Simple mechanical pressure gauges don’t usually work at pressures below 10 Torr, so we need a different way of measuring pressure in our vacuum systems. We use the most inexpensive pressure sensor that works at these levels, which is the T/C gauge. It operates by measuring the thermal conductivity of the gas inside the chamber. As shown in Figure 36, the T/C tube contains a filament heated with an AC constant current and a T/C in contact with the filament. As the pressure decreases, the filament becomes hotter, because the number of gas molecules hitting the wire and conducting heat away from the wire decreases. As temperature rises, the T/C voltage increases and is measured by a sensitive meter that has been adjusted according to the T/C tube’s calibration curve.

Figure 36 A T/C vacuum gauge is able to measure pressures in the range of 1 to 10⁻³ Torr. It operates by measuring the thermal conductivity of the gas inside the chamber. As the pressure drops, the filament becomes hotter, causing the T/C voltage to increase.

Thermocouple gauge readout units (sometimes called T/C gauge controllers) are easy to come by on the surplus market. Make sure that the one you buy is compatible with the T/C gauge tube you intend to use. Alternatively, you may build your own gauge readout, as shown in the schematic diagram of Figure 37. This circuit is designed for the Teledyne Hastings type DV-6M T/C gauge. We use the model that is terminated with a KF-16 flange. The resistance of the heater filament is 18 Ω (don’t test this with an ohmmeter because you will burn the filament). It requires 0.38 VAC to operate at a current of 21 mA. The T/C’s output is 10 mV at high vacuum, and drops as pressure increases (see Figure 38). The DV-6M is useful in the range of 1 mTorr to 1 Torr, but is most sensitive between 10 and 200 mTorr. The filament is connected between pins 3 and 5, but the T/C and heater functions are combined in this tube (unlike the “textbook” diagram of Figure 36), so only one connection (pin 7) carries the T/C’s output signal.

Figure 37 This is a simple readout circuit that you can build to measure pressure with the DV-6M T/C vacuum gauge tube. Before connecting the T/C tube, make sure that the transformer’s output across an 18 Ω, resistor is 0.38 V_RMS.

Figure 38 The output of the T/C output of the DV-6M T/C vacuum gauge varies as a function of pressure. These values are valid when the gas in the vacuum chamber is air, and the current into the filament is calibrated to produce 10 mV at 0.01 mTorr. You can use these scales to calibrate your homemade T/C gauge readout unit.

In the readout circuit of Figure 37, a 555 timer IC controls transistor Q1, which drives the primary of a small audio transformer T1. The output of this transformer is used to heat the DV-6M gauge tube’s filament. An op-amp (U3) is used to amplify the signal from the T/C so that it can be measured with a voltmeter. Before connecting the gauge tube, connect an 18 Ω resistor at the transformer’s output and adjust the voltage across this resistor to exactly 0.38 V_RMS.

A VERY-HIGH-VOLTAGE POWER SUPPLY

The experiments in this chapter will also require us to use very high voltages—well in excess of the 2,000 V that can be obtained from the power supplies we built to power the PMT probe. Fortunately, precise regulation at these voltages is not needed, so a high-voltage power supply that can produce over 100,000 V is easy to build. Figure 39a shows a DC-to-AC inverter that is used to drive the high-voltage multiplier of Figure 39b. In this power supply, a push–pull oscillator drives a TV flyback transformer from an old color TV.^† The original primary of the flyback is not used. Instead, new primaries are made by winding two sets of four turns each of insulated #18 wire around the exposed core of the flyback transformer. Feedback for the oscillator is obtained through an additional coil of 4 turns of #24 wire wound around the core. The application of 12 V at the input of the flyback driver should produce 100 to 200 kVDC (depends on the flyback used) at the output of the flyback’s quintupler.

Figure 39 Schematic diagram of high-voltage power supply. (a) A push–pull oscillator drives a TV flyback to produce AC high voltage. (b) A quintupler is built separately and operated while dipped in pure mineral oil. Terminal B is the negative output if C is connected to the flyback’s AC high-voltage terminal and D to ground. (c) The quintupler is shown (with cover removed) connected to an Information Unlimited GRADRTV1 to produce over 50 kVDC from a 15-V input.

Figure 40 We use two different screens for our homemade CRTs. We built one out of the screen of an oscilloscope CRT that we cut in half. The other is an Ace Glass flask into which we poured a suspension of ZnS phosphor. Both tubes can be connected to our apparatus through #25 Ace-Thred connectors.

The output polarity of the so-called Cockroft–Walton multiplier depends on the way in which its diodes are oriented. Since some experiments call for both polarities, we designed the multiplier to yield either positive or negative output. If the high-voltage AC output of the flyback is connected to point A of the voltage multiplier, and point B is connected to ground, then the output at point D will be positive. If however point C receives the high-voltage AC, and point D is connected to ground, then point B will be negative. The multiplier should be built on a piece of clean perforated board suspended by nylon spacers inside a plastic container. Banana connectors may be installed on the plastic container and connected directly to points A, B, C, and D. The plastic container should then be filled with pure mineral oil (can be purchased at a pharmacy) to completely submerge the multiplier circuit assembly, which prevents high-voltage breakdown between components. Please note that this is a dangerous device! It produces high voltages that can cause very painful or lethal electrical shocks. In addition, spark discharges can be produced that can ignite flammable materials or volatile atmospheres.