Cryogenic refrigeration requirements for superconducting insertion devices in a light source

Abstract

This report discusses cryogenic cooling superconducting insertion devices for modern light sources. The introductory part of the report discusses the difference between wiggler and undulators and how the bore temperature may affect the performance of the magnets. The steps one would take to reduce the gap between the cold magnet pole are discussed.One section of the report is devoted to showing how one would calculate the heat that enters the device. Source of heat include, heat entering through the vacuum chamber, heating due to stray electrons and synchrotron radiation, heating due to image current on the bore, heat flow by conduction and radiation, and heat transfer into the cryostat through the magnet leads.A section of the report is devoted to cooling options such as small cryo-cooler and larger conventional helium refrigerators. This section contains a discussion as to when it is appropriate to use small coolers that do not have J-T circuits. Candidate small cryo-coolers are discussed in this section of the report. Cooling circuits for cooling with a conventional refrigerator are also discussed.A section of the report is devoted to vibration isolation and how this may affect how the cooling is attached to the device. Vibration isolation using straps is compared to vibration isolation using helium heat pipes. The vibration isolation of a conventional refrigeration system is also discussed.Finally, the cool down of an insertion device is discussed. The device can either be cooled down using liquid cryogenic nitrogen and liquid helium or by using the cooler used to keep the devices cold over the long haul.

The potential role of cryogenics in insertion magnets

“…Cooling some classes of permanent magnet materials to cryogenic temperature will increase in remnant field [27]. Other materials such as SmCo 5 and Sm 2 Co 17 can be baked to 120 C (for vacuum reasons) if the magnet is in the beam vacuum, but these materials lose their remnant field at low temperatures. Permanent magnets made of Nd 2 Fe 14 B and Pr 2 Fe 14 B will have higher remnant fields down to 140 K. [28].…”

“…This means that the gap between the magnet poles must be small (< 10 mm). Hence any way of increasing the magnetic field while reducing the magnet gap is of interest to the light source and free-electron laser community [5]. Cryogenic magnets are potential solutions for achieving shorter wave-length of coherent light.…”

Design Issues for Cryogenic Cooling of Short Period Superconducting Undulators

“…Heat into the cold beam tube from most of the sources can be controlled so that the level of heating is only a few watts [6]. Heating due to image currents from the beam may be the largest source of heat into a cold insertion device bore tube.…”

“…Equation 3 is the minimum value of beam heating possible for a smooth crack free bore tube surface. An expression for the transition from ordinary skin effect heat to anomalous skin effect heat can be found in references [6] and [7]. When one looks at the heating per unit length equations one finds that beam heating is proportional to the machine circumference divided by the number of bunches and the beam current I squared.…”

“…The boiling surface ΔT can be estimated by using methods outlined by Frederking in his 1962 paper [12]. An expression for the temperature drop ΔT b from a device to liquid helium through nucleate boiling at 4.2 K can be given by the following expression [6], [13];…”