Plasma x-ray machine? / Electromagnetic Radiation / Forums

Forums

4hv.org :: Forums :: Electromagnetic Radiation

« Previous topic | Next topic »

Plasma x-ray machine?

1 2 next

Move Thread

LAN_403

magnet18

Tue Jun 07 2011, 10:26PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

I was thinking, would it be practical to build an xray machine using a plasma arc as the source of xrays?
I don't know how much they put out (I'm guessing not much) or much about the generated rays or how they could be maximized, it's just an idea.
Thoughts?

Ash Small

Tue Jun 07 2011, 10:53PM

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

A conventional X-ray tube isn't that different in principle to a plasma arc.

While you can get plasmas at atmospheric pressure, most are in a partial vacuum, the same as X-ray tubes.

What is a plasma?......charged particles........same as electrons.

(I've not read the full paper regarding X-rays from arcs, but a conventional X-ray tube is basically an arc, ie electrons moving from cathode to anode)

Proud Mary

Tue Jun 07 2011, 11:03PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you Google plasma x-ray source you will run right into the cutting edge of X-ray research.

The one thing you will notice as you scan the dozens of papers your search brings up, is that almost all the experiments are seriously expensive if they are to be done properly.

But don't let that put you off! The hobbyist can often find a cheaper way that works just as well.

Proud Mary

Tue Jun 07 2011, 11:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

a conventional X-ray tube is basically an arc, ie electrons moving from cathode to anode)

Oh dear me, no! An arc discharge forms during continuous breakdown of (typically) a dielectric gas, such as air. The voltage drop across arcs is usually low, while in a 'conventional X-ray tube' the voltage drop needs must be large, so there will be enough potential difference twixt cathode and anode to impart energy and enthusiasm to the electrons as they blunder helplessly onward while the anode rushes towards them like a cast iron frying pan.

Ash Small

Tue Jun 07 2011, 11:49PM

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

Proud Mary wrote ...

Ash Small wrote ...

a conventional X-ray tube is basically an arc, ie electrons moving from cathode to anode)

Oh dear me, no! An arc discharge forms during continuous breakdown of (typically) a dielectric gas, such as air. .

I absolutely agree. (and thanks for highlighting the difference between a plasma discharge and an arc discharge)

I probably chose my words badly (too much vodka).

The title of this thread is 'plasma X-ray machine'. The point I was trying to make is that what you basically have in a 'conventional' X-ray tube is a plasma.

Therefore a conventional X-ray tube could be described as a 'plasma X-ray machine'.

(Albeit a plasma consisting (almost) entirely of negatively charged particles (electrons).

I'm not sure where Magnet18 got the term 'plasma arc' from, but it's generally aknowledged that a plasma is different to an arc.

Proud Mary

Wed Jun 08 2011, 12:04AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Proud Mary wrote ...

Ash Small wrote ...

a conventional X-ray tube is basically an arc, ie electrons moving from cathode to anode)

Oh dear me, no! An arc discharge forms during continuous breakdown of (typically) a dielectric gas, such as air. .

Keeping it simple, a plasma consists of ionised gas featuring charge carriers which make it conductive.

In a Coolidge tube there is (ideally) no gas at all. Electrons boil off the heated cathode in a sort of dance, and before they can catch their breath they fall under the attractive power of the anode, and accelerate towards it. Were there to be any significant amount of gas present - as in a clapped-out tube that has gone 'soft' for example - the mean free path of the electrons is so short that they make almost no headway before bumping into something, all hope of reaching the anode brought to nothing.

Ash Small

Wed Jun 08 2011, 12:23AM

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

Once again I agree.

Most plasmas, however, exist in partial vacuums. The degree of vacuum, or, conversely, the pressure, can vary.

I'd argue that a plasma at high vacuum (ie very few particles) is not much different from a high current vacuum tube (read 'X-ray tube).

Admittedly, the only positively charged particles in a vacuum tube would originate from the manufacture of the tube, and the vacuum achieved in the manufacturing process and any 'getter' system employed.

But a very low pressure plasma is not much different to a high current vacuum tube. (other than the plasma present in a vacuum tube has an 'overall' negative charge.)

magnet18

Wed Jun 08 2011, 12:52AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

Ash Small wrote ...

I'm not sure where Magnet18 got the term 'plasma arc' from, but it's generally aknowledged that a plasma is different to an arc.

I was thinking the arc formed between anode and cathode, like in a plasma speaker.

But that is irrelevant, please continue your current discussion, I'm enjoying it.

Proud Mary

Wed Jun 08 2011, 02:01AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Most plasmas, however, exist in partial vacuums.

I should have thought that the most common example of a plasma discharge is to be found in electric arc welding at atmospheric pressure.

No matter what the gas pressure, from the abysmally low to the oppressively high, plasma can be made to form wherever there is at least some gas to ionize.

If we consider two electrodes, an anode and a cathode in a 'hard' vacuum (10Eâˆ’9 Torr), there are but two known ways in which a current may flow.

1: electron field emission as we find in the cold cathode X-ray source.

2: vacuum breakdown as we find in flash X-ray sources.

Pinky's Brain

Wed Jun 08 2011, 02:10AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Proud Mary wrote ...

In a Coolidge tube there is (ideally) no gas at all. Electrons boil off the heated cathode in a sort of dance, and before they can catch their breath they fall under the attractive power of the anode, and accelerate towards it. Were there to be any significant amount of gas present - as in a clapped-out tube that has gone 'soft' for example - the mean free path of the electrons is so short that they make almost no headway before bumping into something, all hope of reaching the anode brought to nothing.

With a large enough field you can get runaway electrons even at atmospheric pressure (ie. they keep speeding up, even with the occasional collision). These are probably the source of x-rays in atmospheric discharges.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob