Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 53

Profile measurement of laser microbeam produced by glass capillaries:
tilt dependence
M. Mori,∗1,∗2 T. Ikeda,∗2 Y. Hikima,∗1,∗2 S. Kawamura,∗1,∗2 and W. -G. Jin∗1
Tapered glass capillaries can simultaneously produce
ion and laser microbeams with diameters of several tensof µm.1,2) In the irradiation experiments at the RIKEN
Pelletron facility, for example, H and He ions with energies of a few MeV are used. Their ranges are only about
100 µm in water at most. Since it is diﬃcult to conﬁrm
incidence on the target by using detectors, the use ofa µm-order laser sight system prior to the ion irradiation plays an important role in avoiding mis-shootings.3)
The µm-order laser sight through a capillary was demonstrated with visible light and then extended to UV in
2018.4) This may be applicable to shorter wavelengths
such as X-rays. Moreover, a multi-quantum beam (laser+ ion) or (laser + laser) transmission through a capillary will be feasible. However, the transmission property
is diﬀerent between lasers and ions. Here, a laser beam
bending along the tilted capillary axis is reported, which
is known as the guiding eﬀect for keV-energy ion microbeams through the capillaries without any magnetic
ﬁeld.
Figure 1 shows the experimental setup.
A UV
laser 375 nm in wavelength was extracted from asemiconductor laser source (THORLABS L375P70MLD;
max.70 mW), and it entered a capillary mounted on a
precise stage after reﬂection on two mirrors. The powerof the laser microbeam was measured at a photodiode(OPHIR PD300) after a slit, which shields the scattered
light from the taper part of the capillary. The microbeam
was cut by a knife edge attached on a motorized stage(SURUGA SEIKI XYCV620-G-N) with a scan lengthand minimal step of 2 mm and 0.5 µm, respectively. Theknife edge was located 3 mm downstream of the capillary outlet, of which the corresponding distance from thecenter of the tilting (θ) was 72 mm for a capillary withan outlet of 18 µmϕ.
The proﬁles were obtained by diﬀerentiating the beam
power as a function of the knife-edge position. The peakshift according to θ is shown in Fig. 2. A steeper decrease for the θ(+) tilting, i.e., an asymmetry of theproﬁles in the θ(+) and θ(−) tilting, can be found andmay be attributed to the slightly asymmetric shape ofcapillaries. Satellite peaks are due to higher-order ringsof the Fraunhofer pattern.1) The angles reconstructed

Fig. 1. Experimental setup with the knife-edge method.
∗1∗2

Department of Physics, Toho University
RIKEN Nishina CenterFig. 2. Laser microbeam profiles measured by tilting the capillary in the θ(+) and θ(−) directions.

Fig. 3. Reconstructed angles as a function of θ.

from the measured peak positions are summarized forﬁve outlet sizes, as shown in Fig. 3. The reconstructed
angle is found to be nearly proportional to θ. The solidline shows y = x as a guide to the eye. Most of thereconstructed angles are less than the tilting angles, especially for the outlets of 53, 63, and 80 µmϕ. Such aphenomenon is not found in ion experiments.5,6) This isconsidered a result of diﬀerent transmission mechanismsof ions and light in capillaries. Because ions lose theirenergies during scattering at the inner wall, they cannot
penetrate the capillary window, and only those parallel to the capillary axis contribute to the output beam.
However, light does not lose its energy during scatteringand always contributes to the output beam. We concluded that the knife-edge method is capable of measuring the laser microbeam proﬁle. Further, laser light ispossibly guided by the capillary tilting. The next step
will be to test the dependence on the divergence of theinitial laser beam.
References

1) K. Sato et al., RIKEN Accel. Prog. Rep. 51, 21 (2018).
2) T. Ikeda et al., Quantum Beam Sci. 4, 22 (2020).
3) T. Ikeda et al., RIKEN Accel. Prog. Rep. 51, 245 (2018).
4) S. Kawamura et al., J. Phys. Soc. Jpn. 89, 055002 (2020).
5) M. Ikekame et al., RIKEN Accel. Prog. Rep. 52, 164 (2019).
6) T. Ikeda et al., Nucl. Instrum. Methods Phys. Res. B 470, 42(2020).

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