Iron plasma generation using a Nd:YAG laser pulse of several hundred picoseconds.

We investigated the high intensity plasma generated by using a Nd:YAG laser to apply a laser-produced plasma to the direct plasma injection scheme. The capability of the source to generate high charge state ions strongly depends on the power density of the laser irradiation. Therefore, we focused on using a higher power laser with several hundred picoseconds of pulse width. The iron target was irradiated with the pulsed laser, and the ion current of the laser-produced iron plasma was measured using a Faraday cup and the charge state distribution was investigated using an electrostatic ion analyzer. We found that higher charge state iron ions (up to Fe(21+)) were obtained using a laser pulse of several hundred picoseconds in comparison to those obtained using a laser pulse of several nanoseconds (up to Fe(19+)). We also found that when the laser irradiation area was relatively large, the laser power was absorbed mainly by the contamination on the target surface.

Contribution of material's surface layer on charge state distribution in laser ablation plasma.

To generate laser ablation plasma, a pulse laser is focused onto a solid target making a crater on the surface. However, not all the evaporated material is efficiently converted to hot plasma. Some portion of the evaporated material could be turned to low temperature plasma or just vapor. To investigate the mechanism, we prepared an aluminum target coated by thin carbon layers. Then, we measured the ablation plasma properties with different carbon thicknesses on the aluminum plate. The results showed that C(6+) ions were generated only from the surface layer. The deep layers (over 250 nm from the surface) did not provide high charge state ions. On the other hand, low charge state ions were mainly produced by the deeper layers of the target. Atoms deeper than 1000 nm did not contribute to the ablation plasma formation.

Creation of mixed beam from alloy target and couple of pure targets with laser.

To create mixed species ion beam with laser pulses, we investigated charge state distributions of plasma formed from both Al-Fe alloy targets and pure Al and Fe targets placed close together. With two targets, we observed that the two kinds of atoms were mixed when the interval of two laser pulses was large enough (40 µs). On the other hand, when the interval was 0.0 µs, we observed fewer Fe ions and they did not mix well with the Al ions. The two species were mixed well in the plasma from the alloy target. Furthermore, we observed that specific charge states of Fe ions increased. From the results, it was determined that we can use two pure targets to mix two species whose difference of the drift velocity is large. On the other hand, we must use an alloy target when the drift velocities of the species are close.

Investigation of effect of solenoid magnet on emittances of ion beam from laser ablation plasma.

A magnetic field can increase an ion current of a laser ablation plasma and is expected to control the change of the plasma ion current. However, the magnetic field can also make some fluctuations of the plasma and the effect on the beam emittance and the emission surface is not clear. To investigate the effect of a magnetic field, we extracted the ion beams under three conditions where without magnetic field, with magnetic field, and without magnetic field with higher laser energy to measure the beam distribution in phase space. Then we compared the relations between the plasma ion current density into the extraction gap and the Twiss parameters with each condition. We observed the effect of the magnetic field on the emission surface.

Analyses of the plasma generated by laser irradiation on sputtered target for determination of the thickness used for plasma generation.

In Brookhaven National Laboratory, laser ion source has been developed to provide heavy ion beams by using plasma generation with 1064 nm Nd:YAG laser irradiation onto solid targets. The laser energy is transferred to the target material and creates a crater on the surface. However, only the partial material can be turned into plasma state and the other portion is considered to be just vaporized. Since heat propagation in the target material requires more than typical laser irradiation period, which is typically several ns, only the certain depth of the layers may contribute to form the plasma. As a result, the depth is more than 500 nm because the base material Al ions were detected. On the other hand, the result of comparing each carbon thickness case suggests that the surface carbon layer is not contributed to generate plasma.