RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-8. Accelerator

Supply of 70Zn beam from 18-GHz ECRIS using the micro-oven
K. Ozeki,＊1 Y. Higurashi,＊1 and T. Nakagawa＊1
In the 18-GHz electron cyclotron resonance ion source(ECRIS)1), we have achieved the practical use of themicro-oven to supply high-intensity and stable 48Ca beams
for a long term. 2-4) In the process of the operational tests,
we found that the temperature of our oven could beincreased up to about 1000○C without damaging the oven.
At that temperature, the vapor pressure of ZnO is expected
to reach a level high enough to extract the Zn beam withadequate intensity. Therefore, we conducted the supply testof Zn beam using the micro-oven.
In contrast to the Ca beam, in which the Al powder ismixed to the CaO powder to reduce CaO, only the powderof ZnO was placed in the crucible because the vapor
pressure of ZnO is sufficiently high. The hot liner5), whichplays an important role in reducing the material
consumption rate when supplying the 48Ca beam, is not
used when supplying the 70Zn beam because even if theoven current is increased, the beam intensity hits a peak at
an inadequate level when installing the hot liner into theECRIS.
When increasing the oven current gradually, the water
evaporated first. By increasing the oven current, theproduction of the Zn beam was observed at the oven current
lower than that at which the Ca beam was produced. This
production seems to be due to the metallic Zn existingslightly in ZnO. The production of the Zn beam at this ovencurrent was terminated after a short time. By further
increasing the oven current, the production of the Zn beam
was observed again at the oven current higher than that at
which the Ca beam was produced.
The 70Zn15+ beam produced by the micro-oven was firstsupplied for the experiment at the RIBF, from May to June
2014. The beam intensity at the exit of ECRIS and the ovencurrent are shown in Fig. 1. As an example, the power
applied to the oven for the oven current of 2.45 A (May 16to May 19) is estimated to be 39 W. The RF power fed tothe ECRIS was 550 W. During the experiment, the sudden
increases in pressure in the ECRIS, followed by either theincrease in beam intensity or runaway of the ECRIS,
occurred several times. These phenomena seem to be due tothe grain size of the ZnO (in this experiment, the material
was prepared by chipping sintered ZnO rod. The grain size
was up to about 1 mm in diameter). Assuming that a grain
had a metallic Zn core and a ZnO shell, the inner pressureof the grain increases. The shell becomes thinner with theevaporation of ZnO and cracks at some point to cause asudden increase in pressure in the ECRIS. This instability
problem seems to be prevented by chipping the grains as
fine as possible.

＊1

Because there was a break period (“CGS failure” in Fig.
1), the material was replaced just to be safe. The statuses ofbeam supply before and after the break are summarized inTable 1. The consumption rate all through the experiment
was 0.14 mg/h.
Table 1. Status of beam supply.
Before After
Beam intensity [electric A]
30
33
Amount of ZnO placed in the crucible [mg] 1007
835
Amount of ZnO consumed [mg]5922Consumption rate of ZnO [mg/h]
0.16
0.10
The vapor pressures among Ca, Zn, and ZnO are in thefollowing order: Zn > Ca > ZnO. In order to evaporate ZnO,
an oven temperature higher than that for Ca is required. But
after being decomposed into Zn and O by the plasma, avapor pressure higher than that of Ca is obtained.
Sufficiently low consumption rate without the hot liner
seems to stem from the above relation of vapor pressures.

Fig. 1. Long-term supply of the 70Zn beam for theexperiment at the RIBF. The beam intensity for 70Zn15+(gray) and the oven current (black) are shown.
References

1) T. Nakagawa et al.: Nucl. Instrum. Methods B 226, 392 (2004).
2) K. Ozeki et al.: RIKEN Accel. Prog. Rep. 46, 128 (2013).
3) K. Ozeki et al.: Rev. Sci. Instrum. 85, 02A924 (2014).
4) K. Ozeki et al.: Rev. Sci. Instrum. (to be published).
5) A. Efremov et al.: Rev. Sci. Instrum. 69, 662 (1998).

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