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wiki:design_construction [2022/02/02 16:29]
henri.godfrin@neel.cnrs.fr 		wiki:design_construction [2022/12/15 17:46] (current)
henri.godfrin@neel.cnrs.fr
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    * Small dilution refrigerators ("la Jolla" style) have diameters of a few cm, their 3He flow rate is ~10 to 50 µmol/sec. They are often used as "inserts" in a liquid helium dewar . Due to their small size, the heat exchangers have a small total area, and heat leaks make it difficult to reach temperatures below 10 mK.    * Small dilution refrigerators ("la Jolla" style) have diameters of a few cm, their 3He flow rate is ~10 to 50 µmol/sec. They are often used as "inserts" in a liquid helium dewar . Due to their small size, the heat exchangers have a small total area, and heat leaks make it difficult to reach temperatures below 10 mK.
  
-   * Larger refrigerators with heat exchangers of typical diameters larger than 10 cm ("Grenoble design") can reach temperatures on the order of 2 mK, with flow rates of 100 *mol/s. They can also provide large cooling power, with flow rates up to 10 mmol/s, obviously at higher working temperatures. +   * Larger refrigerators with heat exchangers of typical diameters larger than 10 cm ("Grenoble design") can reach temperatures on the order of 2 mK, with flow rates of 100 µmol/s. They can also provide large cooling power, with flow rates up to 10 mmol/s, obviously at higher working temperatures. 
    * Cooling power at the MC is given by the simple expression    * Cooling power at the MC is given by the simple expression
  
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 **[[1k_pot|Link to Publications HERE]]** **[[1k_pot|Link to Publications HERE]]**
 +
 +===== The 3He condensation line  =====
 +{{ wiki:3He-condensation_HG.JPG?400 |3He-condensation_®HG}} 
  
 ===== The "still"  ===== ===== The "still"  =====
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   * The still plays an important role: the **3He flow rate** of the dilution refrigerator, and hence the cooling power available at the level of the mixing chamber, **is mainly determined by the power applied to the still**.      * The still plays an important role: the **3He flow rate** of the dilution refrigerator, and hence the cooling power available at the level of the mixing chamber, **is mainly determined by the power applied to the still**.   
 +
 +===== The Continuous heat exchanger  =====
 +
 +===== The Step ("discrete") heat exchangers =====
  
 ===== The Mixing Chamber  ===== ===== The Mixing Chamber  =====
-{{ wiki:mixingchamber_hg.jpg?150|Mixing chamber_®HG}}+{{ wiki:mixing_chamber_hg.jpg?170|Mixing chamber_®HG}} {{ wiki:mixingchamber_hg.jpg?150|Mixing chamber_®HG}}  
   * The mixing chamber can be made out of copper, stainless steel, plastics, etc.  Plastic MC are used in the presence of varying magnetic fields, to avoid eddy current heating.    * The mixing chamber can be made out of copper, stainless steel, plastics, etc.  Plastic MC are used in the presence of varying magnetic fields, to avoid eddy current heating. 
 +
   * The MC volume is chosen typically from a few cm3 (in very small refrigerators) to several liters (very large flow rate machines). Modern refrigerators including a large sintered silver heat exchange in the mixing chamber, have volumes of about 100-400 cm3.    * The MC volume is chosen typically from a few cm3 (in very small refrigerators) to several liters (very large flow rate machines). Modern refrigerators including a large sintered silver heat exchange in the mixing chamber, have volumes of about 100-400 cm3. 
  
-==== Cooling power ==== +===== Cooling power of Dilution Refrigerators ===== 
-{{ wiki:DR_cooling_Power.JPG.jpg?250|DR_cooling_Power_®HG.JPG}} +{{wiki:DR_cooling_Power.jpg?350 |DR_cooling_Power_®HG.JPG}}  
 + 
 +  * The figure shows the cooling power of different types of dilution refrigerators.  
 + 
 +  * Cooling power = 82 dn3/dt T^2. This standard formula relates the cooling power in watts to the flow rate expressed in moles/sec (dn3/dt).  
 +It is applicable for temperature T>3 Tmin, where Tmin is the base temperature of the dilution refrigerator (no applied external power).  
 + 
 + 
 +  * Depending on the typical size of the machine (see above), different flow rates can be achieved, and hence different cooling powers. 
 + 
 +  * The performance of refrigerators optimized for very low temperatures is indicated by dashed lines. 
 + 
 +  * Pumped 3He refrigerators have larger cooling powers that dilution refrigerators for T>0.35 K. The are also significantly more user friendly... 
 + 
 + 
 +===== Troubleshooting Dilution Refrigerators ===== 
 +  * 1 K pot hot, pressure is low. Filling capillary blocked. Remove LHe so that the bath level is below the intake, keep 4He pressure in Pot above bath pressure. Having a heater on the 1K pot capillary can help, applying heat pulses... 
 + 
 +  * High inlet pressure. Air? Hydrogen? Water? 
 + 
 +  * Low still pressure. Still empty? Check T_still vs. P_still ! If there is no liquid, the pressure j 
 +  * is low, but the temperature is high. 
 + 
 +  * No cooling power. Interface level in MC? Heat leak to MC? Apply heat and check cooling power at higher temperatures. 3He/4He ratio OK? 
 + 
 +  * Heating spikes, temperature oscillations. Superfluid leak to Vacuum can? Check for spikes in the vaucum can pressure. 
  
wiki/design_construction.1643819361.txt.gz · Last modified: 2022/02/02 16:29 by henri.godfrin@neel.cnrs.fr

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