Vacuum Chamber with Oil Vapor Diffusion Pump

This three-stage vacuum pumping system creates a very strong vacuum, in which charged particles can travel long distances without colliding. It has a large stainless steel vacuum chamber with a transparent cover. When paired with a high voltage power supply, it can be used for experiments involving electron beams, ion beams, etc.

Warnings

  • There is always a chance that a home-made vacuum chamber may implode.
  • If left un-attended, the diffusion pump heater will create a fire hazard.
  • High voltage vacuum experiments have the potential to generate harmful X-Rays.

Components

  • Two-Stage Rotary Vane Pump
  • Foreline Pump Hose
  • Vapor Diffusion Pump
  • Liquid Cooling System
  • Vapor Baffle
  • Gate Valve
  • Vacuum Chamber
  • Variable Leak Valve
  • Convection Gauge
  • Bayard-Alpert Ionization Gauge

Two-Stage Rotary Vane Pump

The two-stage rotary vane pump is a mechanical pump that brings the system pressure down to about 30 microns Hg. It was purchased brand-new from Harbor Freight Tools.

Foreline Pump Hose

Originally, I tried to use an r134a refrigerant charging hose to connect the rotary vane pump to the oil diffusion pump. Unfortunately, the inside diameter of this hose was only about 3/16″ and this created an unacceptably large pressure differential between the diffusion pump exhaust and the rotary vane pump inlet. To solve this problem, I un-screwed the flared fitting that came with the rotary vane pump and connected the pump to a 1/2″ fiber-reinforced flexible plastic tube. A cork-shaped silicone plug was used to join the other end of the tube to the diffusion pump. Nylon barbed tube fittings were used on both ends of the tube.

Oil Vapor Diffusion Pump

I purchased a used 4″ vapor diffusion pump from eBay. It appears to have been salvaged from a JEOL scanning electron microscope, and was in very dirty condition when I received it. This pump was the victim of a careless operator. It is very important to make sure that the rotary vane pump is turned on before the diffusion pump heater, because otherwise the oil will burn and create a huge mess. I dis-assembled the pump and was able to clean out most of the burned oil by scrubbing it with soap water and a wire brush. I purchased a bottle of replacement oil directly from Kurt J Lesker Company.

Liquid Cooling System

The radiator was salvaged from an old Power Mac G5 computer, and the water pump was purchased from eBay. Rubber tubing was used to make all the connections. Two computer fans are used to force air through the radiator. Unfortunately, this system is not large enough to handle the 500W heat output. The radiator becomes too hot to touch after about 20 minutes. A radiator from a small car would probably be more appropriate. Proper cooling will improve the overall performance of the pump by causing the oil vapor to condense as soon as it hits the walls of the pump.

Vapor Baffle

The system functions just fine without a vapor baffle, but I was lucky enough to find a matching baffle for my diffusion pump on eBay. It helps to prevent oil vapor from escaping into the vacuum chamber.

Gate Valve

When the diffusion pump is starting up, it may spew hot oil vapor into the vacuum chamber unless the gate valve is closed. I purchased a used 2″ gate valve from eBay for about $30. I had to replace all of the O-Rings, and I also noticed that vacuum grease had been used on the valve. I purchased a tube of Dow Corning vacuum grease from eBay, and started using it on all of the rubber O-rings and gaskets. A 3/8″ aluminum plate is used to connect the diffusion pump inlet to the gate valve. Nitrile rubber O-rings are used to seal both connections.

Vacuum Chamber

The vacuum chamber is built around a large Bain Marie pot that I purchased from Food Service Warehouse. It is constructed from heavy-gauge sheet metal, and does not have any handles that could potentially leak air into the chamber. Although the cylindrical portion of the pot seems to be structurally sound, I suspect that the flat base could crumple under full vacuum. Therefore, I created a 1/2″ polycarbonate reinforcement plate to take all pressure off the stainless steel base. 14 bolts are evenly spaced around the perimeter of the base, and a 1/16″ nitrile rubber gasket seals the stainless steel chamber to the polycarbonate plate. Some of the bolt holes are sealed with JB-Weld, a slow-setting epoxy resin. After mixing the epoxy, I de-gassed it for about a minute in an ordinary vacuum chamber. The remaining bolt holes are sealed with special screws (from McMaster) that come with built-in O-ring seals. During operation, the vacuum chamber is covered with a 1/2″ polycarbonate plate. This allows the inside of the vacuum chamber to be viewed, and also makes it easy to change between different experiments (by using a different chamber cover for each experiment). The cover seals against the rounded stainless steel edge with a 1/16″ nitrile rubber gasket sheet.

Variable Leak Valve

I purchased a variable leak valve on eBay. This is an unusual item, and I had to wait a while for one of them to come on the market. A variable leak valve can be used to introduce very small amounts of gas into the vacuum chamber. This is necessary for certain experiments, such as the fusor. JB Weld was used to attach the leak valve to a 1/4″ NPT fitting, which was then screwed into a hole that was tapped in the polycarbonate cover. PTFE paste was used to seal the NPT fitting.

Convection Gauge

A convection gauge is fitted to the top of the vacuum chamber cover. The convection gauge has a special fitting that seals against a nickel gasket. I was able to find an adapter that converts the special vacuum fitting on the gauge to a regular NPT fitting, as well as a pack of nickel gaskets. Thanks again, eBay! I drilled and tapped an NPT threaded hole into the chamber cover and sealed the connection with PTFE paste. The Granville Phillips mini convectron gauge actually has most of the electronics built-in. It just needs to be connected to a power supply and the output voltage can be read by an Arduino or even a multimeter.

Bayard-Alpert Ionization Gauge

The ionization gauge is bolted to the bottom of the vacuum chamber base plate, and sealed with an O-ring. It was actually designed to be sealed with a metal gasket, but the O-ring works just fine because it fits inside the larger-diameter circular blade that was originally intended to form a seal. The gauge is connected to electronic instrumentation that I purchased on eBay. It looks like this item was also salvaged from a JEOL electron microscope.

Project Ideas

  • Scanning Electron Microscope
  • Vacuum Tube RF Amplifier
  • X-Ray Generator
  • Mass Spectrometer
  • Fusor

References

Building Scientific Apparatus

Solid State Tesla Coil

The system described in this post is a continuous wave solid state Tesla coil (CW SSTC). As seen in the video above, it produces sparks that look very different from those of a traditional Tesla coil. The continuous wave output gives rise to thicker, brighter, shorter sparks that appear almost sword-like.

The circuit diagram is depicted below. A standard function generator is configured to produce a square wave. The frequency of this square wave must be adjusted to match the resonant frequency of the coil (approximately 600 kHz). Next, the gate drive board amplifies this signal to the appropriate level for driving power MOSFETS. A gate drive transformer isolates this low voltage gate drive board from the H bridge, which runs at 160 volts. The H bridge converts direct current to high frequency alternating current, which drives the air core coil at its resonant frequency.

Table of Contents

  1. Test Equipment – necessary for building and testing this device
  2. Gate Drive Board – a basic circuit for controlling high-power switches
  3. Half H Bridge Board – converts 160VDC to 600kHz alternating current
  4. Primary and Secondary Coil – the main air core transformer (Tesla coil)
  5. Gate Drive Transformer – isolates the gate drive from the H bridge
  6. Completing the System – how to wire together items 1 to 5
  7. Where to Find Parts
  8. Links

1. Test Equipment

Digital Multimeter – A digital multimeter is required for checking supply voltages and identifying damaged components. With care, it can also be used to measure power consumption and analyze circuit performance.

LRC Meter – A precision LRC meter can measure the leakage inductance and magnetizing inductance of the gate drive transformer. It can also be used to characterize the air core transformer. The Vichy DM4070 (available on eBay) has the 1uH accuracy needed for this application. It is possible, but inconvenient, to measure inductance without an LRC meter.

Soldering Iron – A soldering iron is used to assemble the PC boards, and to make a few other connections.

Oscilloscope – If the circuits described here are constructed exactly as described, they should work correctly. In practice, however, minor construction variations can change the behavior of the circuits significantly. An oscilloscope can be used to check for correct operation, and is essential for any experimenter who wishes to design RF circuits from scratch.

Function Generator – An HP 3311A function generator is used to provide the signal for this SSTC. Any function generator can be used as long as it can produce a square wave with a range of 0V to 10V. The frequency must be adjustable from 100kHz to 1MHz. If you can not find one, you can build a simple square wave generator instead.

DC Lab Grade Power Supply – Good lab grade power supplies are available on eBay. A 12VDC regulated plug-in power supply can also be used.

2. Gate Drive Board

The signal from a function generator is not strong enough to drive power transistors. The high performance gate drive circuit described below will amplify the signal to levels that enable high speed switching.

Parts Needed

  • TC4421, TC4422
  • 5 Ohm, 2W resistor
  • 10uF Capacitor
  • 100nF Capacitor
  • Wire

PC Board Fabrication

The PC board can be manufactured by a company such as Osh Park. Upload the following files to their website, and they will process your order.

double_ended_gate_driver_v1

The finished board should look like this, and should measure approximately 1″ x 1″:

Front:  Back: 

PC Board Assembly

  1. The TC4421 and TC4422 ICs are designed for use as a MOSFET gate drive. Together, they form a small H bridge inverter that converts 12VDC to high frequency AC.
  2. The 50 Ohm input resistor should only be used in conjunction with a function generator that is calibrated for a 50 Ohm load. This resistor must be omitted if you are using a regular function generator, a 555 timer, or a 4046 VCO.
  3. The 100nF and 10uF capacitors provide power supply stability, and they should not be omitted. The MOSFETS draw very large currents while turning on and off, and the capacitors can provide this current instantaneously.
  4. The purpose of Rg is to improve the quality of the gate drive waveform. If it is omitted, the parasitic inductance of the circuit can cause ringing in the waveforms. A 5 Ohm resistor can be used for Rg, but experimentation will be needed if an optimal value is desired.
  5. X1 is the gate drive transformer. It will be described later.
  6. Attach wires to all of the inputs on the circuit board. These wires will connect the board to a 12 V power supply, and to a function generator.

3. Half H Bridge Board

The Half H Bridge is a high power inverter circuit. It converts DC to high frequency AC, which is used to drive the Tesla coil.

Parts Needed

  • Half H Bridge Board
  • 2 FPDF16N60 MOSFETs
  • 2 High Speed Diodes
  • 2 Schottky Diodes
  • 2 Polypropylene Film Capacitors

PC Board Fabrication

The PC board can be manufactured by a company such as Osh Park. Upload the following files to their website, and they will process your order.

half_h_v1

The finished board should look like this, and should measure approximately 1″ x 2″:

Front:  Back: 

PC Board Assembly

  1. M1 and M2 are high power MOSFETS. FDPF33N25 from Fairchild Semiconductor
  2. D1 and D2 are ultrafast rectifier diodes. BYC10DX-600 by NXP Semiconductor
  3. D3 and D4 are schottky rectifiers. STPS745 from ST Microelectronics
  4. C1 and C2 are film capacitors. 470nF, 275V. ECQ-U2A474ML from Panasonic
  5. The small components drawn on the board are optional Transient Voltage Suppressors – pairs of back-to-back zener diodes that protect the MOSFET gates from any voltage exceeding a certain threshold. If measures are taken to prevent the waveform from ringing above the gate’s maximum safe voltage, these components can be omitted.

4. Primary and Secondary Coils

The Tesla coil itelf consists of two coils: one with just a few turns, and one with several hundred turns. For this project, I re-used the transformer from a low-power solid state Tesla coil kit sold by Eastern Voltage Research. The parts are, however, readily available.

Parts Needed

  • Coil Form – 4.2″ OD ABS or PVC Pipe, Thin Wall
  • 30AWG Copper Magnet Wire
  • Wire for Primary Coil

Assembly Instructions

  1. The secondary coil can be wound on any cylindrical form of proper diameter. I used a small lathe to wind mine. A 630 turn coil can also be wound by hand.
  2. A thin layer of polyurethane can be applied to secure the windings to the coil.
  3. The primary coil should be separated from the secondary. A slightly larger plastic pipe should be used as the coil form. Only 3 turns are needed.
  4. The top of the secondary should be soldered to a short, thick wire. If the thin copper wire emits a spark, it will melt quickly.
  5. The bottom of the secondary coil should be connected to earth ground. It can be wired to the ground prong of a power outlet, or to a cold water pipe.
  6. The primary coil connects to the half H bridge as shown in the first diagram.

5. Gate Drive Transformer

The gate drive transformer isolates the gate drive board from the H bridge board.

Parts Needed

  • Amidon FT-50A-J Core
  • Wire Wrap Wire

Assembly Instructions

  1. Wire wrap wire is recommended for this transformer. It consists of a thin silver-plated wire with very tough insulation. If copper magnet wire is used instead, make sure to wrap the core in electrical tape so that it does not scratch away the insulation.
  2. Twist 3 2-foot long strands of wire wrap wire together.
  3. Wrap 10 turns of twisted wire on the ferrite core, and strip away the insulation at the ends.

6. Completing the System

Connect together the components as shown in the schematic diagram found at the beginning of this post. Although the SSTC can be connected directly to the power line, it is better to use an isolation transformer for testing purposes. Otherwise, it may not be possible to use an oscilloscope to test the circuit. I used an antique VIZ Isotap II transformer, which includes a built-in fuse. The AC current from this transformer must be rectified before it is fed to the half H bridge. A 10 amp (approximate) rectifier diode or full wave rectifier bridge can be used for this purpose.

The following steps should be followed when powering up the system:

  1. Configure the function generator to produce a 600kHz square wave, swinging from 0V to 10V. Use an oscilloscope to verify that the gate drive board is receiving a proper square wave.
  2. Apply 12VDC to the gate drive board, and make sure that the output is swinging with 12V amplitude (24V peak to peak across the primary coil of the gate drive transformer).
  3. If possible, use an adjustable isolation transformer to power the half H bridge board, starting with a low voltage and working your way up to 160VDC. WARNING: If you are not using an isolation transformer, you can not connect your oscilloscope probe to the secondary side of the gate drive transformer. The oscilloscope probe is grounded.
  4. Adjust the frequency of the function generator by sweeping from around 400kHz to 700kHz. When you hit the resonant frequency, there should be sparks.

7. Where to Find Parts

8. Links

Steve Ward’s Website – Steve Ward does a lot of work with DRSSTCs, which produce longer sparks than the traditional SSTC described here. As far as I know, his site is home to the largest, most powerful SSTCs.

Richie Burnett’s Website – Richie built a similar SSTC to the one described here, but he did so about 15 years ago. His site is still the best source for information on SSTC operating principles and theory. It also contains information on spark gap Tesla coils, Class E SSTCs, and induction heating.

Jan Wagner’s Website – Jan has published many useful circuits and design notes.

Texas Instruments Application Notes

  • SLUP169 – Design and Application Guide for High Speed MOSFET Gate Drive Circuits – Laszlo Balogh
  • SLUP123, SLUP124, SLUP125, SLUP126, SLUP127 – Magnetics Design for Switching Power Supplies – Lloyd Dixon

Eastern Voltage Research – Dan McCauley has written a book on DRSSTCs, and also sells SSTC kits. I purchased some of his products, and found them very interesting.

NOTE: This post was originally published January 2015 (old website).