Measurement of the cooling capacity of an RMC-Cryosystems Model LTS 4.5-025 closed-cycle helium refrigerator

We have measured the cooling capacity of a recently purchased RMC-Cryosystems Model LTS 4.5-025 closed-cycle He refrigerator over the range 4-35 K. We find that the nominal cooling capacity of 250 mW is only met or exceeded over a narrow temperature range around 4.3±0.5 K, and that above this range, there exists a considerable region of much lower cooling capacity, not exceeding ∼100 mW. We believe this behavior results from use of a fixed-aperture Joule-Thompson expansion valve, and might be alleviated if the J-T valve could be adjusted to compensate for changing flow within the 5-20 K temperature range. Present performance may severely limit or prevent effective use in applications where an irreducible heat inflow exists which is greater than ∼100 mW, yet substantially less than the quoted capacity at ∼4 K.