1. Organization of proposed cryolab collaboration with AEGIS
Cryolab (TE/CRG) proposes to collaborate with the AEGIS experiment on two main tasks:
Cryogenics for magnets
Dilution refrigerator
(design, mechanic / heat-transfer calculations, construction, overall lay-out, control system)
Tapio Niinikoski Lionel Metral
Friedrich Haug Laetitia Dufay
Rob van Weelderen Johan Bremer
Thomas Eisel
T. Eisel

2. Cryogenics for AEGIS: how to cool to 0.1 K? Thomas Eisel
T. Eisel 2

3. Outline Presentation Cooling principle – Dilution refrigerator
Dilution refrigerator – layout
Heat load estimation
Heat transfer ultra cold trap – mixing chamber of dilution refrigerator
2 possibilities: sandwich and rod
Heat transfer – heat exchanger
Next steps
T. Eisel

4. Cooling principle – dilution refrigerator (DR)
T. Eisel

5. Dilution refrigerator - layout
One possibility:
“1K pot” and “still” in a chimney with connection to the instrumentation region
Mixing chamber (MC) underneath the 5 electrodes of the ultra cold trap
Only shielded tube-in-tube heat exchanger passes the instrumentation region (gap between the magnets)
Shields are connected to 1K pot and still and possibly in the combination region to IHE
Additional 1K pot to cool the shields around traps (located in the chimney) 1
regions of experiment:
1 e+ accumulation
2 catching
3 instrumentation
4 combination
5 measurement 3 5 2 4
T. Eisel

6. Heat load estimation to ultra cold trap (respective MC)
Thermal radiation (warm to cold surfaces) ~ 1 mW
Primarily coming from the “warm” measurement region
No effort for shielding is too much!
Residual gas ~ 1 mW
Can be reduced by very good vacuum (“new”: mbar)
Insufficient information to estimated the heat loads by:
Thermal conduction (construction material, wires)
Electrical dissipation (sensors)
Other heat dissipation (laser, annihilation, ...)
Heat loads to the ultra cold trap -> have to be transferred into the mixing chamber !!!
Drawback: different potentials of 5 electrodes require electrical insulation (goes together with low thermal conduction)!!!
2 possibilities: sandwich & rod
T. Eisel

7. Sandwich 1st possibility: sandwich of following materials:
Material of electrode (Cu or Al)/In/ Sapphire /In/Cu
Sapphire as electrical insulator and good thermal conductor
Cold Indium joint for good thermal contact between metal and Sapphire
Extrapolation of measurement 5 K:
@5 K low Kapitza resistance between He dilution and copper
Heat of <3.5 mW can be transferred from the electrodes through the sandwich (tight!)
has to be measured under real temperature conditions
T. Eisel

8. Rod
2nd possibility: rod connecting the electrodes with heat exchangers in mixing chamber
Rod material is Cu OFHC and has a much better thermal conduction (no electrical insulator present between cooling source and electrode)
Alumina (Al2O3) is used as electrical insulator to mixing chamber (=ground)
Lower temperature gradient expected for same cooling power
T. Eisel

9. Bumpy way to Rod – standard version
Standard version: Alumina commercially available
Already with a metallisation for convenient brazing, unfortunately with Ni-layer
Magnetic permeability was room temperature -> effect was visible (permeability Ni/Fe = 1/7), measurements at low temperature would be necessary
Besides, difficult to estimate the impact to the homogeneity of the most delicate region of the magnetic field
T. Eisel

10. Bumpy road to the Rod – version 1 custom-made
Manufacturing of parts and joining procedures done at CERN in collaboration with EN-MME
Test active brazing with AgCu between Al2O3 and Cu (OFHC)
design is the key as later realized
Test electron beam welding between Cu and CuBe, incl. metallographic cut
CuBe is harder then Cu OFHC (soft) and is preferred in case of overpressure in the Mixing Chamber
Thermal cycling from 293 to 77 K showed
weak points in the design (see fig.3),
the Alumina broke!!!
Fig. 3
broken rod
Fig. 1 weld
Fig. 2 cut Cu/CuBe
T. Eisel

11. Bumpy road to the Rod – version 2 custom-made
Last rod version
Change in the design
Additional “flexible” Cu ring reduces the stress in the Alumina
Very fast thermal Cycling from 293 to 77 K did not damage the rod
The piece was directly put into liquid N2 and was heated slightly more than ambient temperature with a heat gun
No leak was detected after thermal shocks
T. Eisel

12. Heat transfer – Heat exchanger
Heat transfer at very low temperatures is determined by Kapitza resistance
Kapitza resistance: ΔT = RK Q, with RK ~ T-3.
In general the surface area is increased to overcome the very bad heat transfer
Heat exchangers have sintered surfaces (some m2)
Sinter tests were performed to improve the contact between sinter and substrate (both Cu)
Different surface treatments and filling procedures (powder in mould) have been used
The evaluation is not yet finished
See different surface treatments (right grooved with cutter / left sand blasted)
T. Eisel

13. Next steps Measurements of thermal behaviour
for both: Sandwich & final temperatures of K
Assembly of new MC, solenoid to destroy the superconductivity of In (in sandwich) to achieve a better thermal contact
Implementation in DR, instrumentation (sensor calibration)
Measurement of Kapitza resistance, heat conduction, heat transfer coefficients...
Measurements to electrical insulation between single rods (in ionised He dilution – to be discussed)
Design of the Dilution refrigerator
T. Eisel