rf cavities - how does it work?

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Jack Puntawong
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rf cavities - how does it work?

Post by Jack Puntawong »

I'm researching different ways on how particle accelerator insert energy into the charge particle. However, CERN statement on their LHC confused me a bit.

"A high-power electron beam inside the klystron modulates at 400 MHz. Power is extracted through a rectangular pipe of conducting metal called a waveguide, which leads to the RF cavity.Each cavity can achieve a maximum voltage of 2 MV, making 16 MV per beam."

1. How is the power being extracted ? I thought that a Klystron tube only amplify the current.

2. If I'm not wrong, the waveguide uses the principle of total internal reflection to reduce the effect of power lose.

3. What is the principle behind the power transfer from the RF cavity to the charged particle ?

Thanks!

Kunakorn (Jack) Puntawong
John Futter
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Re: rf cavities - how does it work?

Post by John Futter »

Jack

It is complicated but within the cavity there are areas of high current and other areas of high voltage depending on the dimensions. It is also complicated by the electromagnetic wave having the magnetic field and the electric field orthogonal to each other.
As I understand it these type accelerators are burst mode ie discrete times when this bunching of ions takes place and if you get the frequency and dimensions right you can make each serial cavity add to the ion beam energy.

I think Carl Willis works at a place using this tech --but not sure


you can excite a plasma with a microwave ion source or RF excited ion source very easily and just standard linear acceleration.
I do this at work, Doug Coulter does this on his own system.
I would think that Mr. google will have quite a bit to say on this topic
Jeroen Vriesman
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Re: rf cavities - how does it work?

Post by Jeroen Vriesman »

Hi Jack,

if an electron beam passes a resonator, the voltage over the resonator either accelerates or decelerates the electrons, depending on the polarity of the voltage at that moment.

Somewhat further in space the accelerated electrons will "catch up" with the decelerated electrons ahead in the beam, this will create an electron beam with current modulation, or "bunches", by modulating the speed of the electrons with the first resonator (the buncher), somewhat further in space the beam becomes current modulated.

The magnetic field of this current can be coupled to a second resonator (the catcher), when energy is tapped from that second resonator the induced magnetic field will slow down the electrons in the beam, dampening the current modulation, that's how energy is ectracted from the beam.
Jeroen Vriesman
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Re: rf cavities - how does it work?

Post by Jeroen Vriesman »

to actually answer your question....

A waveguide is just a metal construction to transport the wave, in can be inductively coupled to the resonator (single loop of wire in the resonator connected to a single loop of wire in the waveguide), "coupling loop", or by just a straght piece of wire from the resonator to the waveguide (coupling stub).

you can see some pictures here:
http://www.allaboutcircuits.com/vol_2/chpt_14/8.html

the page shows the coupling to a coax cable, but you can leave out the coax cable an just use a loop in the resonator directly connected to a loop in the waveguide if the distance is not more than a few mm.
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