LN2 Sample Cooling

There are two basic methods of cooling samples using LN2 for vacuum applications, direct cooling and indirect cooling.

In most cases direct contact of the LN2 Dewar with the sample achieves the highest rate of heat transfer and the most rapid cooling to the lowest temperature. If the sample is mounted to a goniometer head or precision gearbox, the plumbing of the LN2 to the Dewar can restrict the motion. In such cases a clamping Dewar may be considered.

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Cooling Diagrams

A clamping Dewar can allow certain degrees of freedom, such as continuous rotation, and still achieve maximum cooling. A heated rotating sample can be cooled by stopping the motion and activating a Dewar that clamps to the sample platen.

The indirect cooling method cools the sample by conduction through a flexible copper rope to a remote LN2 Dewar. The copper rope can be an effective means of cooling. Sample motion and temperature are restricted by the length and number of copper ropes used to achieve the desired level of cooling.

The ability of a cooling system to cool the sample to its lowest temperature is limited by many factors: Direct versus indirect cooling, size of sample, sample heating attachments, limitations required by goniometer, number of copper ropes used, sample transfer requirements, electrical isolation requirements of the sample, and the ability to use a clamping Dewar.

/CLND-1

(Temperature graph showing direct cooling of substrate between heating cycles)

Time-Temperature Test

  • SMR-1 sample mount
  • Resistive heating
  • Direct cooling
  • Screw platen attachment

 


/CLNI-1

(Cooling using in-vacuum Dewar and copper rope connections to sample plate)

Time-Temperature Test

  • STLP-1 sample transfer
  • Molybdenum platen
  • Indirect cooling
          2" x 6" copper rope
          Copper Dewar
          8" x 1/8" feed lines
Cooling Graph

Dewar Materials

Dewars are fabricated from composite, explosively-bonded OFE copper to stainless steel. This material is formed in an explosive process that creates a high quality metallurgical bond. A high explosive charge compresses the copper and stainless plate material at supersonic speeds and at extreme pressures forming a plasma at the contact point. The plasma scours the surfaces to molten conditions as the plates collide and instantaneously cool. The resulting knitted mechanical bond of copper-to-stainless has excellent mechanical and thermal properties for UHV and LN2 to flow in contact with high thermal transfer capacity copper, with stainless steel plumbing (TIG welding) to the stainless steel side of the bonded Dewar. No brazing is required reducing the possibility of contamination and virtual leaks.

Dewar Design

Dewars have an internal phase separator that maximizes thermal transfer. The liquid LN2 contacts the copper and is allowed to boil. Releasing nitrogen gas that is separated and allowed to escape. This technique maximizes contact of the copper with the liquid and keeps boiled-off nitrogen gas (a poor thermal transfer medium) away from the copper heat transfer surfaces. This dramatically lowers the temperature of the Dewar and increases its capacity for thermal transfer.

True OFE copper rope is clamped to the copper side of the Dewar for indirect cooling to the sample.

A 1.33" CF re-entrant seal, with plumbing made to length, may be used to connect the Dewar to the atmosphere side of the LN2 source. The welded plumbing connected to the re-entrant seal eliminates the need for in-vacuum fittings and possible leaks. All Dewars are helium leak-tested at LN2 temperatures prior to shipping. 

The following cooling options may be added to the SM or SMR sample mounts. 

/CLNI-1 LN2 Indirect Cooling

  • 1" O.D., 2" long
  • Explosively-bonded OFE copper to stainless steel
  • Two OFE copper ropes, 12" length
  • Internal phase separator
  • Electrically grounds sample
  • With VCR fittings
  • 1/8" O.D. or 0.093" O.D. supply tubes
  • Includes FLN-133-2 LN2 feedthrough
  • Includes custom supply tube lengths, coils, thermally-isolated mounting hardware, as required

/CDC LN2 Clamping Direct Cooling

  • Actuated clamping annular Dewar
  • Explosively-bonded OFE copper to stainless steel
  • Internal phase separator
  • Electrically grounds sample
  • With VCR fittings
  • 1/8" O.D. or 0.093" O.D. supply tubes
  • Includes FLN-133-2 LN2 feedthrough
  • Includes custom supply tube lengths, coils, thermally-isolated mounting hardware, as required
  • Application depends upon gearbox and range of degrees of freedom

/CLND-1 LN2 Direct Cooling

  • Annular Dewar mounted around 1" sample plate
  • Explosively-bonded OFE copper to stainless steel
  • Internal phase separator
  • Electrically grounds sample
  • With VCR fittings
  • 1/8" O.D. or 0.093" O.D. supply tubes
  • Includes FLN-133-2 LN2 feedthrough
  • Includes custom supply tube lengths, coils, thermally-isolated mounting hardware, as required
  • Application depends upon gearbox and range of degrees of freedom

Options

/CEI-I Ceramic Electrical Isolation

  • Provides electrical isolation
    for indirect cooling

 

/RE-ENTRANT Re-entrant Seal

  • Allows Dewar to be connected to
    atmosphere without in-vacuum
    VCR fittings
  • Fits on 1.33" O.D. flange

 

/CEI-D Ceramic Electrical Isolation

  • Provides electrical isolation
    for direct cooling

Call or email with your requirements and for current pricing.

800.962.2310 / sales@thermionics.com