1. See, for example, the proceedings of any Applied Superconductivity or Cryogenic Engineering Conference, such as “Advances in Cryogenic Engineering”, Vol. 29, Plenum, New York (1984) p. 29ff.
2. CTI, Air Products, CVI, Cryomech, Cryosysterns, and others make small gas refrigerators reaching ~4 K with a J-T loop
3. J. A. Barclay, Magnetic refrigeration for low-temperature applications; in: “Proc. of Third Cryocooler Conference”, N.B.S. SP 698, Boulder, Colorado, (1984) p. 20–32.