Confirmation of leak tightness of the mo sealing in superfluid helium

In SRF cavity application, super-fluid helium leak is a issue. Development of highly reliable and cost effective seal method is very important. We have successfully applied MO seal to SRF cavities. We have found out a leak rate limit: 1*E-10 Pam 3 /s with super-fluid helium. When the helium leak rate is smaller than this value, the cavity gradient is expected to be higher than 25MV/m from vacuum point of view. MO seal satisfies well this level. Tightening torque: 15Nm works to get such a small leak rate. As the flange material, SUS316L and Titanium are useable. Copper gasket looks reliable as seal material.