Dipole Vacuum Chambers Stefan Wilfert GSI Helmholtz-Zentrum fr - - PowerPoint PPT Presentation

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Dipole Vacuum Chambers

Stefan Wilfert GSI Helmholtz-Zentrum für Schwerionenforschung mbH Vacuum systems/ Magnet technology Planckstrasse 1 64291 DARMSTADT

Stefan Wilfert, 5th MAC, 09.05.2011

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I Introduction II Dipole chamber designs

• 1. Chamber with supplementary cooling tubes

(measurements and methods for optimization)

• 2. Chamber with contact cooling

(Measurements and methods for optimization) III Aspects of material choice IV Summary and conclusions

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Introduction Introduction Introduction

Vacuum physical requirements on the magnet chambers

• appr. 885 m of the 1084 m long beam line are operated at cryogenic

temperatures

• appr. 644 m are represented by magnet chambers (dipole, quadrupoles,

sextupoles,….)

• appr. 373 m are represented by dipole chambers

UHV/XHV generation will be realized by cryogenic wall pumping (cryopumping) and additional cryosorption pumps (for H2 +He) effective cryopumping requires wall temperatures lower than 20K Chamber design must be optimized in terms of Field quality (avoiding the distortion of main guidiance field) low wall temperatures during magnet ramping to preserve the effectiveness

• f cryopumping

beam dynamics aspects

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Dipole chamber design Dipole chamber design Dipole chamber design

Great problem: unless special cooling measures, due to the fast magnet ramping eddy currents heat up the chamber wall to temperatures > 80K

80K

Length of chamber: 3.45 m Free aperture: 120 x 60mm2 Wall thickness: 0.3mm Rib thickness: 3.0 mm

Demand: Tcham < 20K

• max. dynamic vacuum pressure:

pH2 ~ 5.10-12 mbar @ 5K, static vacuum pressure: pH2 < 10-12 mbar @ 5K

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Two different dipole chamber layouts Two different dipole chamber layouts Two different dipole chamber layouts

Design II: Beam pipe with contact cooling via the cold yoke Design I: Beam pipe with supplementary cooling tubes Solution: chambers must be cooled

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Dipole chamber design I with different positions of supplementary cooling tubes Dipole chamber design I with different positions of Dipole chamber design I with different positions of supplementary cooling tubes supplementary cooling tubes

19.6° 18.0° electrically insulated electrically insulated electrically insulated

1 2 3 4

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Dipole chamber design I with supplementary cooling tubes Dipole chamber design I Dipole chamber design I with supplementary cooling tubes with supplementary cooling tubes

40 42 44 46 48 50 52 54 56 58 10 12 14 16 18 20 22 24

top & buttom lateral

T (K) x (mm)

Calculated by: Shim (GSI) for 2c cycle

LH picture shows the eddy current density. As shown in RH picture, the optimum position of 4 cooling tubes was calculated computationally. The results shows that the

• ptimum position is 52 mm from the center of magnet. At this

positions the maximum temperatures on the lateral sides and top/bottom, are ~ 16 K.

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Dipole chamber design I with supplementary cooling tubes Dipole chamber design I Dipole chamber design I with supplementary cooling tubes with supplementary cooling tubes

Problem: non- electrically insulated cooling tubes generate additional unwanted harmonics (multipoles) in the magnetic beam guidiance field!

LHe cooling tubes Reinforcing ribs DN125 CF fange

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Calculated by: Shim (GSI) for 2c cycle

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Dipole chamber design I with supplementary cooling tubes Dipole chamber design I Dipole chamber design I with supplementary cooling tubes with supplementary cooling tubes

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Length of chamber: 3.35 m Aperture: 120 x 60mm2 Wall thickness: 0.3mm Rib thickness: 3.0 mm

1

BINP, Novosibirsk

• Fa. Reuter, Alzenau

Straight for BNG Magnet Bend for BINP magnet Test sample by

• Fa. Reuter, Alzenau

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Dipole chamber design I: Measurements on temperature and partial pressure conditions under static conditions Dipole chamber design I: Measurements on temperature Dipole chamber design I: Measurements on temperature and partial pressure conditions under static conditions and partial pressure conditions under static conditions

H2

pH2 = 3·10-12 mbar measured @ RT pH2 = 1.5·10-13 mbar @15K The generated residual gas atmosphere contains (in absense of He) only hydrogen

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Dipole chamber design I: Measurements on temperature and partial pressure conditions under static conditions Dipole chamber design I: Measurements on temperature Dipole chamber design I: Measurements on temperature and partial pressure conditions under static conditions and partial pressure conditions under static conditions

If Tcham > 20K, pressure rises rapidly from 10-12 mbar to 10-6 mbar range, i.e. 6 orders of magnitude!

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Dipole chamber design I: Measurements on temperature and partial pressure conditions during magnet ramping Dipole chamber design I: Measurements on temperature Dipole chamber design I: Measurements on temperature and partial pressure conditions during magnet ramping and partial pressure conditions during magnet ramping

1 2 3 4 5 6 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 1.22s td

magnetic field B [T] time t [sec]

tcyc

magnet ramping cycle

Measurements during magnet ramping cycle similar to 2c (most critical cycle) He mass flow rates through the cooling tubes of (dm/dt)He = 0,33g/s

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Dipole chamber design I: Measurements on temperature and partial pressure conditions during magnet ramping Dipole chamber design I: Measurements on temperature Dipole chamber design I: Measurements on temperature and partial pressure conditions during magnet ramping and partial pressure conditions during magnet ramping

Measurements during different ramping cycles (2a, 3a, 4, 5)

chamber responds

immediately to magnet ramping with an increase in temperature depending on ramping cycle, chamber walls heat-up from 5…6.8K to maximum temperatures of max.12K

• nly He and H2 are released

from the chamber walls all other residual gases remain stuck to the cold walls Helium desorbs completely from the walls (no wall- pumping!!!)

• > Cycles 2a, 3a, 4, and 5

and less critical He mass flow rates through the cooling tubes of (dm/dt)He = 0,33g/s

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Dipole chamber design I with supplementary cooling tubes Dipole chamber design I Dipole chamber design I with supplementary cooling tubes with supplementary cooling tubes

Can we reduce the eddy current losses generated by the cooling tubes by changing the cooling pipe dimension?

Cooling pipe dimension (mm)

• uter

diameter thickness 5 0.5 3.88 3.6 0.3 3.09 time average loss (watt/m) Calculated by: Shim (GSI) for 2c cycle

• 1

1 2 3 4 5 6 7 8 4 6 8 10 12 14 16 18

0.5 mm 0.3 mm

Temperature (K) circumference (cm)

Losses are reduced But temperatures are higher

reduced cooling pipe dimension minimize heat loss by ~20% compared to previous design However, even lower heat loss, the temperature is increased about ∆T ≅ + 1.0 K at the top and lateral side

• f beam tube. As expected, the cooling

efficiency is decreased by smaller heat transfer cross section.

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Methods to reduce the losses induced by cooling tubes Methods to reduce the losses Methods to reduce the losses induced by cooling tubes induced by cooling tubes

In order to reduce eddy current losses and to avoid a conductor loop inside the magnet, the whole cooling tube circuit must be completely electrically insulated from the chamber. Two consequences: i) the outer surface of the cooling tubes has to be covered with a thin ceramic film (e.g. 150µm aluminium oxide Al2O3) ii) Secondly, the electrical loop represented by a closed metallic cooling circuit must be electrically interrupted by the use of an electrical insulating transition piece

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Dipole chamber design I with supplementary cooling tubes Dipole chamber design I Dipole chamber design I with supplementary cooling tubes with supplementary cooling tubes

~ 5 K >18 K

Using a chamber with only 2 cooling tubes (on each lateral side), most of the chamber surface shell is above 18K -> critical for cryopumping

Calculated by: Shim (GSI) for 2c cycle

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Specified dipole chamber design with 4 insulated supplementary cooling tubes to be tendered Specified dipole chamber design with 4 Specified dipole chamber design with 4 insulated supplementary cooling tubes to be tendered insulated supplementary cooling tubes to be tendered

~ 5.5 K ~16 K

cooling tube Al2O3 ceramic layer (~150 µm)

LHe

chamber wall 5.3 mm 5.0 mm T = + 1.7K

Voltage breaker

∆T ≅ + 1.7 K

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Comparison of different dipole chamber designs with supplementary cooling tubes Comparison of different dipole chamber designs Comparison of different dipole chamber designs with supplementary cooling tubes with supplementary cooling tubes

2 4 1 3

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Dipole chamber design II: Beam pipe with contact cooling via cold yoke Dipole chamber design II: Dipole chamber design II: Beam pipe with contact cooling via cold yoke Beam pipe with contact cooling via cold yoke

A major part of the generated eddy-current induced heat will be transfered to the cold mass by the thin copper films coated on the mechanical reinforcing ribs. For a good thermal contact thechamber has to be pressed into the cold yoke. Rib coated with strip-shaped Cu films (~ 40 µm) flexible spring strips attached on the rib edges are used to realize the mechanical-thermal contact to the yoke

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05.05.2011 dfkgajklajklgadg 20 Experimental set-up for investigation of the utilizability of dipole chamber design II with contact cooling to the yoke

Dipole chamber design II: Measurements on temperature conditions during magnet ramping simulation Dipole chamber design II: Measurements on temperature Dipole chamber design II: Measurements on temperature conditions during magnet ramping conditions during magnet ramping simulation simulation

A V

• hmic heating simulates

the eddy current heating

plate heater

15cm

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Dipole chamber design II: Measurements on temperature conditions during magnet ramping simulation Dipole chamber design II: Measurements on temperature Dipole chamber design II: Measurements on temperature conditions during magnet ramping conditions during magnet ramping simulation simulation

Pel/L = 0.75W/ 0.15m = 5 W/m corresponds /\ cycle

• Max. chamber temperature

Tmax ~ 19.2 K @ 2.7W/m Tmax ~ 23.7K @ 5W/m

Too high!

• Max. yoke temperature

Tmax ~ 9.5K @ 5W/m

14:00 16:00 18:00 20:00 22:00 24:00 5 10 15 20 25 30 5 10 15 20 25 30 P

el = 0.5W

P

el = 0.75 W

temperature T [K] time [hh:mm]

FB2-MF2 TY01 TC02 TC03 TC04 TC05 TS01 TS02

Mittelwert FB2-MF2 1,91775 TY01 8,9096 TC02 19,33623 TC03 14,2327 TC04 14,17251 TC05 15,90958 TS01 7,47934 TS02 5,46041 Mittelwert FB2-MF2 1,91428 TY01 9,7022 TC02 23,73865 TC03 17,10299 TC04 17,04158 TC05 19,39807 TS01 8,35266 TS02 5,60784

200µm thin CuBe strips

Pel/L = 0.5W/ 0.15m = 2.7 W/m corresponds ~ 2c cycle

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Dipole chamber design II: Measurements on temperature conditions during magnet ramping simulation Dipole chamber design II: Measurements on temperature Dipole chamber design II: Measurements on temperature conditions during magnet ramping conditions during magnet ramping simulation simulation

How we can improve the performance of the contact cooling?

• Thermal contact resistance of the used chamber was not
• ptimized -> technological improvement is needed
• CuBe used as thermal sheets has a thermal conductivity

at 10K of at least a factor of 100 times lower than those of pure copper -> use of pure copper is reasonable

• Result: improved design with thermal ribs

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Improved dipole chamber design II: Beam pipe with thermal ribs Improved dipole chamber design II: Improved dipole chamber design II: Beam pipe with thermal ribs Beam pipe with thermal ribs

The generated eddy-current induced heat shall be transferred to the cold mass by ‘thermal’ ss ribs coated with OFHC. In this design, the mechanical function of the stabilizing ribs is decoupled from the thermal ribs. Thin ‚thermal‘ Cu ribs (50µm) attached onto the mechanical ribs will be used to realize the thermal contact to the cold yoke

In In preparation preparation

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Magnet chamber material Magnet chamber material Magnet chamber material

Requirement: Relative permeability of chamber material must be low and constant Disadvantage of traditional stainless steel grades of AISI 300 series: relative permeability increases due to: cyclic cooling (spontaneous α‘-martensitic

formation) mechanical stress (stress-induced α‘- martensitic formation, e.g. during quench or chamber vibration during ramping) welding (δ-ferrite formation)

µrel < 1.005, χmag < 5·10-3 at cryogenic temperatures Problem: magnetic properties of beam pipe material at cryogenic temperatures

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Special stainless steel grade for cryogenic applications Bö P506 Special Special stainless steel grade for cryogenic stainless steel grade for cryogenic applications applications B Bö ö P506 P506

Bö P506 (Böhler stainless steel) *) = X 2 Cr Mn Ni Mo N 19 12 11 1

• excellent cryogenic and magnetic

properties

• highly stable austenitic-phase
• very low permeability values of base

material and welding seams between 4K…295K (µrel < 1,005)

• no stress-induced α‘-martensitic

transformation

• extremely low δ-ferrite formation in

weldments

• specific resisitivity at 4K ρel ~ 0.5·10-6 Ωm
• developed by CERN in collaboration with

Böhler Edelstahl (Austria) as beam screen- material for cryogenic LHC sections

Solution: chamber material candidate

• f first choice: Bö P 506

*) S. Sgobba and G. Hochörtler: A new non-magnetic stainless steel for very low temperature applications

• Proc. Int. Cong. Stainless steel Science and Market, Chia Laguna, Sardinia, Italy (1999),
• p. 391-401

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Summary Summary

Cryogenic beam vacuum must be operated at temperatures between 4.5…< 20K Eddy current heating of magnet chambers appears to be a great problem in fast-cycled sc machinery → chambers must be forced-cooled in order to preserve the efficiency of cryopumping Investigation of two different beam chamber designs are in progress: beam pipe with 4 supplementary cooling tubes attached electrically insulated from the outer chamber circumference seems to be the best choice in terms of chamber temperature and field quality

• > preferred chamber design -> to be tendered

Dipole chamber with contact cooling via the cold yoke has the potential to be a good alternative (investigations will be continued)

Special thanks to S. Y. Shim (GSI) for his support!

Thank you for your attention!

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