# Molecular Beam Epitaxy: Water cooling system

Faebian Bastiman

MBE systems typically utilise water cooling to regulate and stabilise effusion sources. This may be integral to the cell, external though individual cooling shrouds or via a water cooled base plate. Water cooling may also be required for the outgas stage, ion pumps, heated view ports and power feedthroughs. Since you have multiple zones that require cooling you must first consider how to configure your water system. You have two choices: series or parallel.

A series system is very simple. You run the inlet of each cell from the outlet of the preceding cell and serially link all parts of the system in turn. This has the advantage that it is cheap and simple. The disadvantage is that the water gradually gets hotter and hotter from cell to cell, hence the final cell in the line will receive much hotter water than the first. Also, effusion cells tend to utilise water cooling shrouds made from 3mm (1/8”) pipe, hence serially connecting all the cells accumulates to a large resistance that the pump must overcome to create a suitable flow. By way of an explanation consider an analogy with a serial electrical circuit (figure 1), here the pressure is switched for potential difference and the litre/minute is current. Thus to achieve the required 0.5 litre/minute (0.5 A) to each cell, a potential difference of 11V must be applied to the serial 22 ohm resistance.

A parallel system is naturally more complicated to implement, but ultimately yields a superior cooling system. Consider the parallel arrangement in figure 2, now the required cooling of 0.5 litre/minute (0.5A) per part is obtained by having larger overall flow (1.5 A) at a lower pressure (3.4 V) due to the lower overall resistance. However, when you take a closer look at the circuit it is obvious there is a serious flaw! Whilst the total flow is 1.5 litre/minute each cell is not receiving the required 0.5 litre/minute. Due to the unique resistance of each cell, the cooling flow is not divided evenly. In this case the 5 ohm cell is receiving 0.68 litre/minute, the 7 ohm cell is receiving 0.48 litre/minute and the 10 ohm cell is receiving an inadequate 0.33 litre/minute.

The first thing we need to do on a parallel system is regulate the flow through each parallel branch. This is done by adding a tap (a variable resistor in our analogy) in line with each cell to provide additional resistance to specific loops. Figure 3 shows a balanced system. Note the pressure penalty incurred in adding the additional resistance.

A parallel loop will typically consist of a dual chambered PTFE/brass manifold to split the water and a series of valves on the outlet side to regulate each loop. An example system is shown in figure 4. To regulate the flow along each branch you will need a flow meter with a 0-10 litre/minute span on the main chiller outlet line and a little patience: (i) close all but one of the outlet vales and turn on the chiller, (ii) make a note of the flow, (iii) open another valve and adjust until the flow is now double the initial flow, (iv) repeat with the other valves. Placing valves on inlet and outlet can be useful, since then an individual cell can be isolated from the rest of the system during unscheduled maintenance.  The manifold and valves will increase the initial outlay by around £1000. In addition a parallel system needs significantly more tubing to distribute the water.

The demands of each individual cooling zone vary. Most effusion cells require 0.5 litre/minute and the same goes for cracker zones. Cells typically have a similar resistance, especially if they are roughly the same size. Consider buying cells with separate water cooling shrouds rather than integral ones, you can then ensure the water cooling shrouds are identical. The initial outlay is higher, but the maintenance cost is lower since the cells are simpler to repair and cheaper to replace. Bulk zones and outgassing zones may need 1 litre/minute and they offer much lower resistance to flow than a normal effusion cell. The bulk zone can be connected in series with the outgas loop and other low resistance loops to increase the overall resistance of that cooling branch.

All in all the MBE system will require 5-10 litres/minute of cooling water depending on the configuration. The cooling water is usually supplied at 8-12°C at a pressure of ~3 bar. Hence a standalone, single phase, direct, vapour-compression water chiller filled with a water-glycol mixture is perfectly suitable for a single MBE system. Suitable chillers include single phase and compact Thermo Scientific Flex1400 (Thermo Scientific) or for larger systems the 3-phase Tricool S2 are an excellent range with customable options (Tricool). A more comprehensive discussion of the types of chillers available can be found in MBE: Water cooling system types.

It is a good idea to back up your chiller on an UPS to ensure security of supply and to have a spare pump available since this is the most common part to fail. Should the system fail the damage wrought can be very expensive, and as such is highly recommended that a backup cooling loop is installed. Tap water is a perfectly viable temporary cooling solution. A flow meter, suitable array of valves and control circuitary can be used to switch to the backup system in the event of a chiller failure. The system can also be configured to automatically switch back to the chiller once flow there is restored. The backup tap water line will utilise ~7,000 litres per day however, so it is only a temporary solution to avoid multi-£10k of damage. The backup loop can also be utilised as a water purging loop with the addition of a N2 line, simplifying purging during routine maintenance. A simple system incorporating these parts is shown in figure 5. The bypass valve is normally closed and the green 3-way valves are normally straight through. The light blue 3-way valve is set to water. If the flow meter detects an interruption to flow (or insufficient flow) the bypass valve is opened and the green 3-way vales are operated to allow the tap water to flow through the cells and into the drain – and hence maintain cooling. Once the chiller is repaired, adequate flow will be detected through the bypass valve and the system can switch back into normal mode. If the chiller is turned off and the blue 3-way valve is set to N2, then the system will enter a purge mode whereby N2 will push all the water from the cells and into the drain. Very useful prior to maintenance. The control flow can be simply set up inside the EPIMAX MBE control software.