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Re: Anode Layer Ion Source Mod?

Posted: Fri Feb 03, 2017 3:13 am
by Dan Knapp
Why would the full power of the accelerating voltage x the ion current be dissipated totally in the source? I would think only a fraction of it in the source.

Re: Anode Layer Ion Source Mod?

Posted: Fri Feb 03, 2017 6:12 am
by John Futter
Because of the electrons returning to the ion source.
They go from earth in this case to terminal which is 60kV.
+ ions out of source equal number or more (due to secondaries) electrons going back the other way.
Simion, Comsol model this quite accurately and has worked in practice with the intercoolers heat transfer between kero / water that we use to cool the source magnets.
We use Alfa Laval plate heatexchangers as intercoolers. Our biggest anode layer ion source consumes 5kW (120kV 40mA )and stopping it wrecking a fortune of Neo magnets is not a trivial task

Re: Anode Layer Ion Source Mod?

Posted: Sat Feb 04, 2017 3:19 am
by Dan Knapp
The energetic ions flowing out of the source are carrying energy which is deposited into whatever they hit. This energy is derived from the ion source power. Thus some of the power is being transferred to whatever the ions ultimately hit.

Re: Anode Layer Ion Source Mod?

Posted: Sat Feb 04, 2017 3:50 am
by Rich Feldman
In the example of a 40 mA 120 kV ion beam, we are putting in 4800 watts of electricity. At first glance, we'd expect the associated heating to be partitioned between the "ion source" unit and the target, in the same proportion as electric potential drops.

As John said earlier: even 40 watts of power dissipation in the source unit is enough to overheat magnets in short order, without liquid cooling.

But as John said more recently, we can't ignore the fraction of 40 mA which is carried by electrons knocked out of the target and accelerated through tens of kV on the way back to e-bombard the ion source. Have I got that more or less straight?

Re: Anode Layer Ion Source Mod?

Posted: Sat Feb 04, 2017 5:48 am
by John Futter
Rich
Yes this is also confirmed by the x-ray flux coming off the ion source. In the 4800 watt case 600 milli sieverts/Hr @ 2 meters form the ion source hence the unit is shielded with a concrete wall 8" thick.
To get x-ray s from the ion source itself you need electrons- lots of them
My gut feel is that for this system we get 3 secondary electrons per arriving ion with one of those going back up the acceleration column all the way to the ion source.
Yes the samples dejour do get hot and this is mitigated with a rotating stage. I have burnt my self quite a few times by bringing the systerm back up to atmosphere (back filled with N2
) and getting them out too soon.

Something I have not got a handle on is the fact
When electrons leave, the cathode cools while the anode heats with the arriving electrons --this I read somewhere or was taught many years ago

Here is a link to the system some time ago
A new ion source since then but you will get the drift I'm presently using the purple door with the hundreds of resistors setting he acceleration plate voltages down the chain from the ion source
http://www.coultersmithing.com/forums/v ... 7&start=10