Although we turned on the cryocoolers for the payload, the temp at DUCT SHIELD BRT1/4 kept gradually increasing, according to the increase of the temp at the 50K BRT HEAD. On the other hand DUCT SHIELD ARM4 started slightly decreasing due to the radiation reduction from the 80K shield.

Actually, we have no way to stop the temperature increase of the 50K REFBRT HEAD.

The final trial is to stop the cryocooler for the DUSCT SHIELD BRT until its HEAD temperature reaches 100K, then to restart. This method was tried for the cryocooler for the DUCT SHIELD ARM in "EY", then the cooling performance was recovered and reported as klog#32861.

Because the temp at DUCT SHIELD BRT4 is approaching the critical temp of 157K, we should perform this in the morning of March 3rd in parallel with the OMC vacuum area recovery.

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Miyoki, Uchiyama, Ushiba, and Kimura discussed the cryocooler's troubles for the DUCT SHIELD ARM in EY and BRT in EX on Saturday and Sunday morning, then we concluded these strategies, considering trying to take any "renewed" sensitivity data using 2 DCPDs and 10W laser power as early as possible. 

The figure 1 shows the past data when the trouble of the cryocooler for the DUCT SHIELD BRT at EX happened.

In this event, the temperature increase continued at the cryocooler head for the DUCT SHIELD BRT, and a sudden enhancement was observed from 78K to 113K. However, the vacuum pressure enhancement seemed to start ~ 2 hours later when the BRT4 temperature reached ~ 157K.

We guess we are now facing the same event. 

At that time, the cooling performance seemed to be recovered after ~ 1 day as shown in Figure 2. However, slight frosting was detected on the AR surface of the mirror and windows. Also the temp enhancement at the head of the cryocooler could be the trigger for the cooling performance recovery as observed in the cryocooler for the DUCT SHIELD ARM in EY.

The BRT4 side of the duct shield is near the mirror. In addition, the duct shield was inserted "inside" the structure of the outer shield of the cryostat itself. So the outgas from the BRT4 area can easily reach the mirror AR side. 

Anyway, the present design of duct shield cooling has no redundancy because there is only one cryocooler for each. We need to consider to add one more cryocooler for each for the reliable commissioning and observation. Otherwise, we will keep losing so much time for the frosting trouble and its recovery.

Or can we recover the frosting on mirrors and windows by only using the IM heaters and window heaters under vacuum conditions?

The temp at EXC1F and maybe 2F increased by 0.3C because of the operation of two more cryocoolers.

Unfortunately, there is no tool for temperature reduction, even if I turned off 1 FFU. I need more reduction of FFU.

The increase rate of the EX BRT Head temp became so slow. The vacuum level at EXT is also very gradually increasing.

ETMX is cooled rapidly (51K: 8K reduction from 59K last morning). So it is better to turn on the heater.

5 days later seems to be the day when water will go out, assuming the BRT HEAD temp increase rate.

So, it is better to take sensitivity with possible achieved improvements.

The comparison between the EY arm and EX BRT which had cooling power reduction troubles.

At present, "EX"_50K_REFBRT_HEAD is around 78.4 K. When "EY"_50K_REFARM_HEAD reached this 78.4K, the "EY"_DUCT_SHIELD_ARM4 reached ~145 K. Meanwhile, the "EX"_DUCT_SHIELD_BRT4 reached 155.7K, which is 13 K larger than 145 K. 

Although there is a difference in position between the Arm and the BRT sides, the EX BRT side seems to have more heat invasion.