RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

70-m laser beam transport for the RIBF-PALIS experiment
T. Sonoda,∗1 H. Iimura,∗2 M. Reponen,∗3 M. Wada,∗1 I. Katayama,∗1 V. Sonnenschein,∗4 T. Takamatsu,∗4Tomita,∗4 and T.M. Kojima∗1∗1H. TomitaLaser beam transport is essential part of the operation of PALIS.1,2) Figure 1 shows a stereoscopic viewof the PALIS laser beam transport line. Because ofthe strong radiation in the beam line area, one cannot access to PALIS when BigRIPS is in operation.
Therefore, the laser light source, which needs appropriate tuning during the experiment, was placed in
the human-accessible area. As a result, the laser lightsource and the ionization area for PALIS are on different ﬂoors and diﬀerent building foundations, whichleads to a total laser beam path of up to 70 m. Henceeven small beam ﬂuctuations can cause the laser beam
to miss the distant downstream optics.
We designed and implemented a simple optical system consisting of several mirrors equipped with compact stepping motor actuators, lenses, beam spot
screens, and network cameras. The system enables
multistep resonant laser ionization in a gas cell andgas jet with a collinear overlap of a few millimeters diameter between the laser photons and atomic beam at
70 m away from the laser light source.
The optical components used for the long laser beamtransportation are only four mirrors and one lens tosimplify the whole system. The mirrors are vapourdeposited aluminum ﬂat mirrors that were designed forhigh reﬂectivity at any incident angle. To avoid divergence due to the long distance, the beam size has to besuﬃciently large. The beam size at the exit of the dyelaser is approximately 0.8 mm horizontally and 2 mmvertically. Then it expands to 7 mm × 17 mm using
a telescope consisting of a concave lens and an achromatic lens. The laser beam size is ﬁnally collimatedto 3 mmφ by a concave lens to irradiate the resonancephotoionization volume. In order to control the mirror
angle remotely, each mirror has an actuator equippedwith a high-performance compact stepping motor forboth the horizontal axis and vertical axis. The singlepulse travel of the stepping motor is 1 μ m. All stepping motors can be remotely controlled by a PC with
an automation program written in LabVIEW. The remote control interface is integrated with the LAN platform, so that control is possible from any place wherea LAN connection is available. Using a dichroic mirror
and polarizing cube beam splitter, laser beams with
diﬀerent wavelengths are combined at the starting location in the laser laboratory and transported to theionization volume.
We have achieved a transmission eﬃciency of 53 %
∗1
∗2
∗3
∗4

RIKEN Nishina CenterJapan Atomic Energy Agency
Department of Physics, University of Jyvaskyla
Faculty of Engineering, Nagoya University- 200 -Fig. 1. Stereoscopic view of the 70-m laser beam transportline on the RIBF experimental ﬂoors. Multi-color laser
beams are sent from the PALIS laser laboratory (B1F)
to the PALIS located in the second focal plane chamberin the fragment separator BigRIPS (B2F-F2).

from B1F laser laboratory to BigRIPS F2 room, forwhich the total ﬂight path corresponds to about 66 m.
We also conﬁrmed that the position stability of thelaser beam stays within a permissible range for a dedicated resonant ionization experiment. The system will
be ﬁnalized in several on-line experiments for robustand eﬃcient handling operations.
References
1) T. Sonoda et al., AIP Conf. Proc. Vol. 1104 132(2009).
2) T. Sonoda et al., Nucl. Instr. and Meth. B295 1(2013).