Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 50 (2017)

Long term operation of low charge state laser ion source†
M. Okamura,䠆1,䠆2 S. Ikeda,䠆1,䠆3 and T. Kanesue䠆1In March 2014, a laser ion source (LIS) that delivers
high-brightness low charge state heavy ions for the hadron
accelerator complex in Brookhaven National Laboratory(BNL) was comissioned. Since then, the LIS has provided
many heavy ion species successfully. The induced
low-charge-state (mostly singly charged) beams are injectedto the electron beam ion Source (EBIS),1) where ions areheavily ionized, e.g., Au32+, to fit to the ensuring
accelerator’s Q/M acceptance. To provide various species2)
to NASA space radiation laboratory (NSRL), more than ten
materials are installed in the laser irradiation chamber, andthe target can be changed in a few seconds. In 2015, we
installed another laser and an additional disk-shaped target
to provide a gold beam to the relativistic heavy ion collider(RHIC). Now the LIS provides beams to two user facilities
simultaneously.
Although the LIS was a newly conceptualized ion
source and we encountered many minor difficulties asexpected, we could provide various heavy-ion beams
with almost no down time during the past runs. Table 1shows the operation days. Run 17 has already started,and the LIS is providing beams to the following
accelerators now.
Table 1: Total days of operation.

We have experienced many types of the failures. Initially,
we planned to rotate the gold disk target with a constantvelocity. However, the continuous revolution of themechanical shafts destroyed some bearings since they are ina vacuum condition. Therefore, we employed the stepscanning of the rotating target.
By applying anintermittent rest condition, accumulated heat can be
conducted from bearings. We also replaced some ceramicball bearings to Vespel bushings.
In 2016, we found heavy accumulation on the gold target.
An enlarged photo of the accumulation is shown in Fig. 1.

The shown build-up was formed from evaporated material,
since we had operated the beam or almost two hundred days
continuously. Unfortunately, some chunks of gold werestuck on the target surface and caused unstable beam
condition. For the next run, we modified the target coverand installed a carbon fiber brush to scrape off theaccumulations.

Fig. 1. Accumulation caused by vaporized gold.
The vacuum window for the laser path is one of thevulnerable parts in a laser ion source. The window is 1.8 m
away from the target at an angle of 30° from the plasma
expanding axis. Since the plasma expands within about 20°.
the off-set position of the window helps to reduce direct
exposure from the dense plasma. Even under this condition,
after one season’s exposure, the window was darkened. The
outside surface has AR coating, but the inner surface has nocoating since a slight accumulation could easily dirupt theAR condition. On the laser path, the deposited laser energy
helps to keep clean the inner surface of the window. The
laser transmission on the laser spot reduced to 4 %, theremaining area absorbs 9 % of the laser energy.
We also experienced an energy drop in one of the laser
systems. The laser had been in standby mode for a month ata steady temperature of 38 °C. This warm water condition
could cultivate algae in the water circulation system anddegrade the reflectors installed around the laser flash lamps.
Now, we avoid a long standby mode and apply UVsanitization when the cooling water is replaced.
The LIS has operated successfully since 2014. Although
we had some failures, those experiences help us improve
the overall reliability of the LIS.
References

Condensed from the article in Proceedings of North American Particle
Accelerator Conference, MOA4I01 (2016)1
Collider-Accelerator Department, Brookhaven National Laboratory
2
RIKEN Nishina Center
3
Radiation Laboratory, RIKEN1) J. Alessi et al., Rey. Sci. Insrtum., 81, 02A509 (2010).
2) M. Okamura et al., Rev. Sci. Instrum., 87, 02A901 (2016).

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