Technical Design Report Super Fragment Separator

2.4.12 Local Cryogenics
2.4.12.1 Cryogenics for dipole stages
DRAFT
There are 28 superconducting dipoles units in the Pre- and in the Main-Separator including the
three branches (High-Energy-Branch, Low-Energy-Branch, and Ring-Branch) and in the experimental
area of the Super-FRS (Magnetic Spectrometer and Energy-Buncher) as well. The superconducting
dipole consists of a warm iron yoke and a warm bore. Only the upper and the lower
coils are housed in one cryostat. According to the lattice design, always three dipole units of each
dipole stage are always grouped together and will be supplied in parallel by one feedbox unit with
liquid helium.
The flow scheme which is shown in Figure 2.4.129 demonstrates the following cooling concepts.
Supercritical helium (5 K, 3.0 bar, pink line in the cryogenic transfer line) is expanded through the
mass flow rate control valve, which works also as a J-T valve (FCV J-T) and its discharging
pressure is variable between 1.2 bar and 1.5 bar (or even higher) as requested by the magnets. To
re-condensate the flash gas produced by the control valve (FCV J-T), the discharging flow (dark
blue) will be re-cooled through a heat exchanger in the so called subcooler. The heat exchanger is
immersed in a liquid helium bath whose liquid level and temperature are adjusted by two pressure-control
valves (PCV): a) one is on the circuit for collecting the return flow (light blue) from
the magnet cryostats and b) another is on the circuit for the vapor return (yellow) from the subcooler.
The helium flow at the outlet of the subcooler should contain only one-phase liquid helium
that allows the helium supply to be equally distributed into three streams in parallel, each feeding
one magnet cryostat. The liquid helium is transferred under the discharging pressure to each
magnet cryostat via the so called jumper that makes the connection between the feedbox unit
(subcooler and cryogenic transfer line) and the magnet cryostats.
Liquid helium is fed from the bottom of the coil container in the magnet cryostat and circulated
through the flow channels around the coils. The heat load (steady state heat in-leaks, Joule heating
of the instrumentation cables, SC wire junctions and AC loss during ramping, etc.) in the magnet
cryostat may cause the evaporation of liquid helium. Therefore two-phase helium may be present
in the return flow which comes out from the top on the coil container. A small fraction of such flow
is used for cooling the resistive current leads whose cold ends are seated in the so called chimney
helium vessel (light blue). The rest of the return flow (possibly two-phase helium) from one
magnet cryostat merges with the other two streams before it enters into the subcooler.
The cooldown of the magnets can be carried out on the base of a group. The cold helium gas is
tuned in its flow rate by the cooldown control valve (violet) in a circuit which bypasses the subcooler.
The control of the inlet temperature of the cooldown flow is also possible by mixing the
cold helium with 300 K warm helium at required flow rates. The flow control valve (FCV) in the
warm circuit that connects the helium vessel to the insulated quench gas collection/cooldown/warmup
gas return line allows fine-tuning the cooldown flow through individual
cold mass at request. Therefore the cooldown speed and the temperature gradient over the individual
cold mass structures are controllable with respect to the specification. The warmup of the
magnets can be done also on the base of a group by circulating the 300 K helium gas through both
the 4 K and 50 K shield loops of the magnet cryostat.
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