One of the pressing demands in our western society is the safety and maintenance of our nuclear legacy. In Germany the dismantling, safe processing and storage of nuclear waste have resulted in a multi-national research program. One of the findings was that nondestructive testing methods and material selective imaging of compound large objects is possible using thermal and fast neutrons. They also identified that a powerful, safe, and compact neutron source would be required.
Since the advent of ultra-intense lasers many applications have been investigated using the unique parameter of laser-driven secondary sources. Recently, we have demonstrated the realization of a short-pulse laser- driven neutron source with beam intensities orders of magnitude above earlier attempts. Those sources can lead to a compact and potentially mobile neutron source with a large number of applications. The neutron source is based on a so-called pitcher-catcher approach, where a short burst of energetic, laser-driven deuteron ions are converted into a short pulse of neutrons in a subsequent converter material. Here, in addition to neutron conversion via (d,n) reactions the breakup of energetic deuterons and a non-thermal pre-compound reaction in the converter material result in a directed beam of neutrons and an increase in neutron numbers compared to earlier attempts. The neutron record number so far were produced using high-contrast, high-energy short pulse lasers of a few hundred femtosecond pulse duration and invoking relativistic transparency in solids as a new ion acceleration mechanism.
I will present the underlying mechanism of creating an intense pulsed and highly directed beam of neutrons using ultra-intense lasers and the recent experimental results using laser systems in the US and in Europe. Furthermore, I will focus on a few examples of using such sources for applications that are either important for the security of our countries or will have large economical potential in industrial applications. These range from the remote sensing of illicit nuclear material in cargo to the non-destructive analysis of large civil constructions using compact laser systems.
The initial ion pulse that is converted into neutrons is of only a few picosecond duration and thus the neutron burst is generated with sub-nanosecond pulse length. This allows for precise energy resolution of the neutrons using time-of flight techniques. As the ion acceleration is of only a few millimeter in length, replacing ten's to hundred's of meter of conventional acceleration length the source is extremely compact. As the ion beams are extremely intense and do not suffer from space charge deterioration the fast neutron source is capable of point projection imaging with an initial source size of around a millimeter. As the pulse duration is short, high energy resolution can be achieved at a much reduced time-of-flight length offering more compact systems.
As future laser systems largely increase not only peak power, but also already offer repetition rates up the kHz applications in research and industry become visible.
To fully take advantage of those new sources new detector systems should keep up with enhanced spatial and temporal resolution. Also they need to withstand the unavoidable x-ray flash and electro-magnetic pulse originating from the laser plasma interaction. We have started to explore novel detectors and tested them at high-energy short pulse laser systems for fast and thermal neutron beams.
Great progress has been made in recent years in realizing compact, laser-based neutron generators. These devices, however, are inapplicable for conducting energy-resolved fast-neutron radiography because of the electromagnetic noise produced by the interaction of a strong laser field with matter. To overcome this limitation, we developed a novel neutron time-of-flight detector, largely immune to electromagnetic noise. The detector is based on plastic scintillator, only a few mm in size, which is coupled to a silicon photo-multiplier by a long optical fiber.
I will present results we obtained at the Trident Laser Facility at Los Alamos National Laboratory during the summer of 2016. Using this detector, we recorded high resolution, low-background fast neutron spectra generated by the interaction of laser accelerated deuterons with Beryllium. The quality of these spectra was sufficient to resolve the unique neutron absorption spectra of different elements and thus it is the first demonstration of laser-based fast neutron spectroscopy.
I will discuss how this achievement paves the way to realizing compact neutron radiography systems for research, security, and commercial applications.
With PHELIX (Petawatt High Energy Laser for heavy Ion EXperiments) a high energy/ultra-high intensity
laser system is currently under construction at the GSI (Gesellschaft für SchwerIonenforschung, Germany). In
combination with the high current high energy ion accelerator facility this will provide worldwide unique experimental
opportunities in the field of dense plasma physics and inertial fusion research. In the long pulse mode the laser system
will provide laser pulses of up to 5 kJ in 1-10 ns pulses. In the high intensity mode pulse powers in excess of 1 PW will
be achieved. For this the well known technique of chirped pulse amplification (CPA) will be implemented. A new CPA
stretcher-compressor setup for the PHELIX laser was calculated and designed. A 4-pass single-grating stretcher and
a 4-pass single-grating test compressor, both with a full transmission bandwidth of 16 nm, as well as the compact
single-pass compressor for the final pulse compression will be presented. Spatial chirp and spectral phase aberrations of
the stretcher were optimized. We discuss the dependence of critical alignment tolerances on the angle of incidence and
show the effects on the temporal pulse shape.
We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.
The unique combination of an intense heavy ion beam accelerator and a high energy laser opens the possibility of exploring new physics taking advantage of the synergy of both facilities. A variety of new fields can be addressed with this combination in plasma physics, atomic physics, nuclear- and astro-physics as well as material research. In addition, using CPA-technology, laser pulses with a pulse power of up to a petawatt opens the door to explore the regime of fully relativistic plasmas. Therefore the Gesellschaft fuer Schwerionenforschung is augmenting the current high intensity upgrade of the heavy ion accelerator facility with the construction of PHELIX. Designed with two pulse-generating front ends and send to multiple experimental areas PHELIX will serve as a highly versatile laser system for various applications. In this report, we present the design of the laser system and some key experiments that can be performed with this combination for the first time.
At the Z6 experimental area of the Gsesellshaft fur Schwerionenforschung in Darmstadt experiments with the nhelix laser facility were carried out to determine the plasma parameter sch as temperature, degree of ionization and expansion dynamics for laser heated targets, which are used for the ion beam-plasma-interaction experimental series. Spatially resolved x-ray spectroscopy with spherically bent mica crystals showed well collimated jets of He- and H-like ions emerging out of the front and rear surface of the target with energies in the MeV range.
The LLNL Petawatt Laser has achieved focused intensities up to 6 by 1020 W/cm2. IN plasmas created by this laser, the quiver energy of target electrons exceed several MeV. Recent experiments revealed an intense, collimated beam of high-energy is converted into protons which leads to an energy content of 30J in a pulse of less than 10 ps. The beam shows a broad particle energy spectrum with a sharp cut off and an almost mono-energetic part above 55 MeV. With their short pulse duration, high particle energy and large luminosity these beams are promising candidates in numerous applications, such as short-pulse injectors for laser accelerators or as the ignitor for fast ignition ICF. Using intense proton beams the fast ignitor concept may also become more attractive in heavy-ion fusion due to the possibility to work with indirectly driven targets. Finally, the acceleration is not restricted to protons and the use of tailored target surfaces may allow to accelerate more massive ions to similar energy per nucleon.
For the development of a heavy ion driven inertial confinement fusion scenario a detailed knowledge of the interaction processes of the ions with the converter material is crucial. As this converter will be predominantly in the plasma state one of the main topics of the plasma physics group at Gesellschaft fuer Schwerionenforschung (GSI) is the interaction of heavy ions with dense hot plasma. Based on the latest result on interaction experiments with laser generated plasma targets presented here and concerning the high current upgrade of GSI a new high energy laser system is proposed. It will serve as a driver for interaction experiments with heavy ions as well as a diagnostic tool for heavy ion generated plasmas. In addition, with the combination of high current heavy ion beams and intense lasers innovative, fundamental research in the field of high energy density physics will be accessible for the first time.