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The Art of Cryogenics in High Magnetic Fields

The Art of Cryogenics in High Magnetic Fields. Eric Palm – NHMFL NHMFL – Summer School 2009. Outline. Issues Eddy Currents Magnetic Helium Cooling the Sample Thermometry Cryogenic systems Vacuum can – exchange gas VTI – Variable Temperature Insert He-3 Systems Dilution Refrigerators.

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The Art of Cryogenics in High Magnetic Fields

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  1. The Art of Cryogenics in High Magnetic Fields Eric Palm – NHMFL NHMFL – Summer School 2009

  2. Outline • Issues Eddy CurrentsMagnetic HeliumCooling the SampleThermometry • Cryogenic systemsVacuum can – exchange gasVTI – Variable Temperature InsertHe-3 SystemsDilution Refrigerators

  3. Resistivity - ρ r IssuesEddy Current Heating L Use high resistivity materials – brass, SS, titanium, G10, plastics, etc Small radius – slots Low ripple, slower ramp rates Use eddy currents in outside shields to reduce ripple inside

  4. Helium – diamagnetic “ low field seeker” • drop helium into magnetic field it will levitate above field center • Liquid helium more magnetic than gaseous helium – more dense • Bubbles form in LHe they are trapped at field center • Force proportional to Field x Field gradient product (B dB/dZ) • At B dB/dZ greater than 21 T2/cm • In MagLab resistive magnets starts ~ 20T • Hybrid magnet starts ~ 32T • Impossible to recondense He-3 at high fields FB IssuesMagnetic Helium mg Field Center

  5. IssuesCooling the sample • Have to remove Heat from the sample • Cooling in liquid more effective than gas • Flowing gas better than static gas • At low temperatures – phonon mismatch between materials with different masses – essentially Snell’s law for phonons. Very small cone (3 deg) cone for transmission. Must have large surface area. Must limit power – femtowatts! • Self heating by as much as 2K while frig still at 20mK!

  6. IssuesThermometry • Magnetic field affects sensors • Pt good to 10K • Cernox good to a 3K • RuO ok at low temps worse at high temps • Capacitance thermometers – if you really need stable temperatures • Thermometry below in high fields 300mK still an open challenge

  7. Vacuum Can • Simple can • Exchange gas provides cooling • Super insulation or spacers prevent touches/ thermal shorts • Slow, inefficient use of time • Simple and cheap

  8. Variable Temperature Insert VTI • Valve control flow from LHe bath • Vaporizer controls temperature of gas • Easy fast temperature changes • Very stable temperatures • Good cooling power • Expensive – lose sample space

  9. He-3 System – Double wall can • Condense He-3 into bottom,then pump it back out • Simple, cheap, easy, saves space • Not good base, requires care to get good results • Pump the entire bath to condense He-3 – not efficient • Prone to icing cans into the dewar

  10. He-3 System – Sorption pumpedTop-loading • Condense He-3 into bottom,then pump it back out, with Sorb! • High pumping speed, low temps • Pump 1K Pot – not bath, efficient • Use copper (outer) tails to eliminate bubble problem, split copper on inner tail • He-3 will stratify in thin narrow space, need thermal short CharcoalSorb 1K Pot

  11. He-3 System – Sorption pumpedBottom-loading • Condense He-3 into bottom,then pump it back out, with Sorb! • High pumping speed, low temps • Pump 1K Pot – not bath, efficient • Sample in Vacuum • Must provide thermal link to cool sample – Copper or Silver • Fight eddy currents • Good for heat capacity measurements • Delicate – pain in the butt CharcoalSorb 1K Pot Vacuum Joint

  12. Dilution Refrigerator • Below 780mK spontaneous separate into He-3 and He-4 rich • But 7% He-3 mix into He-4 phase To Pump • Pump out He-3 from end • He-3 “evaporates” across phase separation line • Dilution – absorbs heat – cooling!

  13. Dilution Refrigerator Base temp depends on cooling power (flow rate)heat exchangers Challenge to make work in very high magnetic fields Top-loading even more challenging Top-loading crucial for user facility 1K Pot Still Heat Exchangers Mixing Chamber

  14. Conclusions • Devil is in the details • We have worked out the details • You don’t have to be an expert • But: Learn the basic principlesRead and follow the instructions / recipesDon’t try to go outside the box / boundaries

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