Molecular Beam Epitaxy: AC power

Faebian Bastiman

With a peak demand of 18kVA (240V x 25A x 3 phases) an MBE machine has the power requirement of a small industrial facility. The actual power demand involves a peak demand, operating remand and a base demand. The peak demand corresponds to when the system is baking, the operating remand is when the DC power supplies for the cells are at operating power and the base demand is essentially the demand in an idle state. You can work out your demand with a power meter or a loop meter and some systematic data gathering on each piece of hardware. Some typical values are shown in the table in Figure 1. Note the power demand is per unit and the total is the sum-product of all units.

What this shows is that the operation demand is roughly 200% the idle demand, and in turn the peak demand is roughly 110% the operation demand. What this means is that machine’s power demand can be met with one 240V x 32A x 3phase line for normal operation, and one 240V x 20A x 3 phase line for the bakeout. That will leave a safety margin with plenty of room for expansion. Decoupling the power supply can be of great advantage when you come to backing up your AC power. Note: You may find your own demand is significantly higher if you are running old (pre-2000) hardware. Also your peak demand will be significantly higher if you do not employ some eco-friendly practices like turning off the water chiller during bakeout.

AC power v4

Should you back up your AC power? That is a good question. Imagine a power cut. Everything goes off. You are in darkness. Pumps start winding down, cells crash toward room temperature, the vacuum rapid deteriorates, the cyro pump pressure rises rapidly, and worst of all you cannot see what is happening, because all the power is off! If it is just a brown out the power may come back on after a few seconds or a minute. It may switch on an off several times in rapid succession. It may stay off for several hours.

In an absolute worst case the power cut and/or surge will have damaged some of your hardware and you will not be able to restart the whole system. However, assuming all hardware survives, the worst case of prolonged power outage is that you will need to restart your system from “cold”. This means you Ga, Bi and Al will be frozen solid and your vacuum may be anywhere from 10-3 to 10-9 mBar depending on how well you leak detected. You have two options:

  1. Simply restarting/reheating everything is a viable option. If you have an Al insert or a double-walled crucible you can be confident the cell will survive.
    • Best case: The system will need a day or so to outgas
    • Worst case: You lose a cell or two (Ga/Al/Bi). Repair cost: £10k, time: 4 weeks
  2. You vent the system. Replace the crucibles and rebake. Repair cost £750 – £2250, time: 2 weeks.
  3. Back everything up on a 3 phase UPS and a LPG/diesel generator and simply keep growing

Option 3 is perhaps something you will not have considered. You do not need to back up the bake out 20A-3Phase line since it is non-critical, but the 32A-3Phase line is definetly worth backing up. The backup comes in two phases:

Firstly the UPS: A 32A-3Phase UPS costs a mere £2.5k, about 25% of that cell you may lose. The UPS is a stop gap solution, it is not intended for continuous operation. It is intended to hold for a number of minutes, 15 minutes max, until power is restored. UPS’ also buffer against power surges and fluctuations and instability, protecting your delicate equipment.

Secondly the generator: A 16kVA generator is capable of sustained operation. You will of course be consuming fuel, and a backup generator is less efficient than a power station, but you can operate indefinitely in the absence of all external power. You can in fact carry on growing if you wish. A 16kVA generator costs £7.5k, about 75% of that cell you may lose.

And so, for the price of a single effusion cell you can protect your system against the potentially very damaging and destructive effects of a mains power failure. Should you back up your AC power? I will leave you to decide.

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3 thoughts on “Molecular Beam Epitaxy: AC power

  1. Pingback: MBE Design and Build: Bake out controller | Dr. Faebian Bastiman

  2. Pingback: Molecular Beam Epitaxy: Initial Outlay | Dr. Faebian Bastiman

  3. Pingback: Molecular Beam Epitaxy: Run costs | Dr. Faebian Bastiman

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