application of time-of-flight Faraday cups and SiC detectors for the measurement of currents of fast ions emitted by laser-produced plasmas is reported. Presented analysis of signals of ion detectors reflects the design and construction of the detector used. A similarity relation between output signals of ion collectors and semiconductor detectors is established. Optimization of the diagnostic system is discussed with respect to the emission time of electromagnetic pulses interfering with signals induced by the fastest ions accelerated up to velocities of 107 m/s. The experimental campaign on laser-driven ion acceleration was performed at the PALS facility in Prague

Fast neutrons available through beam-target reactions driven by TW lasers

Laser accelerated deuterons with energy ranging from 0.1 to 2 MeV were exploited as drivers of the $7$Li(d,n)$8$Be nuclear fusion reaction. Deuterons were emitted by CD$2$ plasma produced with focused laser intensity of ~ 3x10$16$ W/cm$2$. Deuterons accelerated upstream the laser beam impinged on a LiF catcher target producing there D-Li neutrons with energy > 13 MeV. Time-resolved signals of scintillation detectors were analysed with respect to the arrival time of fast neutrons at five scintillation detectors positioned around the target chamber. This analysis made it possible to determine energy of both the fusing deuterons and fusion neutrons. The energy spectrum of deuterons was also determined from the time-resolved charge density of ions derived from ion collector signals. Besides the neutrons produced through the 7Li(d,n)8Be fusion reaction, neutrons produced via the $2$H(d,n)$3$He reactions driven by the laser - CD$2$-plasma interaction were observed. The total dose of neutrons was determined employing high-sensitive bubble detectors (BD-PND). Since only a small fraction of generated fast deuterons were hitting the LiF catcher target, the maximum yield of neutrons from both the primary and secondary targets was ~ 3.5x10$8$ neutrons/shot, which gives a normalized yield of about 5.8x10$5$ neutrons/J. This value should still grow up when increasing the area of the catcher LiF target

MCNP calculations of neutron emission anisotropy caused by the GIT-12 hardware

The MCNP6 and MCNPX calculations for the GIT-12 device in Tomsk were performed to determine the influence of the gas-puff hardware on the neutron emission anisotropy and the neutron scattering rate. A monoenergetic 2.45 MeV neutron source and F1 and F6 tallies were declared in the simulation input. A comparison between MCNP results and the measured data was made. Differences between MCNPX and MCNP6 output data were investigated. In the experiment, two nTOF scintillation detectors with the Bicron BC-408 scintillator were used to measure the neutron waveform. Four bubble BD-PND detectors were used to estimate the amount of neutrons in different places around the neutron source

Target current: a useful parameter for characterizing laser ablation

AbstractA current flowing between the ground and target exposed to the nanosecond laser radiation is analyzed to complete characteristics of laser ablation. Three phases of the target current are distinguished. During the ignition phase, the electron emission is driven by the laser pulse and the positive charge generated on the target is balanced by electrons coming from the ground through the target holder. At post-pulse times, a peaked waveform of the target current is typical for the active phase of the plasma and can give information on the material composition of the ablated surface layers. The afterglow phase is determined by a current of electrons flowing from the target to the ground. Experiment shows that the time-resolved target current is very sensitive to the actual composition of the surface layer of irradiated target and laser parameters

Target current: a useful parameter for characterizing laser ablation

AbstractA current flowing between the ground and target exposed to the nanosecond laser radiation is analyzed to complete characteristics of laser ablation. Three phases of the target current are distinguished. During the ignition phase, the electron emission is driven by the laser pulse and the positive charge generated on the target is balanced by electrons coming from the ground through the target holder. At post-pulse times, a peaked waveform of the target current is typical for the active phase of the plasma and can give information on the material composition of the ablated surface layers. The afterglow phase is determined by a current of electrons flowing from the target to the ground. Experiment shows that the time-resolved target current is very sensitive to the actual composition of the surface layer of irradiated target and laser parameters

Simulating trends in soil organic carbon in long-term experiments using the NCSOIL and NCSWAP models

NCSOIL simulates C and N transformations in 4 soil organic pools: Pool I labile, Pool I resistant, Pool II, and Pool III, with half-lives of 2, 17, 115 days and about 150 years, respectively. Pool I labile and Pool I resistant represent the microbial biomass. Pool I and Pool II represent the potentially mineralizable N, or the biologically active soil organic matter. The sum of Pools I, Pool II, and Pool III - the soil organic matter - corresponds to the total organic matter minus residues. Each residue is described by 2 pools. NCSOIL is a stand-alone model. It is also a module of NCSWAP, a larger model which encompasses the soil-water-air-plant system. A number of systems and treatments, including the Rothamsted nitrate treatment and the Calhoun tracer C data were simulated. The initial level of Pool II and the decay rate constant of Pool III were calibrated on the basis of measured total soil organic matter and above-ground production. Simulated data were sensitive to above-ground production as it controlled residues input to soil. Model performance, based on total soil organic matter only, is discussed elsewhere in this issue. Most decay rate constants for Pool III ranged from 1.0E - 5 to 3.0E - 5 d-1. Rate constants outside this range were associated with peculiarities of the soil or agronomic practices. Levels of biologically active organic matter (Pool I plus Pool II) in the top soil layers ranged from 4 to 108 μg N g-1. They were consistent with those reported for the potentially mineralizable nitrogen and reflected the agronomic practice and soil fertility level better than did the total soil organic matter. The simulated biologically active organic matter presented a I year periodic cycle. In the future, a major challenge in modelling studies will be to free simulations from the calibration process and to devise experimental methods which will provide initial values relevant to the dynamic requirements of the model.</p

Simulating trends in soil organic carbon in long-term experiments using the CANDY model

CANDY (CArbon and Nitrogen DYnamics) is a simulation system based on long-term experiments of organic matter turnover and nitrogen dynamics at Bad Lauchstadt, Germany. Key driving variables are soil physical properties, meteorological data and management information. The main application of the CANDY model is the calculation of short-term dynamics of nitrogen transformation and long-term dynamics of organic matter turnover in arable soils. This paper concentrates on the evaluation of the model in simulating carbon dynamics in long-term experiments representing different land uses and very different geographical sites. The experimental data came from data sets that were made available to modellers at a workshop held at Rothamsted in 1995. In this paper we describe how the different data sets were modelled and provide a qualitative assessment of model performance. The performance of several models, including CANDY, are compared quantitatively in Smith et al. (1997). Our results show that the mathematical basis of the model, its consideration of a biological time base and its calculation of the 'reproducing carbon' are applicable over a wide range of sites and land-use scenarios. Most of the standard parameters can be used for other sites and land-use systems.</p