The peak partial pressure of H2O in the IXC was around 19:00 on April 9.
Currently, the partial pressure of H2O shows a decreasing trend.
A trend graph of the partial pressures of the various gases for the three days is attached.

IX: 4K_REF3_50K_HEAD performance seemed to be recovered. (Fig.1)

By the way, there might be a typical temperature at which the cryocooler cooling performance is automatically recovered: ~120K in both cases of IX_4K_REF3_50K_HEAD and EX_50K_REFBRT_HEAD (Fig.2).

 The behavior of the cold head around 120 K may be influenced by atmospheric components
that can be contamination components of helium gas and the boiling point of hydrocarbons (methane 112 K).
The hydrocarbons are imagined to be derived from products from the lubricating oil used in the helium compressor.

• EX: (Fig.3)
  • EX_DUCT_SHIELD_BRT4 temp was over the critical temp again because the EX_40K_REFBRT_HEAD temp fluctuates around 85 K.
  • However, the water partial pressure did not change in this time(Fig.2), while the water pressure started increasing just before EX_DUCT_SHIELD_BRT4 temp reached the critical temp in the last case. What is the difference?
• IX: (Fig.1)
  • IX_4K_REF3_50K_HEAD temp kept decreasing.

Figures show temperatures in EX, EY, IY, and IX on 29th April

1. EX(Fig.1) : trouble in the cryocooler for BRT is ongoing as you know.
   • All cryocoolers are working. So even the temperature of the outer shield of the cryostat is lower than the critical temperature(157K). 
   • 50K_REFBRT_HEAD temp is now fluctuating around 88K. So, the DUCT_SHIELD_BRT4/3 temperatures are also fluctuating around just "above" the critical temperature. While, the  DUCT_SHIELD_BRT2/1 temperatures are also fluctuating around "below" the critical temperature
   • The pressure at EXT is fluctuating around 7 x 10^-6 Pa.
2. EY(Fig.2) : slight trouble in the cryocooler for ARM side
   • One of the cryocoolers for the cryo-payload is not operating.  However, even the temperature of the outer shield of the cryostat is lower than the critical temp(157K) as EX.
   • 50K_REFARM_HEAD temperature is fluctuating around 64K that is larger than 50K_REFBRT_HEAD temp by 10K. 
3. IY(Fig.3) : Almost perfect performance as in 2023.
   • Two cryocoolers for the cryo-payload are not operating.
   • However, even the temperature of the outer shield of the cryostat is lower than the critical temp(157K) as EX/Y !
   • The vacuum level at BSYGV is gradually decreasing. The same situation was observed just after the OMC activities.
4. IX(Fig.4) : temp of 4K_REF3_50K_HEAD showed a drastic increase by 40K! In addition, 4K_REF1_50K_HEAD also showed drastic changes by 20K to 100K about 10 days ago.
   • Two cryocoolers for the cryo-payload are not operating.
   • The temperature of the outer shield of the cryostat is "higher" than the critical temp(157K), different with the IY case.
   • 4K_REF3_50K_HEAD temp is now fluctuating around 90 K (It should be less than 70 K at least, compared with other cryostats) after reaching 120 K.
   • 4K_REF1_50K_HEAD temp is now decreasing to the level of 85K  (It should be less than 70 K at least, compared with other cryostats) after fluctuating around 100 K.
   • 4K_REF1_4K_HEAD temp is also recovered at the level of ~30 K.
   • 100K and 120K seem to be important temps for the automatic recovery of the cooling performance. CH4 is now suspected as the impurity in the He gas. One more possible gas?

The difference of 120K and 100K might come from the slight He gas pressure difference between components.

EX 50K REFBRT HEAD temp is now around 95K, which is comparable to the last case. 

The vacuum level at EXC and EXT is worsening as in the previous case. H2O partial pressure is also gradually increasing.

In the last case, after the HEAD temp fluctuated around 95K for 2 days, its cooling performance was recovered. In the case of IX REF1 cooler, 4 days for the recovery after fluctuating around 100 K. In the case of IX REF3 cooler, its cooling performance was recovered just after the HEAD temp reached ~ 120 K.

Different from the last case, not only the BRT4/3 temps but also the BRT2 temp is expected to be over the critical temp (157K) in this time.

I am now hesitating whether we should increase the EX REFBRT HEAD temp to 120K for the lucky quick recovery as IX REF3 cooler.

　As the attached graph shows, the gradient of the H2O partial pressure in the EXC increase changed around 3:20 a.m. on May 2.
The H2O partial pressure continued to rise.

As the attached graph shows, at about 3:21 p.m. on May 2, the H2O partial pressure in the IYC increased, but recovered to the previous level in about 4 hours.

This is just because of the EX 50K REFBRT HEAD temp rapidly increased from 97K to 122K for 3 hours. The vacuum level was almost the same to those in the previous case. (Fig.1)

However, the temperature enhancement up to ~ 120K and the start of gradual decrease are the same as the performance in other cryocoolers, which encountered the same cooling performance reduction trouble, such as IX 4K REF3 50K HEAD about 22 days ago, EX 50K REFBRT HEAD about one and a half years ago.

In the case of   IX 4K REF3 50K HEAD trouble about 22 days ago,  it took 4 days to be below 90K, and 10 days to reach 83 K. (Fig.2)

Although this is just my guess, water in some tiny dead-end space came out because of the shrinking caused by the cooling.

EX 50K REFBRT HEAD temp is now decreasing from 122 K to 100 K.  However, the vacuum pressure mainly dominated by H2O was 3 times larger than the vacuum pressure in the same kind of incident in March because the temperatures at BRT1/2 were also over the critical temperature in addition to on BRT3/4. So, the OPLEV SUM values seemed to decrease by ~ 15% because of H2O frosting. We need careful check for the HR side. At least 6.4Mpc sensitivity was obtained in this morning.

I also compared the same kind of incident on August 12th, 2023, as Fig.1(cryocooler head temps) and Fig2 (Oplev SUM), and this time on May 1st, 2025, as Fig.3 and Fig.4. 

In both cases, all cryocooler were operated. The temp decreasing trend just after the enhancement up to ~ 120K seems to be roughly the same. The temps on BRT1-4 are also the same with each other.

One remarkable difference is the vacuum pressure level during the incidents. At EXC vacuum gauge,  2.5x10^-4 Pa was the worst on Aug 12th, 2023, while 3.5x10^-5 Pa at this time. About 1/10 outgases in this time compared with the case on Aug 12, 2023. The reason is not clear at all.

The other remarkable difference is the SUM variation during outgassing. In the case of Aug 12, 2023, the obvious sinusoidal changes were observed because of H2O frosting on windows. However, the mean value once increased and came back. On the other hand, in this case on May 1st, 2025, no such sinusoidal changes, but only a reduction of SUM.

Anyway, H2O frosting on the AR side at this time is expected to be less than the case on Aug 12th, 2023. Also, we can expect the EX 50K REFBRT HEAD temp will reach ~80K for 10 days, assuming the same performance as the case on Aug 12th, 2023.

120K is the magical temperature for the self-recovery of the cryocooler performance.

If the cryocooler is much effective than TMP for absorbing H2O, it is worth installing a cryocooler at the BRT side for the emergency case.

In the past, Windows' frosting source had always come from the warmed duct shield, except for the warm-up timing of the 2-layer radiation shields themselves.

So very simple solution to avoid such a nasty (Tobacchiri) situation, short ducts should be put around windows, avoiding contact with other structures, such as the vacuum tank, and other layer radiation shields? (pdf file)

If the outer shield's temperature is over the critical temperature, no frosting on windows on the outer shield, while some amount of frosting on windows on the inner shield. If the outer shield's temperature is under the critical temperature, no frosting on windows on the inner shield, while some amount of frosting on windows on the outer shield. It is necessary to know the frosting rate on windows on the inner/outer shields according to several possible cases.

In the incident on 12th, August 2023, the EX_50K_REFBRT_HEAD temperature reached ~ 120K, then decreased by ~30K, then kept fluctuating around 90K for ~3.5 days, then started decreasing to ~ 70 K. This process took ~ 11 days.

At this time, the EX_50K_REFBRT_HEAD temperature reached to ~ 120K, then also decreased by ~20K, then also kept fluctuating around 95 K for 3.5 days, then started decreasing at present. A very similar performance is now observed.

Figures show temperatures in EX, EY, IY, and IX.

1. EX(Fig.1) : trouble in the cryocooler for BRT is ongoing as you know.
   • All cryocoolers are now operating.
   • 50K_REFBRT_HEAD temp once reached around 120K, then started decreasing. This phenomenon is similar to the incident on August 12th, 2023.
   • DUCT_SHIELD_BRT1~4 temps were once over the critical temp (Tc:157K). At present, the only BRT1 temp is below Tc.
   • The pressure at EYT once increased and decreased now.
   • Oplev values seemed to be decrease because of the water frosting.
   • Is it true that the temp of EX_4K_REF2_4K_HEAD is around 11K and EX_4K_REF4_4K_HEAD is around 9K ! 
2. EY(Fig.2) : slight trouble in the cryocooler for ARM side
   • 5 cryocoolers are now operated, except for EY_4K_REF4.
   • 50K_REFARM_HEAD temperature is now fluctuating around 68K. The slight enhancement was recently observed. We need monitoring.
   • Is it true that the temp of EY_4K_REF2_4K_HEAD is around 8.5K ! 
3. IY(Fig.3) : Almost perfect performance as in 2023.
   • All cryocoolers are now operating.
   • The vacuum level at IYA became very stable after operating the last two cryocoolers for cryo-payloads, maybe because the Nitrogen absorption became effective.
   • Is it true that the temp of IY_4K_REF2_4K_HEAD is around 8.5K  ! 
4. IX(Fig.4) : temp of 4K_REF3_50K_HEAD recently shows a drastic increase by 40K!  While, 4K_HEAD temp did not show a drastic increase.
   • All cryocoolers are now operating.
   • 4K_REF1/3_50K_HEAD temps once increased at the level of 100K/120K. However, their cooling performance is being recovered.

At present, the cooling performance recovery of EX_50K_REFBRT, and frosting on windows on the EX 2-layer radiation shields are of concern.

IX 4K REF2/4 cryocooler seems to be strange because their 2nd head temps are now around 20K, while others are around 10 K.

This is the reason why the IX payload temp cooling is slower than others, especially than IY.

>Anyway, H2O frosting on the AR side at this time is expected to be less than the case on Aug 12th, 2023. Also, we can expect the EX 50K REFBRT HEAD temp will reach ~80K for 10 days, assuming the same performance as the case on Aug 12th, 2023.

The EX 50K REFBRT HEAD temp is around 90K, which is higher by 10 K compared with the incident on August 12th, 2023. Please come back! Because of this higher temperature, the BRT2 temp cannot be below the critical temp.

It might be effective to recover the cooling power by replacing the flexure tubes again with those that were sent back to Sumitomo Heavy Industry for its gas components inspection after cleaning their inside and refill He gas in them again.