A diagrammatic representation of the apparatus arrangement is given in Figure 2.1.
The vacuum chamber was made with as small an internal volume as possible, while
still allowing reasonable adjustment of the apparatus within. The object was to
produce a background pressure of < 1 x 10® mbar. This was achieved not only by
minimising the surface area to be pumped and the length of the vacuum seals
required, but also by careful selection of the materials used in the manufacture of the
apparatus positioned within the chamber. Although PTFE is UHV compatible it is
porous and out-gases solvent for several days after it has been cleaned. Ceramic was
the preferred insulating material, since it out-gases less than PTFE. All the chamber
ports were sealed with OFHC copper gaskets, except the laser entrance and exit
windows which were sealed with annealed copper gaskets.
channel plate signal
To voltage supplies . for the detector 4" To the voltage supplies for the TOF mass
spectrometer Gas in to IG electronics / to RGA satelite control unit Heating element feedthrough _ Thermocouple feedthrough Turbo molecular pump
Figure 2.1: The vacuum chamber system.
The chamber was in the form of a four way symmetrical cross, as shown in Figure
2.1, with four main flanges (diameter 200mm): two vertical, S I and 82, and two
horizontal, T and B. The laser exit and entrance ports, E1W and E2W, (diameter
67mm) were on opposite sides of the vertices of the cross. The internal apparatus
electronic feed-throughs for the time of flight mass spectrometer and detector, PS, were mounted on this flange as was the gas leak valve, LV, (VG Model : MD6). The
Residual Gas Analyser, RGA, (Model : 160-121-001, discussed in section 2.3) and ionization gauge, IG, ( VG Model : VIG 22) were mounted on flange 81, as well as the feed-through, HL, for the internal heating bulb (Osram Xenophot HLX 150W). The remaining flange, 82, was an access flange upon which were mounted the gas heating filament and thermocouple feed-throughs.
The Balzer’s turbo pump (Model ; TPU 450 H) was mounted on the bottom flange, B. The system was backed with a Balzer’s diaphragm pump (Model : MD4) for an oil free pumping system. Both pumps were controlled by a Balzer’s electronic drive unit (Model: TCP 380).
To initiate the pumping of the chamber from atmospheric pressure the turbo pump electronic drive unit was turned on. This started the turbo pump and backing diaphragm pump simultaneously. When the pumps were used to pump the chamber from atmospheric pressure the current driving the turbo blades was 7.15 A initially. A real-time value of the current was shown on the display panel on the front of the electronic drive unit. As the pressure in the chamber decreased so too did the current. When the pressure was lower than 1x10"® mbars the current driving the turbo blades, was 1.1 A, with a blade rotation frequency of 760 Hz.
The blades in the turbo pump rotate faster in lower pressures, as there is less resistance to their movement, and thus, if the frequency was still not at the optimum value, within the specified time duration, a gross leak is indicated. An automatic electronic trip, to shut down the pumping system, was employed such that if the optimum operation frequency of the rotating blades was not achieved within forty minutes the turbo and diaphragm pumps were turned off. When the system is shut down, either by the electronic trip or by manually switching off the electronic drive unit, the diaphragm pump stops pumping immediately. Power loss to the turbo pump, however, results in a decrease in the blade rotation frequency and a decline in the pumping speed until pumping eventually terminates.
The blade frequency decreases very slowly, initially, as the blades are in vacuum and there is very little resistance to their movement. An argon gas supply is attached to the magnetic vent valve. Once the rotation speed has decreased to 185 Hz, which
takes approximately 10 minutes, the magnetic venting valve (Model: TSF 012) opens automatically and an inert argon atmosphere fills the chamber, minimising the exposure of the channel plates (see Section 2.8) to oxygen and water, which reduces their efficiency. The rotation rate decreases very rapidly as the argon atmosphere hinders the turbo blade's movement. With an initial chamber pressure of 5 x 10^ mbar, and an argon gas head pressure of 10 kPa, atmospheric pressure within the chamber is achieved in approximately 15 minutes after the shutdown of the electronic drive control unit.
The ionization gauge measures the ambient pressure in the chamber. The ionization gauge control unit (Model: IGC 27) trip facility was used to terminate the mains voltage to all the power units supplying the apparatus within the chamber if the pressure rose above 1x10 ® mbar. The trip was only employed when the experiment was running; otherwise it was overridden.
When the turbo pump blade frequency reached the working value of 760 Hz, the ionization control unit was initiated in order that any pressure change could be monitored. Once the pressure in the chamber fell to below 1 x 10 ® mbar the residual gas analyser could be employed.