The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DESIGN OF THE MUON TARGET AT J-PARC
S.Makimura, Y.Miyake, K.Nishiyama, and K.Nagamine, KEK, Tsukuba, Japan
Abstract
At J-PARC muon science facility, the most intense
pulsed muon beam will be produced by 3 GeV/1 MW/25
Hz proton beam. As a muon target, we are going to adopt
an isotropic graphite (IG-43) with a thickness of 20mm,
which shape is like a disk. As a frame, copper is adopted
and a stainless steel tube is buried in the copper frame.
The heat loss, which is generated by the proton beam, isestimated to be 3.3kW on graphite and 600W on thecopper frame in NMTC/JAM [1]. Figure 1 shows aschematic view of the target and the picture of the target.

Fig.1: The schematic drawing of the target and the pictureof the target
In the first section, static properties, calculated by thefinite element methods ANSYS are reported. Thermal
problem and stress from the gradient of the temperature
are discussed. In the second section, dynamic properties
such as mechanical shockwave and thermal transient
response are described. In the third section, theexperiments to test the calculations by ANSYS aredemonstrated.

STATIC PROPERTIESAt first, the static properties such as temperature andstress, were evaluated. The target is placed in the vacuum
chamber, therefore almost all of the heat must be removedby the flow of water in the copper frame. Considering
thermal properties of graphite, thermal conductivity ofgraphite under neutron irradiation has to be taken into
account. Figure 2 shows temperature dependence of thethermal conductivity of graphite in cases of the variousconditions of neutron-irradiation [2].

Fig.2: Temperature dependence of the thermal
conductivity of graphite in cases of the various conditionsof neutron-irradiation.
Thus, the gradient of the graphite temperature is muchenhanced under the neutron irradiation. By the calculationof the finite element methods ANSYS, the maximum
temperature of graphite (on the centre of target) reaches
upto 1500 degC. This value is much below the melting
point of graphite. At this temperature the vapour pressureis also negligible small. At the first design, we adoptedone turn of the stainless steel tube, resulting in the surface
temperature of the water tube to be more than 250 degC.
In order to get rid of boiling, three turns of water tube areadopted. Consequently the temperature of the surface ofthe tube became about 110 degC, which is lower than theboiling point under the 0.3MPa to be 140 degC.
Because of the large gradient of the temperature, thethermal stress must be considered. If the copper is justplaced around graphite, the stress on graphite is very
close to the strength to stress. Therefore in order to absorb
the thermal stress, the titanium layer is adopted as anintermediate material between the graphite and the copper.
Table 1 shows the stress on graphite in case that there isno titanium layer as a stress absorber and in case that
there is a titanium layer with a thickness of 2 mm.
Table1: The stress on graphite in case of no titanium
layer and in case of 2 mm titanium layer. As a reference,the strengths of graphite are also on the table.
Strength(Stress)
Temperature (degC)

No Ti

Ti
2mm

1462

1492

Tensile stress (MPa)

3735

8

Compressive stress(MPa)904042Share stress (MPa)18176In case that the thickness of titanium layer is 2 mm, thecompressive stress is 42 MPa (compared with the strength90 MPa) the tensile stress is 8 MPa (compared with theThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

strength 37 MPa), and the share stress is 6 MPa(compared with the strength 18 MPa). Figure 3 shows thedistribution of the maximum principal stress (S1) and theminimum principal stress (S3) in case that the thicknessof the titanium layer is 2 mm. Now the stress isconcentrating on the titanium layer, and it must bechecked whether the stress on the layer is less than theelastic limit of titanium, that is 190 MPa. It’s also
calculated that the stress is 150 MPa by ANSYS.

Fig.3: The distribution of the maximum principal stress(S1) and the minimum principal stress (S3). The thicknessof the titanium layer is 2 mm.

Fig.4: The rises of the temperature in a proton pulse on
the centre of graphite in the cases of 700 degC and themaximum temperature.
In the accidental operation, the water flow was assumed
to be stopped. This issue is important to construct the safeinterlock system. Even if we have the interlock signal of
the water flow to stop the proton beam, the target musthave the two stages of interlock system. Another signal
should be prepared for the case that the proton beam
couldn’t be stopped by the signal of the water flow.
Figure 5 shows the response of the temperature on thetarget and the frame when the water flow stops. As theflow of water doesn't stop, the temperature of graphite
raises gradually, on the other hands the copper frame
raises rapidly. When the temperature of the copper frame
reaches 300 degC in 20 seconds, the temperature of thegraphite is still 1470 degC. When the copper frame
reaches 680 degC (melting point of silver blazing) in 70
seconds, graphite is 1660 degC. In 120 seconds, thecopper frame reaches 1040 degC and starts to melt. From
this calculation, the upper limit of the temperature of thecopper frame is less than 680 degC.

DYNAMIC PROPERTIES
Next, the dynamic properties such as the mechanicaltransient response, so-called shockwave, and the thermaltransient response were evaluated. The pressure increment
in a pulse, which is induced by a sudden increment of thetemperature, is estimated to be only 0.6 MPa by a thermoelastic calculation [3]. From the calculation by ANSYS, it
is checked that there is no amplification of stress.
The thermal response were analysed for two cases, thenormal operation and the accidental operation. In thenormal operation, at first it was estimated how long it
takes from the room temperature (30 degC) to themaximum temperature (1460 degC) and from themaximum temperature to R.T. Calculating by ANSYS, it'sproved that it takes 60 seconds from the R.T. to themaximum temperature and 100 seconds from themaximum temperature to the R.T. And there was no
extraordinary gradient of temperature, which will produce
a local stress.
Then it was estimated how much temperature willchange in a proton pulse. Under the maximum
temperature, the rise of the temperature in a pulse is 6
degC. The repeated stress induced in a pulse is smallenough to be 0.1 MPa. It won't lead to the fatigue of therepeated stress. Figure 4 shows the rises of thetemperature in a proton pulse on the centre of graphite in
the cases of 700 degC and the maximum temperature.

Fig.5: The response of the temperature on the target andthe frame when the flow of water stops.
But in the actual operation, the water flow will notrecover soon after the proton beam stops. In this condition,
the enthalpy that is on the centre of the target will spreadto outside. Figure 6 shows the spread of the enthalpy
when the proton beam stops at the temperature of thecopper frame to be 500 degC. At least the temperature of
the copper frame must be less than 500 degC in theinterlock system. Finally the temperature of the copper
frame to stop the proton beam is set to 200 degC

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Fig.6: The spread of the enthalpy when the proton beamstops at the temperature of the copper frame to be 500
degC.

EXPERIMENTS
So far, according to the calculated design, we made alot of efforts to produce a prototype of muon target. At
first silver blazing and casting methods were tried to bondthe stainless tube to the copper frame, resulting in afailures. The silver blazing cannot acquire enough thermaltransfer because whole water tube cannot be covered. Inthe casting, it was impossible to fix the tube in the precise
position, as the deformed stainless steel has the residualstress. Finally it turned out that HIP process worked very
well. Then titanium layer and graphite was bonded tocopper frame by silver blazing in vacuum simultaneously.
Since HIP was achieved at the temperature of 850 degC,
silver blazing, which has the melting point of 680 degC
was adopted. Figure 1 shows the picture of the producedtarget.
On the next step, the target was heated by two electron
beams by both sides. Approximately 4 kW of heatgeneration by the proton beam is calculated on the target.
In this experiments, it was succeeded to supply 4.2 kW ofheat on the target. Figure 7 shows a picture of the filament.

In this experiment, we should consider that the raw
material without the neutron irradiated was used and theheat generation is just on the surface. But on thecalculation of ANSYS, the temperature of copper frame isalmost same as the operation of proton beam. There are
two purposes in this experiment. The first is to check thebonding of each interface. The second is to acquire thethermal transfer coefficient between the surface of thewater tube and water. In general, the value from theexperience was used for this parameter [4]. However it
depends on a lot of conditions, such as the material, theshape of tube, the roughness of the surface, and so on.
Therefore it’s better to have the experiments in the sameconditions. In order to check this parameter, the heatgeneration and the temperature of copper frame ismeasured in the electron beam experiment. In thisexperiment, it’s proved that the value of thermal transfer
coefficient was 2000 W/m2/K, while thermal transfer
coefficient was supposed to be 3800 W/m2/K from theexperiences. On the other hands, the bonded silver
blazing looked well. In figure 8, the temperatures
measured in the experiments are plotted, and the solid line
is a calculation assuming thermal transfer coefficient to be2000 W/m2/K.

Fig.8: The temperatures measured in the experiments areplotted, and the solid line is a calculation assuming
thermal transfer coefficient to be 2000 W/m2/K.

REFERENCES

Fig.7: A picture of the filament[1] N.Kawamura, KEK-MSL Report 2000 (2000)13.
[2] T.Maruyama and M.Harayama, “Neutron irradiation
effect on the thermal conductivity and dimensionalchange of graphite materials,” Journal of Nuclear
Materials 195 (1992) 44-50.
[3] Z.Tang and K.Anderson, “Shock Waves in P-bar
Target,” FERMILAB-TM-1763 (1991)[4] JSME Data Book, “Heat Transfer”, The 4th Edition