WARNING: The use of high voltages is perhaps the most dangerous part of working with lasers - while laser radiation can cause instant blindness, high voltages can cause instant death! Do not attempt to experiment without adequate safeguards and safety practices including insulation around exposed parts operating at high potentials. As well, design charging systems for as low a current as possible to reduce the risk of electrocution. The author specifically disclaims any and all liabilities associated with the construction and use of such devices. Designs presented here are in the interests of providing information on operational principles only and do not represent safe nor ANSI safety compliant designs.
The traditional approach is to use a small neon-tube transformer with a variable autotransfomer (a Variac) used to control the AC input voltage to the transformer. While larger transformers may be rated at 15kV at up to 60mA this is overkill for most applications (not to mention overkill in the lethal sense - 60mA will ensure almost certain death !). Smaller transformers may be found which output 8kV at 5mA and are better suited to capacitor charging applications. Similar transformers may be found in old photocopiers. Note also that a neon sign transformer is often center-tapped: the secondary consists of two output terminals with the center point connected to the case, which MUST be grounded (these can be identified by the two ceramic insulators on the sides of the transformer). This poses a problem for many applications since either both terminals on the target capacitor must be run electrically 'hot' (this is dangerous) or only one-half of the transformer used with the case ground being common with the capacitor ground. Many of the smaller transformers are single-ended with the case being ground and a single high-voltage terminal protruding from the unit - these are the best bet for a supply of this kind.
Transformers output AC which must be rectified for caapcitor charging. A single Varo VG-20 diode or a diode scavenged from an old TV (usually a long, tubular ceramic type) works well for this purpose. The transformer output lead is wired in series with the diode. Add a 50K wirewound resistor in series as well with the diode as well to limit charge current to protect the system when the capacitor discharges and prevent the possibility of a continuous discharge (e.g. across the spark gap of a nitrogen laser). Rectified, the DC output of a transformer will be 1.4 times (20.5) the AC output so that a 5kV AC transformer will ultimately charge a capacitor to 7.1kV. For higher voltages, an old colour TV tripler can be used. These boxes contain several capacitors and diodes and triple the AC input voltage allowing the generation of higher voltages (up to 20kV). There are frequently four leads on a tripler: AC input, high-voltage output, ground, and focus. Leave the focus lead unconnected and be sure to connect the ground to the transformer ground (usually the case) which is also common with the capacitor. Again, use a series resistor on the output to limit short-circuit current. Unfortunately, voltage triplers are hard to find nowadays since most are integral with the flyback transformer in modern monitors and TVs.
An alternative to an AC powered transformer is the use of a small flyback from a TV or monitor. This is my favourite approach since current is low for safety's sake. Newer flybacks incorporate a voltage tripler and will produce over 20kV on their own while older flybacks produce up to 10kV AC which can then be rectified or multiplied externally.
A driver circuit is required which drives the flyback with a pulsed DC signal. One such circuit is shown below:
A 555 is used as an oscillator, optimally around 15.76KHz (in North America). Power pins on the chip (pin 1 to ground, pin 8 to +12V) are not shown on the diagram. The output from the 555 is then switched by PNP transistor Q1 and by the large 2N3055 NPN power transistor. Q2 must be adequately heatsinked as well since it may well switch over an amp during operation. Output terminals are then connected to a primary winding of the flyback. The average flyback has up to ten primary terminals and one secondary (the secondary identified as the thick, insulated, wire protruding from the top of the unit). Choosing the correct primary terminals is usually a matter of trial-and-error with the effect evaluated by the size of the arc produced! Connect a 12V supply to the circuit (a bench-type supply works well here), set the oscillator frequency to something in the audio band (you will hear the transformer when you connect it to the flyback), place the secondary lead within a few millimeters of the +12V ground terminal (the circuit uses positive ground, the negative lead is switched in this case), and with a little luck a purple spark (in the case of a flyback without an integral multiplier), or a noisy intermittent white arc will be seen from the output to ground - if not, simple change terminals on the primary of the flyback. When the output is optimized, vary the frequency of the oscillator by varying R2 until it is just beyond hearing range ... this will be close to the optimal frequency for operation (R2 can also be used as a sort of voltage control).
With an 10mm arc, about 10kV is available. For a flyback without an integral multiplier an external diode is required, when a flyback with an integral multiplier is used the output is already DC and may be used to charge a capacitor directly.
Other possibilites for high voltage supplies include those scavenged from electrostatic air cleaners (which often run at 8 to 10kV but with low currents under 500mA), portable ionizers (up to 7kV but again, low current), drum charging supplies from old photocopiers and laser printers (often a convenient module which runs from 24V and outputs 5 to 7kV - I have used one of these to drive my TEA laser), HeNe laser tube supplies, and even a modified bug-zapper. One inexpensive bug zapper, shaped like a badminton raquet and run from two AA batteries, was measured to output 1.6kV DC. The AC output from the transformer could be used to drive a multiplier to produce much higher voltages. The multiplier could even be built from several of these inexpensive units. A tripler would produce 4.8kV, enough to operate a small TEA nitrogen laser.
Sam's Laser FAQ outlines many designs for high voltage supplies.