Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Development of high-temperature oven for 28-GHz ECR ion source
J. Ohnishi,＊1 Y. Higurashi,＊1 M. Kidera,＊1 K. Ozeki,＊1 A. Uchiyama,＊1 and T. Nakagawa＊1
U35+ ions extracted from the 28-GHz superconducting
ECR ion source1) are used to supply uranium beams to theRIBF. Although we have thus far used the sputtering
method, in which uranium is supplied in the ion source
plasma by directly inserting a metal uranium rod, we began
developing a high-temperature oven2) with the aim ofincreasing and stabilizing the beams. Because the ovenmethod uses UO2, a crucible must be heated to atemperature higher than 1900°C to supply an appropriate
amount of UO2 vapor to the inside of the ion source.
Figures 1 and 2 show the dimensions of the crucible andillustrate the oven in its entirety. The crucible isjoule-heated with a large DC electric current. The crucible,
made by machining a tungsten rod, is supported with upperand lower water-cooled copper blocks. The electric current
and cooling water are supplied through brass double pipes.
The crucible was designed by performing the electric,thermal, and structural analyses simultaneously usingANSYS.3) Figure 3 shows the temperature distribution ofthe oven, calculated by ANSYS. The boundary conditions
are as follows: The temperature of the cooling water is27°C, the heat transfer coefficient from the water to thecopper block is 5000 W/m2/K, and the voltage between theupper and lower copper blocks is 1.25 V. The radiation
coefficient of tungsten was assumed to be 0.25. The electric
current was calculated to be 439 A. The maximumtemperature of the body is 2041°C, and the temperatures ofthe bottom and the cap are 1960−2000°C.
The oven is placed in a solenoid magnetic field ofapproximately 3.3 T, which is orthogonal to the axis of thecrucible. Therefore, if an electric current of 450 A flows
through the crucible, the crucible is subjected to anelectromagnetic force of approximately 40 N. According to
the ANSYS calculation, a maximum stress of 160 MPa is
generated around the tapered parts on the crucible bodysides of the upper and lower rods. Since the temperature ofthese tapered parts increases to higher than 1800°C, it wasexpected that this stress level could result in thedeformation and destruction of the crucible with thedecrease in the tungsten’s strength. In fact, bends in theupper and lower rods were observed after operation.
We installed the oven loaded with UO2 in the 28 ECR ion
source and tested the generation of uranium beams in April
2013 after a temperature rise test and temperature
measurement in a test chamber. In the first test, the oven
was operated for 42h and a U35+ beam current of 140 A
was successfully obtained at an RF power of approximately
3 kW. After the first test, operation tests of the oven wereexecuted intermittently from July to December. The
operation time was a total of 29 days. Although we could
maintain a U35+ beam current of 50−80 A at an RF powerof 1.5 kW for a maximum of one week, the beam currents＊1

often decreased to less than half in 7−8h. This decrease
resulted from UO2 blocking the crucible ejection hole.
Since the cause of the ejection hole’s blockage was assumed
to be that the temperatures of the cap of the crucible and theupper part of the hole are lower than the temperature of thebottom, we reduced the thicknesses of the cap and brim by
0.2 mm. Figure 1 shows the schematic after this reduction.
Presently, we have just started to test the crucible of the newdesign. We are also investigating the use of rhenium insteadof tungsten, which has better creep characteristic strength athigh temperature for solving the bends and fracture of therods of the crucible.

Fig. 1. Schematic of the tungsten crucible. The axis of thecrucible is oriented vertically.

Fig. 2. Schematic of the crucible and support.

Fig. 3. Temperature distribution of the oven calculated byANSYS.

References

1) Y. Higurashi, et al., Rev. Sci. Instr. 83, 02A308 (2012).
2) J. Ohnishi, et al., Rev. Sci. Instr. 85, 02A941 (2014).
3) http://www.ansys.com/

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