Annular enclosure provided with an arrangement of recesses or protrusions to reduce mechanical resonance

A cryostat for housing a solenoidal superconducting magnet, the cryostat comprising:

• a liquid cryogen vessel for containing liquid cryogen;
• and
• an outer vacuum chamber surrounding the liquid cryogen vessel;
• wherein the outer vacuum chamber comprises an annular enclosure comprising:
• a cylindrical outer shell;
• a cylindrical inner shell;
• annular end caps which join said outer shell to said inner shell, forming said enclosure;
• and
• a non-random pattern of (a) recesses having a bottom surface made of a material of the annular enclosure and (b) protrusions having a top surface made of the material of the annular enclosure and a wall, made of the material of the annular enclosure, descending from the top surface to the bottom surface, wherein the recesses are located between the protrusions;
• wherein said pattern is asymmetrical in all axes, whereby the tendency of the material of the annular enclosure to mechanically resonate is reduced.