[Amps] trying to understand 1296 cavity amp

[jmltinc at aol.com]

ok...  whew!  I was hoping I wasn't crazy.  So you agree that when that 2c39 anode connection is poked through that anode plate, that there is no metal to metal connection to the inside of the cavity??
 

** There is no mechanical connection between the tube and the cavity. I too have been puzzled by how the RF is coupled to the cavity. 

I turned to Kermit Carlson, W9XA. He is an Engineering Physicist at Fermilab National Accelerator Laboratory. He designs and implements the cavity amplifiers used in the LINAC accelerator (of the Tevatron) and the new Muon Lab.

Here is his explaination:

John;
 
The upper cavity connection to the plate is separated  from the rest of the cavity by a plate of kapton. This  makes the surface of the anode in the tube actually a portion of the anode resonator cavity. It actually is easier if we draw the construction - this provides an extrememly low inductance connection and makes the tube -part- of the  cavity.... did this help?

I wrote back:
I think so Kermit. I thought it would be capacitively coupled to the cavity. You are telling me the opposite. I can imagine the assembly operating as a waveguide (with the tube as the radiating  element). Is this a TEM mode?

To which he returned:
I am not 100% sure if this is a full TEM mode or a TE mode inside the cavity without looking at the design - but yes, the outer area of the anode is actually at relatively low RF potential, but high DC potential. It is the internal electron beam inside the tube in an area of E-field reasonance for the cavity that allows RF in the tube to be coupled from the cavity to a load by either an inductive loop or an E-probe.  

The Kapton sheet acts as a capacitor from the tube anode (the fingerstock and about a 2x2" square of brass) to the outside of the reasonator cavity. This does decouple the RF from the anode and allows it to "flow" along the inside dimmensions of the cavity, and to not be coupled outside the cavity.

Hope this helps Gary.

-John, N9RF







gary
 
> To: amps at contesting.com
> Date: Fri, 20 Nov 2009 18:29:12 -0500
> From: jmltinc at aol.com
> Subject: Re: [Amps] trying to understand 1296 cavity amp
> 
> 
> Different, isn't it? There is no physical connection between the plate and the cavity.
> 
> My Elmer, a microwaver and builder extraordinaire, once called it "Black Magic". Not satisfied with that, I scoured a half of a dozen books for the answer. Found several articles on how to build them, but nothing regarding the "Black Magic" part.
> 
> My guess the tube is capacitively coupled to the cavity. Perhaps someone can confirm or debunk that.
> 
> If not, I have a friend at Fermilab that can tell me as that is his gig.
> 
> -J
> 
> 
> 
> 
> -----Original Message-----
> From: Gary Patterson <gpatterson53 at hotmail.com>
> To: jmltinc at aol.com
> Sent: Fri, Nov 20, 2009 5:07 pm
> Subject: RE: [Amps] trying to understand 1296 cavity amp
> 
> 
> 
> 
> Thanks John. My problem is there is no physical connection between the 2c39 plate and the cavity to allow skin effect action to put rf into the cavity.
> 
> 73
> 
> > To: amps at contesting.com
> > Date: Fri, 20 Nov 2009 15:05:27 -0500
> > From: jmltinc at aol.com
> > Subject: Re: [Amps] trying to understand 1296 cavity amp
> > 
> > 
> > Gary,
> > 
> > Although the anode connection (fingerstock of the anode by-pass plate) is on the outside of the cavity, the tuned circuit is the inside of the cavity. Due to skin effect, RF cannot not pass through the walls of the cavity (or through the cross section of the fingerstock). 
> > 
> > As the outside of the cavity is not a tuned circuit; a large, coherent (in-phase) RF signal cannot develop on the anode by-pass plate nor the anode cooler, and both are essentially at DC potential.
> > 
> > -John, N9RF
> > 
> > 
> > 
> > 
> > -----Original Message-----
> > From: Gary Patterson <gpatterson53 at hotmail.com>
> > To: amps at contesting.com
> > Sent: Fri, Nov 20, 2009 9:48 am
> > Subject: [Amps] trying to understand 1296 cavity amp
> > 
> > 
> > 
> > I have a n6ca 2c39 1296 cavity amp and have trouble understanding how it works. 
> > he 2c39 plate has a large teflon sheet capacitor to ground. I dont understand 
> > ow any rf gets into the cavity when the plate is dead shorted with this 
> > apacitor. any help?
> > 
> > gary
> > w4af
> > 
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