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

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

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

Cooling principle – dilution refrigerator (DR) T. Eisel

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

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”: 10-12 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

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 results @ 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

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

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 measured @ 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

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

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

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

Next steps Measurements of thermal behaviour for both: Sandwich & rod @ final temperatures of 0.05... 0.1 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