The bake out runs infrequently, but has a huge power demand when operating. In MBE: AC Power, we discussed the power requirements for an MBE system. I suggested a 32A 3PNE line was needed to operate the system, whereas a dedicated 20A 3PNE line was suggested for the bake out. This article provides a design of the control system for the bake out utilising a dedicated 20A 3PNE line.
The bake out can be performed in two general ways:
- A box or tent with a combination of 2.5kW fan heaters and 1kW ceramic heaters
- With heat wraps or a heater jacket
The principle of the bake out is fairly simple: heat the system to the target temperature for X hours, then cool down. The heat and cool ramp rates should not exceed 1°C /min in order to avoid thermal stress. Furthermore the heat ramp should suspend if the pressure in the chamber being heated exceeds 5.0 x 10-6 mBar. The control hardware of choice for managing the temperature and pressure requirements of a bake out is the Epimax PVCx ion gauge controller. The PVCx is a multi-purpose tool: primarily it functions as an ion gauge controller, it has units of mBar, Pascal, Torr and Amps thus it further functions as a picoammeter for beam flux measurements, it can operate a pirani, be utilised as an 8 channel normally open/normally closed (NO/NC) relay/trip switch and control the bake out. All these controls and trip conditions can be configurable via either the PVCx menu or via serial comms.
The basic components of the bake out control system are shown in Figure 1. The MBE chamber (depicted in blue) has an ion gauge monitoring the internal pressure and a k-type thermocouple monitoring the external temperature. The entire chamber is surrounded by an insulating SS box or fibreglass tent. Two heaters are mounted inside the box/tent: Firstly a 2.5kW fan heater (BesTec) and secondly a 1KW ceramic heater (VG Scienta). The actual number of heaters will depend on the size of your system. The AC heater power is simply controlled via NO contactors rated at 32A with a 24Vdc coil (RS). The PVCx then actuates the contactors with its relays by modulating the external 24Vdc supply.
The fan contactor is on whenever the bakeout is above ambient temperature (i.e. constantly during bakeout) whereas the heating contactors make and break several times a minute to regulate the temperature. The fans can therefore simply be controlled by an AC switch and manually turned on before and off after the bake (Figure 2).
Alternatively the bakeout can be performed with heat wraps (VG Scienta) or custom made jackets (sadly currenty no know supplier). The jacket essentially comprises a heater element attached to a fibreglass fabric covering an inner fibregalss insulating layer (essentially loft insulation) that is custom shaped to fit the contours of the system. The bake out is therefore more efficient since the heat is injected view conduction into the chamber’s metal body and the is more readily retained. The jacket can be attached more swiftly than the wraps which need to be laboriously assembled each bake out, or left as a permanent feature for convenience at the cost of aestheticism. It is a good idea to use some insulation, even if is just Al foil to cover the heat wraps to retain the heat. In this alternative bake out the PVCx simply makes and breaks the contactor to control the power to the heat wraps/jacket rather than the heaters (Figure 3).
The actual heater configuration, power demand and wiring will depend on your system configuration. Figure 4 shows my wiring diagram for a custom built bakeout heater for a Riber 32P system. The system comprises a dedicated 20A 3PNE line, a rack mounted miniature circuit breaker (MCB) and fuse panel, a rack mounted 5 x contactor array, a DIN rail mounted distribution enclosure (FIBOX) mounted on the Riber 32P frame, 2 x 2.5kW heaters, 4 x 1kW heaters and 4 x 1kW heat wraps. The bake out controls the temperature and pressure of each chamber: MBE, preparation (PREP) and the fast entry lock (FEL) with 3 PVCx controllers. The rack mounted switching panel utilises 5 x 3-way switches to enable the system to be set into manual/automatic/off. Note: for clarity I did not include the EARTH connections (green), but any electrician reading this can rest assured all heaters, fans, racks and the MBE system itself were suitably earthed.