Neutron Generation and Spectroscopy
for plasma diagnostics and neutron sources
Laser intensities exceeding 1019 W/cm2 as delivered by ATLAS-10 correspond to laser electric fields in excess of 1013 V/m. These fields are a million times higher than typical field strengths in conventional particle accelerators. If such a laser pulse interacts with matter, electrons are rapidly ripped off the atoms. These electrons are accelerated to relativistic energies over lengths scales of a few millionths of a meter (µm). In conventional accelerators, the same energy gain would require acceleration lengths of a few meters. This leads to the phenomenon that the electrons are removed from the laser focus, leaving behind the positively charged ions. They remain stationary because with their high mass they cannot follow the oscillating laser field. This sets up a strong space charge field of 10 12-1013 V/m, which in turn lives long enough to accelerate the ions to MeV energies. In the case that the laser hits a thin foil, fast electrons that penetrate the target can set up a similar space charge field at the back surface, which rips ions off and accelerates them to MeV energies as well.
We can now use secondary fusion neutrons to analyze the properties of these
ions, especially the ones that are accelerated in the laser focus. Most of
these ions cannot penetrate the target, and therefore are very hard to detect
with standard methods. However, if these ions are deuterons (2
H-nuclei), and the target contains deuterium as well, the fast ions
can trigger the fusion reaction d(d,n)3He, where two deuterons
fuse to generate a 0.8 MeV Helium-nucleus and a 2.45 MeV neutron, both with
fixed energy in the center-of-mass frame. In the laboratory, we measure a
shifted neutron energy, which contains information on the original deuterons's energy and/or direction. By measuring the energy of many
neutrons simultaneusly, it is now possible to reconstruct the original ion distribution.
The experiment here at Garching measured the mean neutron energy under different directions. We could detect that the highest neutron energies were always detected behind the target, indicating that the accelerated ions are predominantly running into the target. This was a surprise and was not suggested by model calculations. The neutron yield in these experiments was ~40000 neutrons/shot. Since the neutrons are generated in a tiny volume of mm3 in a time of a few picosecods, this neutron source is among the most brilliant ever constructed, and the neutron flux can reach 1016-1018 neutrons / (cm2 s), depending on the distance from the source. The short duration would allow for highly dynamic processes to be studied with these neutrons
. A second experiment was performed at the Jena 10-TW laser, a system similar to ATLAS. This time the target consisted of small heavy water droplets, and a secondary deuterated plastic target ("catcher"). The laser accelerates ions from two sources:
- Laser focus, leading to prompt fusion reactions in the droplet, and
- from the rear droplet surface, fusing in the catcher and generating delayed neutrons.
The separation of the two neutron signals is a measure for the energy of the ions from the rear, while the width of the first neutron signal corresponds to the energy of the ions from the focus.
This experiment for the first time resolves the open question for the dominant acceleration mechanism and gives a quantitative answer about the properties of ions from the two sources. It was shown that the acceleration from the rear surface is much more dominant, even if like under our condition the target is spherical rather than flat.
This experiment counted single neutrons, with much less than one neutron/shot, so the spectra were accumulated over many shots. With a mure powerful laser and more neutrons, the same experiment can be done with a single shot. In experiments done at the LULI 100 TW laser, which delivers 20-30J in 400 fs, a CD2 foil target was used instead of the droplets, but the principle is the same.
