Isotopic purification of He4

✍ Scribed by M. Atkins; P.V.E. McClintock

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 208 KB
• Volume: 16
• Category: Article
• ISSN: 0011-2275
• DOI: 10.1016/0011-2275(76)90250-2

✦ Synopsis

Short-time ripples of the feedback system are thus removed. We chose R2 ,¢ Rl, in order that Tt2 ~ Ttl. If R2 were of the same order as RI, the second RC circuit would be more efficient, but the minimum temperature of the stabilized part of the thermostat would be further increased, which is hardly acceptable. Now, r ~--R l (Cl + C2) ~ 10 s when the upper and lower part of the thermostat weigh 50 and 100 grammes respectively. Actually, R2 was not a copper wire, but a brass bar of 4 mm length and 5 mm diameter, giving R2 ~ 500 K W -1 at 1 K. The thermostat was attached to the mixing chamber by means of four Teflon bars. The stability of this thermostat is also shown in Fig. 2. First, experiments with a copper wire of 0.4 mm ¢ were performed. It appeared that the variations in T were about -+ 0.3 mK, almost independent of the temperature. When a copper wire of 0.59 mm ¢ was used, the stability improved again, see Fig. 2. One could wonder whether an extra electronic RC time of about 1 second in the feedback system provides the same stability as adding the second (thermal) RC circuit with this time constant. This was tried, but with a negative result. The stability of the whole system was significantly better with a thermal stabilization than with the extra electronic stabilization.