Upgrade of the CERN Cryogenic Station for Superfluid Helium Testing of Prototype LHC Superconducting Magnets

Abstract

The cryogenic infrastructure of the station for testing LHC prototype superconducting magnets in superfluid helium below 2 K has been upgraded. Liquid nitrogen precooling has permitted to increase the liquefaction capacity of the refrigerator. The addition of cold centrifugal compressors with a pressure ratio of 3:1 has boosted the capacity of the warm pumping unit. To ensure adaptation of the pumping capacity, a heater-and-valve box allows to bypass the cold compressors. This box also comprises a 32 kW electrical heater for warming up the low-pressure gaseous helium before it enters the volumetric warm pumping unit. Possible impurities in the helium returning from the subatmospheric circuits are trapped in a freeze-out helium cleaner. Automatic process control and supervision permit unattended operation and optimal management of the helium inventory.

A Cryogenic Axial-Centrifugal Compressor for Superfluid Helium Refrigeration

“…To stimulate development of this technology, CERN has procured from Linde, with ATEKO and PBS as subcontractors, a prototype Cold Compressor Unit (CCU) with a nominal flowrate of 18 g/s @ 1 kPa inlet pressure, with a pressure ratio of 3 and with an isentropic efficiency better than 60 %. These specifications also meet the need for a booster stage enabling to triple the capacity of an existing warm pumping unit (WPU) which provides 1.8 K refrigeration to the CERN cryogenic test station, an upgrade described in a companion paper [2]. The compliance of the system allows to handle low flow continuously varying over the range from 6 to 18 g/s, with a pressure ratio of 1 to 3 and with an isentropic efficiency better than 0.5.…”

Large Cryogenic Infrastructure for LHC Superconducting Magnet and Cryogenic Component Tests: Layout, Commissioning and Operational Experience

“…In order to lower the helium temperature down to 1.8 K for the magnet test station and the String, a common pumping line connects to a very low pressure pumping system [3]. It is composed of a cold compressor unit (CCU1; one centrifugal compressor), a 30 kW heater and a warm pumping unit (WPU1; three roots in series followed by three rough pumps in parallel) giving a pumping capacity of 18 g/s at 1 kPa.…”

“…The new 18 kW at 4.5 K refrigerator, part of the refrigeration system for the LHC and installed in Point 18 [1], has been integrated to the existing cryogenic infrastructure in the SM18 building [2,3] to provide the necessary cooling and gas recovery for the largest cryogenic test facility at CERN, hereafter called Zone 18. During the LHC construction, this area has been and will be used for magnet cryostating and testing as well as for testing different equipments like the RF-cavity modules, the String experiment [4], prototype HTS current leads [5] and the 1.8 K refrigeration units [6,7].…”