Takeshi Kanesue

Magnetic plasma confinement for laser ion source

A laser ion source (LIS) can easily provide a high current beam. However, it has been difficult to obtain a longer beam pulse while keeping a high current. On occasion, longer beam pulses are required by certain applications. For example, more than 10 micros of beam pulse is required for injecting highly charged beams to a large sized synchrotron. To extend beam pulse width, a solenoid field was applied at the drift space of the LIS at Brookhaven National Laboratory. The solenoid field suppressed the diverging angle of the expanding plasma and the beam pulse was widened. Also, it was observed that the plasma state was conserved after passing through a few hundred gauss of the 480 mm length solenoid field.

Features of ion generation using Nd-glass laser

Charge state and energy distributions of ions generated by a 3J∕30ns Nd-glass laser were measured at a distance of 3.7m from the target for seven different elements of the Periodic Table and for two different laser power densities (of the order of 1011 and 1012W∕cm2). Two groups of elements were found: highly charged ions with ionization potentials in the range of 500–1000eV were registered for all elements between C12 and Fe56; at the same time, ions with about-one-order-less ionization potentials were registered for elements between Ge74 and Ta181. The most probable reason for such a big difference is the recombination losses of ions during laser-produced plasma expansion into vacuum. Verification of recombination losses in the case of Ta181 target has shown no losses at distances longer than 32.5cm from the target, so recombination processes should take place at shorter distances. Current densities, pulse durations, energy ranges, and numbers of ions with different charge states were found for all elem...

Laser ion source with solenoid field

Pulse length extension of highly charged ion beam generated from a laser ion source is experimentally demonstrated. The laser ion source (LIS) has been recognized as one of the most powerful heavy ion source. However, it was difficult to provide long pulse beams. By applying a solenoid field (90 mT, 1 m) at plasma drifting section, a pulse length of carbon ion beam reached 3.2 μs which was 4.4 times longer than the width from a conventional LIS. The particle number of carbon ions accelerated by a radio frequency quadrupole linear accelerator was 1.2 × 1011, which was provided by a single 1 J Nd-YAG laser shot. A laser ion source with solenoid field could be used in a next generation heavy ion accelerator.

Design study of primary ion provider for relativistic heavy ion collider electron beam ion source

Brookhaven National Laboratory has developed the new preinjector system, electron beam ion source (EBIS) for relativistic heavy ion collider (RHIC) and National Aeronautics and Space Administration Space Radiation Laboratory. Design of primary ion provider is an essential problem since it is required to supply beams with different ion species to multiple users simultaneously. The laser ion source with a defocused laser can provide a low charge state and low emittance ion beam, and is a candidate for the primary ion source for RHIC-EBIS. We show a suitable design with appropriate drift length and solenoid, which helps to keep sufficient total charge number with longer pulse length. The whole design of primary ion source, as well as optics arrangement, solid targets configuration and heating about target, is presented.

Feasibility study of a laser ion source for primary ion injection into the Relativistic Heavy Ion Collider electron beam ion source

Charge state 1+ions are required as a primary ion source for Relativistic Heavy Ion Collider-electron beam ion source (RHIC-EBIS) at BNL and laser ion source (LIS) is a candidate as one of the external ion source since low energy and low charge state ions can be generated by lower power density laser irradiation onto solid target surface. Plasma properties of (27)Al, (56)Fe, and (181)Ta using the second harmonics of Nd:yttrium aluminum garnet laser (0.73 J5.5 ns and 532 nm wavelength) for low charge state ion generation was measured. Charge state distribution of Ta was optimized for 1+with estimated laser power density of 9.1 x 10(8) Wcm(2) on the target. It has been shown that the LIS can produce sufficient ion charge with the appropriate pulse structure to satisfy injection requirements of the RHIC EBIS.

Laser plasma in a magnetic field.

Laser ion source (LIS) is a candidate among various heavy ion sources. A high density plasma produced by Nd:yttrium aluminum garnet laser with drift velocity realizes high current and high charge state ion beams. In order to obtain higher beam current, we made experiments using the LIS with a magnetic field by which a confinement effect can make higher beam current. We measured total current by Faraday cup and analyzed charge distribution by electrostatic ion analyzer. It is shown that the ion beam charge state is higher by a permanent magnet.

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.

Drift distance survey in direct plasma injection scheme for high current beam production

In a laser ion source, plasma drift distance is one of the most important design parameters. Ion current density and beam pulse width are defined by plasma drift distance between a laser target and beam extraction position. In direct plasma injection scheme, which uses a laser ion source and a radio frequency quadrupole linac, we can apply relatively higher electric field at beam extraction due to the unique shape of a positively biased electrode. However, when we aim at very high current acceleration such as several tens of milliamperes, we observed mismatched beam extraction conditions. We tested three different ion current at ion extraction region by changing plasma drift distance to study better extraction condition. In this experiment, C(6+) beam was accelerated. We confirmed that matching condition can be improved by controlling plasma drift distance.

Experimental results of DPIS with a new RFQ

We have developed a new heavy ion production system which uses a combination of an RFQ and a laser ion source. Induced plasma by a laser shot is delivered to the RFQ without an extraction electrode. We named this new idea ‘direct plasma injection scheme (DPIS)’. In 2004, a new RFQ was built for demonstrating the capability of the DPIS. After a few months of commissioning period, we could obtain more than 60 mA of carbon beam from the RFQ. This new scheme could be applied to cancer therapy facilities and high energy nuclear physics accelerator complexes.

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.