Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Transportation of laser beams for PALIS
H. Iimura, 1, 2 T. Sonoda, 1 and M. Wada 1
PArasitic RI-beam production by Laser Ion-Source(PALIS)1) is under construction as a part of the slow
RI-beam facility, SLOWRI, at RIBF. The PALIS is basedon resonant photoionization of reaction products cached in agas cell. According to the present plan, the gas cell will be
installed in the vicinity of the slit at the F2 focal plane inBigRIPS. However, the laser system for PALIS is set up ina room on a different floor located at approximately 50 m in
the horizontal and 10 m in the vertical direction from the F2chamber of BigRIPS. Thus, to transport the laser beams
across this long distance we designed an optics system.
The laser system newly installed for the PALIS consistsof two dye lasers (Spectra-Physics Credo) pumped by a
Nd:YAG laser (EdgeWave IS) at a repetition rate of 10 kHz.
We will use a two-step two-color scheme or a three-step
two- or three-color scheme for the resonant ionization ofatoms. For example, in case of the three-color scheme, theNd:YAG laser will be used in the third step from anintermediate state to the ionization continuum.
The planned optics system is shown in Fig. 1. Three setsof this system will be installed to transport threedifferent-color laser beams independently. We prepared a
few spares of each lens and mirror to exchange themaccording to the wavelength of laser. Three laser beams
overlap each other at the PALIS gas cell. Because mirror
M3 is close (~1 m) to the gas cell, it reflects all three laser
beams; this is different from other optical components thatare used for only one laser beam.
Because the beam size of the laser is small (0.8 mmhorizontally and 2 mm vertically) at the exit of the dye laser,

Fig. 1. Planned optics system for transportation of laser
beams to PALIS.
12RIKEN Nishina Center
Applied Nuclear Physics Laboratory, Japan Atomic Energy Agency- 209 -

it diverges after long distance transportation (withdivergence of 1 mrad). Therefore, we expand the laser beamto approximately 7 mm x 17 mm using an expander
consisting of a concave lens (L1) and an achromatic lens
(L2). The estimated divergence of this beam is 0.1 mrad,
and the beam size does not change significantly after atransportation distance of 45 m. Then the laser beam isfocused with a long focal length using a combination ofconvex (L3) and concave (L4) lenses. Finally, the laser
beam is injected into the PALIS gas cell in the F2 chamberof BigRIPS. The resonant photoionization occurs inside theSextuPole Ion Guide (SPIG) to which atoms move from thegas cell. We designed the optics system such that the beamsize of laser changes to 3 mm x 3 mm along the 25-cm
length of SPIG and the laser beam matches the 3-mm inside
diameter of SPIG.
Regarding the intensity of the laser beam, outputs of thedye lasers are less than 15 W, or in case of using a second
harmonic generator, they are less than 2 W. In many of theionization schemes, the intensity of ions is not saturated
with these laser powers. Therefore, higher transport
efficiency of the laser beams is necessary to achieve higher
intensity of ions. The designed optics system uses aminimum number of optical components, and the transport
efficiency estimated from the transmission and reflectanceof the optical components is approximately 50 % at awavelength of 350 nm. If we use an optical fiber instead,the efficiency lowers to approximately 10 %.
The experimental room where BigRIPS is located cannot
be entered when the RI beams are injected, although thelaser room can be entered at any time. To handle the laser
beams without entering the experimental room, we placed
actuators to change the angles of 2-inch mirrors M1 and M2.
These actuators can be controlled via Ethernet by acomputer. Additionally, several CCD cameras will be
installed to monitor the laser beam spots from a distance.
We also plan to place a photo detector inside the gas cell to
finally confirm that the laser beams pass through aperturesof the PALIS.
Transportation of dye laser beam is currently being
examined for attenuation of the intensity, spatial fluctuationof the beam spot, and so on. Besides the dye lasers, anarrow band-width injection-locked Ti/Sapphire laser2) isbeing developed for the in-source laser spectroscopy.
Transportation of this laser beam is also planned.
References

1) T. Sonoda et al: Nucl. Instr. Meth. B 295, 1 (2013).
2) T. Takatsuka et al: Nucl. Instr. Meth. B 317, 586 (2013).