Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
III-b CRYOGENICS He4 CRYOSTATS The fundamental request for the equipment is to keep the evaporation rate of a cryogen as low as possible isolation (mostly vacuum technology). low temperature leaks absorption at cooled surfaces – danger of explosion and cold burns – safety hazards Rigorously follow the cryogenic safety rules!. Cool down phase Amount of cryoliquid necessary to refrigerate 1 Kg of material using: only latent heat (latent heat + gas enthalpy): Cryogen N2 He4 He4 T 30077 774.2 3004.2 Al 1.0(0.63) 3.2(0.2) 66(1.6) SS 0.53(0.33) 1.4(0.1) 34(0.8) Cu 0.46(0.28) 2.2(0.16) 32(0.8) Conclusion: 1) Use Lq N2 to precool. 2) Cool slowly. Running (low temperature) phase of the experiment Main sources of heat losses in the cryostats: a) Heat conduction – minimized by use of proper material (thin wall tubes, etc.) b) Heat radiation Q(W)=5.67×10-12 A cm 2 T14 -T24 for the case when absorbing and radiating area are the same (and equal to A), and for both emissivities ε=1 (black body approximation). For ε 1 multiply by a factor: -1 ε1 +ε 2 -1 N.B. For 12<<1 the factor is: /2. ε1 ε 2 To reduce the radiation effect it is important to keep surfaces clean and “shiny” (e.g., gold plated). This results not only in low emission but also in high reflection. ε values for various materials: Au 0.01 - 0.03 Ag 0.02 - 0.03 Cu 0.02 - 0.6 Al 0.02 - 0.3 Cs 0.05 - 0.1 Plexiglas 0.1 - 0.9 To minimize radiation losses: Never expose directly low temperature parts to 300K (room temperature) radiation. Note that 10 cm2 at 300K evaporates 0.7 l/h of lq. He, while 10 cm2 at 77K evaporates only 0.003 l/h. Use radiation shields or baffles thermally anchored at intermediate temperatures. c) Conduction by residual gas in the vacuum space. Mostly He gas – all the rest is “frozen out”. Q W 0.02aA [cm2 ] p[mbar] ΔT[K] a = accommodation coefficient, can be as small as 0.025 for clean surfaces. d) Other heat sources (vibrations, e-m radiation, para-orto transitions etc) are only important at mK and K ranges. He4 cryostats Cryostat: Invented by J. Dewar for public demonstrations. 1. Double wall glass Dewar system (Fig. 32a). Implosion danger! Protect and handle with care. There is a problem of Helium diffusion through glass 2. Metal Dewars. (See Fig: 32c, Lq N2 Dewar Fig: 32 b) Stainless steel. Al + fiberglass. 3. Spectroscopy Dewars – fiberglass (problem of Helium diffusion!) Superisolation - mylar +Al rate < 0.1 l/h 4. Cryostats for T > 4.2K, see Fig: 32d. Temperature control by He flow control, heater, or both. 5. Variable temperature cryostats for 1.3K < T <4.2K 1. Pumping on the main He bath- Non economical – 40% of He will be lost on cooling He from 4.2K to 1.3K. 2. Pumping on a small fraction in a separate container 3. continuously operating He4 evaporation cryostat see Fig: 32e,f Self regulating device. Steady state operation is reached when: Q He ( The required impedance is: Z = 1 n L) = Q tube (h) + Qext 2 Δp Vη p - pressure drop required to cause a volume flow rate dV/dt of a medium with viscosity of . Typical p ~ mbar-bar. Z too large – He chamber will run dry. Z too small – the required temperature will not be reached. Typical: Z ~1011cm-3, can be obtained with 1m (several m) long capillary with Φid≈ 0.05 (0.1) mm. Alternative: A short capillary with inserted wire that is tightly fitting into it. Problems: blocking of the capillary by solidified air during the cool down (to avoid use pure pressurized He4 gas) and operation (use Cu powder filter). Main sources of problems- dirty cryogen, air leaks (air at 4.2 K is solid!). Advantages: Continuously operating cryostat can be refilled without interrupting the system operation. Continuously operating He4 dewar can be inserted into a large storage dewar (e.g., NW 50 neck) for a continuous supply of liquid helium. Operates from 1.3-300 K with very low cryogen consumption and excellent temperature stability, see 33 a. Auxiliary Equipment Storage dewars, see fig. 33b Transfer tube, see fig. 33 c Level meters Resistive Capacitive Acoustic, see fig 33 d. Commercially available cryostats. Fig. 34 – detachable tail, cold finger optional Fig 35 – Flow cryostat a)flow transfer tube +cryostat, b) magnet cryostat, optical and blind Several pages of drawings Closed cycle cryostat – no cryogen required. Figure Fig 36 – a commercial example of a closed cycle cryostat. Average cryocooler offers about 2W of cooling power at 20 K, reaches the minimum temperature of 810 K at the cold finger of the two-stage cooler. The first stage cools down to 60-80 K and cools the radiation shields. Problems: Noise and vibration problems. Strong temperature fluctuations (0.1-0.5 K) at lowest temperatures associated with the thermodynamic cycle. Solution: use a thick Pb washer at the cold finger to increase thermal inertia and reduce fluctuations. Operation: Samples can be mounted in vacuum or in exchange gas. Operation of a crygenic laboratory. Recover all He gas – use He recovery system Care for leaks and cleanness of the operation System works in overpressure. Tb(He4) > 4.2 K. Helium-3 cryostats. 0.3 – 1 K range Advantages of He3: higher vapour pressure, no superfluid transition in this temperature range Disadvantages: cost, low heat of evaporation. As a result: usage only well below 1 K. In all He3 systems the He3 gas is liquefied by a pumped He4 bath. The advantage and use thermal shielding by He4 and thermal anchoring to He4 bath. He3 cryostats of increasing sophistication are shown in figs 37 a-c. a) Non-recirculating with pumped main He4 bath b) Non-recirculating with continuously operating He4 cooler c) Recilculating with He4 evaporator Foils of figures Pumping He3: low vapour pressure at mK range → large pump is needed → large diameter of pumping tubes required → large radiation input to the system. Example: to maintain 0.5(0.3) K we need to pump down to 0.2 mbar (2 microbar). At these pressures already roots + booster pump set-up is needed. For cooling power of about 1mW the required evaporation rate is: V Q 7cm3 of lq. He3/h 3l of gas He3/h at 1 bar. L We need pumps with s 15(15000) m3/h! Assuming laminar flow regime we need tubes with the diameter of 3(10) cm and few meters length. This would cause huge heat flow along the pumping lines. Solution: Internal sorption (charcoal) pumping – see fig 37d. Modern He3 cryostats are typically made in a form of dipsticks to a storage He 4 vessel, see fig 37 e. The pumping power of an activated charcoal see fig. 37f. One can maintain 025 K at power input below 0.01 W and 0.4 K at 1 mW. Commercial He3 spectroscopic system – fig. 37 a