Molecular Beam Epitaxy: LN2 system

Faebian Bastiman

LN2 usage can represent the major expense in LN2 operation. Designing an efficient LN2 cooling system for an MBE operation is more involved than a simple fluid transport system. At one end the system should comprise a large dedicated external storage tank and the other end consists of the cryo panel of the MBE system. There are two main loss mechanisms that must be minimised in order to transport the LN2 from one end of the system to the other.

Firstly there is the compound effect of connection loss (via heat loss) and N2 gas heating. The heat loss results in evaporation of LN2 into N2 within the pipework. The actual LN2 lost at the point of heat loss is fractional compared to the “heating” the “warm” N2 gas causes to the “cold” LN2. This may seem like an odd concept, but the N2 gas is indeed hot compared to the LN2. The poorly insulated connection that resulted in heat loss is localised at a single point on the system, however the N2 gas permeates the entire pipework system and constantly converts LN2 to N2, which in turn exacerbates the problem. In order to minimise this loss mechanism the entire transport system, including valves, pipework and connections, must be vacuum insulated. Moreover, the integrity of the vacuum insulations must be regularly checked and maintained.

The second form of loss is flash loss. Flash loss is a foreign concept to those outside the LN2 community. It is a loss induced by reducing the pressure of the LN2. The change in pressure causes a fraction of the LN2 to “flash off” as N2. The reason for the dedicated tank is now clear: The tank must store the LN2 as close to atmospheric pressure as possible. A small pressure must be maintained in order to ensure the LN2 can reach the destination with the intended flow rate. The lower the LN2 storage pressure, the lower the flash loss.

The presence of N2 and LN2 in the system is termed “two phase flow”. Two phase flow implies the cooling is inefficient since the LN2-cyro panel contact area is reduced in the presence of N2 gas. Hence a “phase separator” is typically employed in order to separate the gas and liquid phases and ensure only LN2 reaches the cryo panel. The phase separator stores the LN2 at atmospheric pressure (the most efficient pressure) and uses gravity to feed LN2 into the cryo panel. It is important to remember that N2 gas will be generated inside the cryo panel. The cryo panel is the only “intended” heat loss present on the system. Additional triaxial pipes ensure that the N2 gas generated in the cryo panel can leave without interrupting the LN2 entering the cryo panel. Thus the cryo panel is optimally cooled and achieves optimum efficiency resulting in an optimal vacuum and (hopefully) optimal samples.

The ideal LN2 system would therefore comprise:

  1. a low pressure LN2 tank
  2. a vacuum insulated tank connection valve
  3. vacuum insulated pipework
  4. a vacuum insulated phase separator connection valve
  5. an atmospheric pressure phase separator
  6. a vacuum insulated triax LN2 feed hose and a vacuum insulated return hose from the phase separator to the cryo panel
  7. the cryo panel
  8. a stainless steel N2 gas exhaust line from the phase separator to the outside world

LN2 system crop2

The volume of the tank (1) will depend on your individual usage (typically 100 – 250L /system/day) and the frequency of your deliveries. Expect to pay £10-20k for the tank. The valves (2 and 4) are generally included. The pipework (3) varies by manufacturer (~£300/m) and typically requires a small, dedicated pump to ensure vacuum integrity. The phase separator (5) is around £16k and the hoses (6) are around £5k. The entire system will therefore cost around £50k. Which may seem like a lot, but to put it in perspective it roughly equates to the amount of money you would waste in the first two years of operation of an inefficient system. All that remains is to choose your manufacturer: VBCVBSDeMaCo or PECO.

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