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best ways to minimize HV arcing/ spraying?

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Vlad

Sun Jul 12 2015, 12:05PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
Location:
Posts: 65

I am using a high frequency, high voltage (<25 KV) power supply to attempt driving an x-ray tube. The x-ray tube is located within a lead shield and the tube is about 18" away from the HV supply. I have a 40 KV TV wire going from the output of the supply, into the front of the lead shielding, and coupled to the tube. However, I am getting a lot of losses and HV spraying/ arcing which greatly reduce voltage to the tube. I do have the HV lead/ tube connection thoroughly siliconed, but not helping much. What is the best way to minimize this and get full power to the tube? As a last resort, I could place the flyback/ multiplier under oil, along with the tube maybe, but this is a rotating anode type and needs to have the thin window unfiltered and if in a container filled with oil that wouldn't be the case.

Thanks in advance for any help.

Dr. Slack

Sun Jul 12 2015, 12:45PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Big conductors rather than small conductors is the general way, it results in a smaller field gradient at the conductor surface. So for instance, use the outer of a piece of coax to haul the voltage from one point to another, or a bit of pipe. Top all pointy rods with a ball. Do an images search for high voltage switchgear stations, and just feast your eyes on all of the rounding-off, and large diameters which are of interest at 25kV, as well as the large separations and field equalising conductors which are less important at that voltage

Ash Small

Sun Jul 12 2015, 01:01PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Auto ignition lead may be a cheap solution here.

Loads of different types and diameters are available. Most is 7mm diameter, but some is 10mm.

Cores vary as well. low resistance solid copper cores are usually the best.

They do deteriorate with age so maybe best to buy new, rather than rip some out of an old car.

GrantX

Sun Jul 12 2015, 03:09PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

It is very problematic when you have a HV feed-through in a metal case and high frequency AC. In addition to rounding off all exposed HV terminals, you might also need to widen the opening in the lead shielding a little (even though this may increase X-ray leakage slightly). This should allow enough room for a nylon cable gland and MANY layers of dual-wall heatshrink tubing ;)

With HF AC it usually comes down to the choice between oil immersion, a massive layer of dry insulation or a huge air gap. DIY dry insulation for tens of kilovolts and tens of kilohertz is sometimes a gamble, but for small areas it's usually worth it.

Dr. Slack

Sun Jul 12 2015, 04:10PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Ah, hadn't spotted you were going through a shield with the conductor.

In line with Ash's ignition cables, always press readily available cheap commercial stuff into service where you can. A spark plug makes a wonderful feedthrough, and is good to >30kV, although here you don't need its high pressure performance. File off the ground electrode (no filings in the gap between the shell and the centre please (you could fill the gap with wax before you file to keep them out)) and at least one end is already plug-compatible with ignition leads.

Ash Small

Sun Jul 12 2015, 09:07PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Most modern spark plugs have built in resistors for RF/EMI suppression.

In some cases that may not be an issue, but generally you want to minimize resistance, so use an unsuppressed plug eg for ford model 'T', or similar, or an unsuppressed racing plug.

Unsuppressed plug caps are available too if you are using plugs as feedthroughs, again they are generally used by the vintage and racing enthusiasts.

Silicon grease is also available for corona suppression.

Vlad

Sun Jul 12 2015, 11:15PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
Location:
Posts: 65

Thanks, guys, but this isn't a feedthrough case-- the HV lead enters the front of the cylindrical shield and attaches to the tube. Currently, the tube rests on the shield floor, but I will be elevating it slightly so it isn't touching the shield. Wire/ shield distance is very close, however, which was the reason for all the silicone around the wire/tube connection. Right now, the connection is made with 40KV rated TV wire.

Sulaiman

Mon Jul 13 2015, 02:00AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You could thread your HV cable through some silicone tubing e.g.

GrantX

Mon Jul 13 2015, 06:01AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Sulaiman wrote ...

You could thread your HV cable through some silicone tubing e.g.

Since there's no small feedthrough to worry about you'll probably get good results with some silicone tubing. I was using regular PVC hookup wire threaded through some blue automotive silicone tubing for one of my neon sign transformers (15 kV).

Vlad

Tue Jul 14 2015, 02:08PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
Location:
Posts: 65

Thanks, guys, read all replies so far and I like the idea of the silicone tubing and/or spark plug wire. However, there still is another option I have been considering. Since the HVPS is a dual based 2N3055 design that drives a flyback followed by multiplier, I am thinking about simply placing the flyback, multiplier, and tube in a plastic junction box container or similar that would fit into the shielding, and then filling with oil. My question here concerns the coupling between the 3055 circuit and flyback primary: how far away can these be and not have any adverse affects? Only the three items above would be under oil; the 3055 circuit and driver transformer would not be. Thanks.

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