High Power Test of Mizuno - Ohtsuka Dry Load S. Matsumoto and T. Higo Report made on

Mizuno-Ohtsuka Drawing 800mmX160mmX20mm Wgt=10kg

Mizuno-Ohtuka Dry Load

High Power Test Evaluate the high power performance. Specify its characteristics rigorously. (Still it does not have the official spec!) Check the performance of the load under the air cooled condition as well as under water cooled by the water-cooled “heat sink” attached on it. Two new loads (MODL#? and # ) and one used (KDum005) were prepared (ex-situ prebaked) for the high power tests. One new load has been tested (Dec. – Dec. 24) and we report here on this load.

Ex-Situ bake The load was prebaked before its installation to KT-1 station. Why? Because we expect the reduction of outgas during the conditioning process and it should result in saving the time needed for the processing. The bake temperature was set to be 200deg (automatically controlled within less than a few degrees). Bake continued until the gas pressure in the load until being equilibrium. It usually takes a few days. RF Load (under baked) To Roughing pump (TMP)

Load launched at KT Waveguide Valve RF Load Directional Coupler downstream Directional Coupler upstream Cold Cathode Gauge 30l/s Ion pump

Test started on Dec 11. Conditioning process. Input Power goes up and down to “bake out” the outgas Test started. Pulse width Vac (Load head)

Evaluate performance under constant power starting on Dec. 14: Observe how often the trips(breakdowns) take place under air- cooled condition.

Performance Table (1) (as an air-cooled load) Pulse Width(ns) Power (MW) Heat Deposit (W) Surface Temp (C)* Breakdowns / Duration (at Pos 3)0 /12Hr (5)0 / 16Hr (5)0 / 3Hr (5)0 / 23Hr (4)0 / 24Hr * Surface temp = Maximum Temp Raised among 9 monitor points. The maximum position number is specified in the bracket.

Summary as an air-cooled load We have not observed any breakdown event in the load during whole its performance test. The net heat deposit of 150W is a good criterion for the maximum spec of the load under air-cooled condition though it is sure that the load can handle more power than this. The operation limit eventually comes not from the occurrence of breakdown events but from the surface temperature being too high. Although there is no strict limit on the maximum surface temp, it should be fair not to exceed 100deg C for safety reason. This naturally sets the upper limit of net heat deposit into the load (150W). Two days’ conditioning process was required before the vacuum pressure being well lowered and stabilized. We applied a normal processing technique; starting with pulses of short width and of small peak power and increase the width and peak gradually as the outgas from the load being “exhausted”.

Water-cooled attachment (Heat sink)

Load with the attachment at KT The water-cooled attachment is bolted and clamped together with the load as shown in this picture. The water flow was set to be11.5 Littre/min. Clamps Bolts

Performance test with the attachment 53MW / 100ns Looks to need some processing for higher heat deposit Set 100ns/53MW 24Hr

Performance test with the attachment 55MW / 200ns Reach stably 53MW Sudden increase of width requires some processing against the load. 24Hr Set 200ns/55MW (from 100ns/53MW)

Performance test with the attachment 39MW / 360ns Scout at 45MW Tried to set 360ns/50MW, but could not. Gave up. 24Hr Set 360ns/39MW

Performance test with the attachment 39MW / 500ns Suffers severe breakdowns Need some time to recover 24Hr Set 500ns/39MW

Performance Table (2) (with the water-cooled attachment) Pulse Width(ns) Power (MW) Heat Deposit (W) Surface Temp (C)* Breakdowns / Duration (at Pos 4)7 /22Hr (4)8/ 32Hr (4)0 / 18.5Hr (4)2 / 2.5Hr (4)8 / 24Hr * See the note appeared in “performance table (1)”.

Performance of the load with the water-cooled attachment In both the cases of100ns /53MW and 200ns / 55MW, we have rather many breakdowns in spite of their relatively light or moderate heat deposit (265W and 550W respectively). We may simply say that we have overestimated those breakdown rates. By taking into account of 360ns/39MW (702W) case, the E- field in the 100ns/200ns cases should be 40% higher than 360ns case and this should make the breakdown rate larger significantly. The case of 500ns /39MW shows a “severe” breakdown event which causes other successive breakdowns in the recovery process and it takes some time to recover the load to accept the nominal power. It is concluded that the heat deposit 975W should be “heavy” enough for the load.

Summary as a load with water- cooled attachment It was fully demonstrated that the water-cooled attachment works fine to suppress the surface temperature raise. The water-cooled attachment improves the performance of the load. 750W heat deposit or 50MW peak power is a good criterion to specify the performance limit of the load. The limit can be set by how often the occurrence of breakdown events one can allow. Current criterion gives the breakdown rate of one BD per a few hours at most.

Surface Temp Measurement Load set at KT-1 with 9 thermocouples on its surface Launched at KT-1 50MW X-band Station

Surface temps (1): air-cooled cond. (at 9 monitor points)

Surface temps (2): water-cooled. (at 9 monitor points)

Surface temp Monitor The temperature raise of each point is monotonically dependent on the power deposit in the case of water-cooled while it is not in the caseof air-cooled. The highest temperature raise is found around point 4 or 5 in both air and water cooled cases. The raise should be reflected the RF loss distribution along the RF path in the load.

Heat conductivity MaterialHeat conductivity (Watts/meter.K) Copper350 Aluminum230 SS15 Water0.63 Air0.024 There are some heat transfer sheets (or paste) whose conductivity varies from a few (silicon rubber), several () and more than 1000(carbon graphite sheet). The unit of conductivity is Watts/meter/K.

Possible option: Water Cooling Pros: Make the fabrication process simpler since the load has no water channel while the plate has it. Flexible usage of the load can be possible. It depends on the actual net heat deposit on the load. Cons: The contact between the plate and the load may not be perfect. The heat from the load is not possibly transferred so well. The heat sheet may damage due to radiation (X-ray). The “naked” load can be sandwiched with cooling plates being clamped together. It is better to insert a piece of Heat Sheet between the load and the plate for better thermal conductivity.

Conclusions Mizuno-Ohtsuka dry load shows excellent performance under air-cooled condition upto about 150W heat deposit. Although the load is probable to work at more than this power, it should be regarded as its actual operation limit since its surface gets more than 60deg C temp raise maximum.. 150W should be a good criterion of the maximum spec of the load under air-cooled condition The water-cooled attachment improves much the spec of the load. It was shown the power of 360ns, 39MW, 50pps (=700W heat deposit into the load) can be safely handled. 750W heat deposit or 50MW peak power is a good criterion to specify the performance limit of the load. There happens sometimes (typically once per several hours) breakdown events in the load at this heat deposit or peak power and this sets the specified maximum power while the surface temp is kept moderate (about 30deg C temp raise.) The observation of the surface temp shows the RF loss in the load is not uniformly distributed. Point 4, 5 and 6 are the “lossy” places. We prebaked (Ex-situ bake, 200deg C for a few days) the load before launched in the station for the test since we expect this helps the reduction of outgassing in the conditioning process and we can save the time of processing.