Download 3_2._Cryogenics

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
30077
774.2
3004.2
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 12<<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