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wiki:design_construction [2022/02/02 15:48] henri.godfrin@neel.cnrs.fr |
wiki:design_construction [2022/12/15 17:46] (current) henri.godfrin@neel.cnrs.fr |
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There exist many different types of dilution refrigerators, | There exist many different types of dilution refrigerators, | ||
- | ===== Classical dilution refrigerators ===== | + | ===== Classical |
{{ wiki: | {{ wiki: | ||
* 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 " | * 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 " | ||
- | * Larger refrigerators with heat exchangers of typical diameters larger than 10 cm (" | + | * Larger refrigerators with heat exchangers of typical diameters larger than 10 cm (" |
* 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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**[[articles_on_dilution_refrigeration|Link to Publications HERE]]** | **[[articles_on_dilution_refrigeration|Link to Publications HERE]]** | ||
- | ===== Pulse-tube based dilution refrigerators ===== | + | |
- | {{ wiki: | + | ===== Pulse-tube based (" |
- | * Dilution refrigerators can be pre-cooled by Cryo-Coolers, | + | {{ wiki: |
+ | * Dilution refrigerators can be pre-cooled by Cryo-Coolers, | ||
+ | * The first units used Gifford Mac-Mahon coolers, but the large level of vibration end up making these machines a curiosity. | ||
+ | * When pulse-tube coolers reached temperatures below 4 K, several groups developed [[pulse-tube_dr|pulse-tube dilution units]]. The machine must be carefully optimized, the thermodynamics limit for condensation | ||
+ | * The possibility to obtain large " | ||
+ | * The success of this type of machines results therefore from a combination of the low temperatures achieved by modern pulse-tube cryocoolers, | ||
**[[pulse-tube_dr|Link to Publications HERE]]** | **[[pulse-tube_dr|Link to Publications HERE]]** | ||
===== "1K pot" and "no 1K pot" dilution refrigerators ===== | ===== "1K pot" and "no 1K pot" dilution refrigerators ===== | ||
- | The design of 1K pots as well as systems pre-cooled without 1K pot (compressed 3He) are discussed in several publications: | + | {{ wiki: |
+ | * The incoming 3He has to be pre-cooled to relatively low temperatures to satisfy the thermodynamic requirement that the enthalpy of the 3He gas leaving the still, must be higher than the enthalpy of the 3He gas entering the dilution set-up, just before the thermalization on the still, if any cooling is to be obtained. | ||
+ | * A separate cooling stage, the "1K pot" , is traditionally used in "wet refrigerators" | ||
+ | * The 1K pot is often a source of problems, the feeding capillary may get blocked by dust, or by small ice or solid nitrogen, argon or oil particles. Different systems of filters have been tried, but the best is to check the bath is leak-free to the room, and that clean helium storage Dewars are used. | ||
+ | * The design of 1K pots as well as systems pre-cooled without 1K pot (compressed 3He) is discussed in several publications: | ||
**[[1k_pot|Link to Publications HERE]]** | **[[1k_pot|Link to Publications HERE]]** | ||
+ | |||
+ | ===== The 3He condensation line ===== | ||
+ | {{ wiki: | ||
===== The " | ===== The " | ||
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* The still plays an important role: the **3He flow rate** of the dilution refrigerator, | * The still plays an important role: the **3He flow rate** of the dilution refrigerator, | ||
+ | |||
+ | ===== The Continuous heat exchanger | ||
+ | |||
+ | ===== The Step (" | ||
===== The Mixing Chamber | ===== The Mixing Chamber | ||
- | {{ wiki: | + | {{ wiki: |
* 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 of Dilution Refrigerators ===== | ||
+ | {{wiki: | ||
+ | |||
+ | * 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/ | ||
+ | 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. | ||
+ | |||
+ |