S. Bedacht

Pre-plasma formation in experiments using petawatt lasers.

We used time-resolved shadowgraphy to characterize the pre-plasma formation in solid-target interaction experiments with micrometer-scale accuracy. We performed quantitative measurements of the plasma density for amplified spontaneous emission (ASE) levels ranging from 2 · 10(-7) to 10(-10) backed with 2-dimensional hydrodynamic simulations. We find that ASE levels above 10(-9) are able to create a significant pre-plasma plume that features a plasma canal driving a self-focusing of the laser beam. For ASE levels of 10(-10), no ASE pre-plasma could be detected.

Simultaneous observation of angularly separated laser-driven proton beams accelerated via two different mechanisms

We present experimental data showing an angular separation of laser accelerated proton beams. Using flat plastic targets with thicknesses ranging from 200 nm to 1200 nm, a laser intensity of 6×1020 W cm−2 incident with an angle of 10°, we observe accelerated protons in target normal direction with cutoff energies around 30 MeV independent from the target thickness. For the best match of laser and target conditions, an additional proton signature is detected along the laser axis with a maximum energy of 65 MeV. These different beams can be attributed to two acceleration mechanisms acting simultaneously, i.e., target normal sheath acceleration and acceleration based on relativistic transparency, e.g., laser breakout afterburner, respectively.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

A novel experimental setup for energy loss and charge state measurements in dense moderately coupled plasma using laser-heated hohlraum targets

We report on a new experimental setup for ion energy loss measurements in dense moderately coupled plasma which has recently been developed and tested at GSI Darmstadt. A partially ionized, moderately coupled carbon plasma (ne ≤ 0.8• 1022 cm-3, Te = 15 eV, z = 2.5, Γ = 0.5) is generated by volumetrical heating of two thin carbon foils with soft X-rays. This plasma is then probed by a bunched heavy ion beam. For that purpose, a special double gold hohlraum target of sub-millimeter size has been developed which efficiently converts intense laser light into thermal radiation and guarantees a gold-free interaction path for the ion beam traversing the carbon plasma. This setup allows to do precise energy loss measurements in non-ideal plasma at the level of 10 percent solid-state density.

In-situ formation of solidified hydrogen thin-membrane targets using a pulse tube cryocooler

An account is given of the Central Laser Facilitys work to produce a cryogenic hydrogen targetry system using a pulse tube cryocooler. Due to the increasing demand for low Z thin laser targets, CLF (in collaboration with TUD) have been developing a system which allows the production of solid hydrogen membranes by engineering a design which can achieve this remotely; enabling the gas injection, condensation and solidification of hydrogen without compromising the vacuum of the target chamber. A dynamic sealing mechanism was integrated which allows targets to be grown and then remotely exposed to open vacuum for laser interaction. Further research was conducted on the survivability of the cryogenic targets which concluded that a warm gas effect causes temperature spiking when exposing the solidified hydrogen to the outer vacuum. This effect was shown to be mitigated by improving the pumping capacity of the environment and reducing the minimum temperature obtainable on the target mount. This was achieved by developing a two-stage radiation shield encased with superinsulating blanketing; reducing the base temperature from 14 ± 0.5 K to 7.2 ± 0.2 K about the coldhead as well as improving temperature control stability following the installation of a high-performance temperature controller and sensor apparatus. The system was delivered experimentally and in July 2014 the first laser shots were taken upon hydrogen targets in the Vulcan TAP facility.

Temperature measurement of hohlraum radiation for energy loss experiments in indirectly laser heated carbon plasma

For ion energy loss measurements in plasmas with near solid densities, an indirect laser heating scheme for carbon foils has been developed at GSI Helmholtzzentrum für Schwerionenforschung GmbH (Darmstadt, Germany). To achieve an electron density of 10^{22}cm^{3} and an electron temperature of 10-30eV, two carbon foils with an areal density of 100μg/cm^{2} heated in a double-hohlraum configuration have been chosen. In this paper we present the results of temperature measurements of both primary and secondary hohlraums for two different hohlraum designs. They were heated by the PHELIX laser with a wavelength of 527nm and an energy of 150J in 1.5ns. For this purpose the temperature has been investigated by an x-ray streak camera with a transmission grating as the dispersive element.

Simulations of the energy loss of ions at the stopping-power maximum in a laser-induced plasma

Simulations have been performed to study the energy loss of carbon ions in a hot, laser-generated plasma in the velocity region of the stopping-power maximum. In this parameter range, discrepancies of up to 30% exist between the various stopping theories and hardly any experimental data are available. The considered plasma, created by irradiating a thin carbon foil with two high-energy laser beams, is fully-ionized with a temperature of nearly 200 eV. To study the interaction at the maximum stopping power, Monte-Carlo calculations of the ion charge state in the plasma are carried out at a projectile energy of 0.5 MeV per nucleon. The predictions of various stopping-power theories are compared and experimental campaigns are planned for a first-time theory benchmarking in this low-velocity range.

Ion energy loss in plasma beyond the linear interaction regime

W. Cayzac 1, A. Ortner2, V. Bagnoud3,4, M.M. Basko5, S. Bedacht 2, A. Blǎzevíc3,4, S. Busold3,4, O. Deppert 2, M. Ehret2, S. Faik10, A. Frank4, D.O. Gericke6, L. Hallo7, J. Helfrich2, E. Kjartansson2, A. Knetsch8, D. Kraus9, G. Malka1, K. Pepitone7, T. Rienecker 10, G. Schaumann 2, D. Schumacher 3, An. Tauschwitz 10, J. Vorberger 12, F. Wagner 2, and M. Roth2 1Univ. Bordeaux-CEA-CNRS CELIA UMR 5107; 2Technical University of Darmstadt; 3GSI; 4Helmholtz institute Jena;5KIAM Moscow; 6University of Warwick; 7CEA/CESTA; 8University of Hamburg & CFEL;9University of California; 10University of Frankfurt;11HIC for FAIR; 12MPI for physics of complex systems

Energy loss measurements of heavy ions in dense weakly coupled plasma generated by volumetric heating with hohlraum generated x-rays

In two experimental campaigns in 2012 precise measurements of the energy loss of Ca ions in dense, partly ionized carbon plasma have been carried out. An unexpected hight increase of the stopping power in this weakly coupled plasma (Γ ≈ 0.5) has been measured. This was the first time, that ion stopping in such a plasma was measured. A 1 ns long, frequency doubled (λ = 527 nm) laser pulse with a total energy of 150 J is converted in a spherical cavity into X-rays with a radiation temperature of Tr ≈ 100 eV. This Plackian radiation heats up a secondary cylindrical hohlraum to a radiation temperature of T r ≈ 30 eV. These soft X-rays are then used to heat volumetrically two thin carbon foils into a dense plasma state. A weakly coupled carbon plasma with an electron temperature of 5 eV to 15 eV and electron density of up to 5 · 10 cm−3 and an ionization degree of 2 to 4 is generated. The properties of the primary as well as of the secondary hohlraum have been extensively studied by RALEF2D hydro-simulations [2, 3] and characterized in several experimental campaigns [4]. It has been shown, that no gold from the walls of the cylindrical hohlraum flows into the ion path for about 5 ns, which allows us to study the interaction of an ion beam in a pure carbon plasma. Fig. 1 shows the experimental setup used at the Z6 target area at GSI. The radiation temperature is measured with an X-ray streak spectrometer [4] and the electron density of the carbon plasma is characterized with a multi-frame interferometry. The ion energy loss is determined by a time of flight measurement of the delayed ion bunches. Details