4.3.1 Experimental Setup
Figure 4.3 shows a schematic drawing of the experimental setup in this measurement.
A Gifford-McMahon (GM) cryocooler (Suzuki Shokan Co. Ltd.), which has a cooling power of 0.7 W at 4 K is utilized to cool down the test samples. A 15 mm thick copper plate is attached to the 2nd stage of the cryocooler as a sample stage. The test sample is
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set on the sample holder bolted on the sample stage. A film heater with 25 Ω is glued on the 2nd stage to adjust the base temperature during the measurement.
In this experiment, the thermal oscillation from the GM cycles is found to be about 200 K at low temperature around 4.5 K so that the measurement at low temperature becomes very difficult. In order to reduce the effect of thermal oscillation, a glass fiber reinforced plastic (GFRP) dumper with a thickness of 6 mm is inserted between the sample stage and the sample holder [82]. The thermal oscillation is reduced from 200 mK to less than 20 mK at the sample holder.
Sample stage
Wires (AWG36) Heater Wires Aluminum tape
Film Heater 2nd stage
Super Insulators
40 K Shield
Figure 4.4: Development of measurement system with cryocooler. Left: The wires used in measurement are fixed on the sample and 2nd stage with aluminum tape, and well-wound on the cylinder to avoid the heat inleak from the conduction through wires. Right: Both 4 K and 40 K are covered with the SI to reduce the thermal radiation. The bolt joints in the 40 K shield are filled with the cryogenic grease to enhance the thermal contact.
A thermal shield made of aluminum plate is attached to the first stage of the cryocooler to be cooled at 40 K. As shown in Fig. 4.4, totally 12 layers of superinsulation (SI) covers the 40 K shield to reduce the thermal radiation from the room temperature to 40 K stage.
An additional thermal shield made of 2 mm thick aluminum plate covered by 12 layers of SI is attached to the 4 K stage, and a resistive heater is adhered on the 4 K stage to
Chapter 4. Measurement of Insulation Thermal Conductivity with Irradiation
warm up both the 4 K stage and the thermal shield to reduce the thermal radiation to the test sample.
A resistive heater of 125 Ω is glued to a thin copper plate with the epoxy glue (Stycast 2850FTJ), and the plate is bolted on the top of the test sample to input the heat so that thermal conduction can be measured with the temperature gradient in the sample.
Two temperature sensors (LakeShore, Cernox CU-1050) named Tu and Tl are bolted on the surface of aluminum bars with a distance of 10 mm from the surface of the BT prepreg tape. Additional heater plate is bolted on the sample to directly adjust the sample temperature. Temperature at the sample holder and the sample stage is monitored by the same type temperature sensors named Th and Tb during the measurement.
Cryogenic grease (Apiezon N grease) is applied to enhance the thermal contact at temperature sensors, the sample heater, sample holder and so on. Fine phosphor bronze wires (LakeShore, QL-36) that connect to the sensors and the sample heater are wound on the stainless bobbin keeping enough length between the sensors and the thermal anchor to prevent the heat leak. These wires are fixed on the sample stage with aluminum adhesive tape to make a thermal anchor as shown in Fig. 4.4.
4.3.2 Measurement System
A temperature controller (LakeShore, Model 335) is used to readout two temperature sensors of Tu and Tl, and to control the heaters on the sample base and the sample holder.
The sample temperature at Tlis controlled to be stable with the sample holder heater in a closed loop. The temperature at the sample stage and the sample holder is measured with another temperature monitor (LakeShore, Model 218). A DC current source (Keithley, Model 6221) is used as a current supply to the sample heater of which voltage is measured with a nanovoltmeter (Keithley, Model 2182A). The heater power is measured in four wire configuration. In this measurement, the temperature and heater power is measured with the sampling rate of 10 Hz.
4.3.3 Gamma-ray Irradiation
Unlike the neutron irradiation in metals, the major mechanism for the organic material is the ionization since the ionization can break the chemical bonds, which influences the material properties. Thus the recovery of radiation damage in an organic material can be ignored by annealing effects, and the irradiation test is usually performed at room temperature. In this work, the irradiation effect on thermal conductivity of insulation is investigated with the gamma ray irradiation because the uncharged particle of gamma ray is domain source to cause the ionization in the insulation at PCS magnets. The peak energy deposition in PCS is expected to be about 1 MGy for 280 day operation by the PHITS simulation as described in chapter 2. Thus three samples are irradiated by 60Co gamma ray at Takasaki Advanced Radiation Research Institute of the National Institutes for Quantum and Radiological Science and Technology (QST) with the accumulated dose of 0.2, 1 and 5 MGy, respectively. In QST, the radiation source of 60Co has the radioactivity of 8517 TBq measured in 4/Jan/2017.
Glass ampule
Test sample
10 cm
Irradiation source
Figure 4.5: Sample setup for the irradiation. The test sample is set far from the irradiation source and irradiated at vacuum.
As shown in Fig. 4.5, to prevent the oxidization of the insulation tape, the samples are enclosed in a glass ampule and irradiated at vacuum. The samples are settled on a support frame, and positioned with a distance of 10 cm far from the 60Co source where the dose rate is measured to be 10 kGy/h. The thermal conductivity measurement is performed on each sample before irradiation to compare with the thermal conductivity after irradiation.
4.3.4 Reference Measurement
Due to the higher thermal resistance in the insulation layer, the heat leakage from the sample heater and the sensors could affect the quality of measurement. Although the lead wires are aligned with the consideration of heat leakage, the heat leakage should be investigated to ensure the reliability of this measurement system.
To validate a heat leakage in this setup, a reference measurement on stainless steel (SUS304) has been performed. As shown in Fig. 4.6, we choose an I-shaped SUS304 sample with a length of 140 mm and a cross sectional area of 10 × 10 mm2, of which the thermal resistance is relatively higher than the test sample of insulation tapes. The other equipments are same as that in the measurement for insulation tape. Figure 4.7 shows the result of SUS304 measured from 5 K to 24 K. The measured thermal conductivity of SUS304 is in good agreement with the reference data derived from the NIST material properties database [83] within the measurement error. Thereby we confirm that the heat leakage is well suppressed in this measurement system.
Chapter 4. Measurement of Insulation Thermal Conductivity with Irradiation
Thermosensor Heater plate
SUS bobbins 30 mm
Sample (SUS304)
Figure 4.6: Experimental setup for the reference measurement. An I-shaped SUS304 sample is bolted on the sample holder. The temperature difference between two temperature sensors is measured by adjusting the heater power.