Re: Superconducting 1/4 wave resonator

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

One can make a number of very interesting Tesla Coil like phenomena with
open 1/4 wave transmission lines.  For practical sizes, the frequencies
need to be pretty high (but if you've got one of  those surplus 13 MHz (1/4
wave = 5.8m) or 27 MHz (1/4 wave = 2.75m) ISM band RF heaters....).  An
interesting experiment is to build a transmission line out of something
like copper pipe and feed it with a suitable high power RF source and
tuning network.  You can make a wonderful RF arc at the end which is pretty
quiet (it sort of just hisses).  A 100W ham transmitter (suitably modified
so that you don't radiate in the ham bands....)makes a fine source (and,
hey, you could AM modulate it!)

I've tried it with a "two wire" line made from 1/2" hard copper pipe
supported on PVC pipe standoffs, and it's kind of cool to see the arc move
back and forth as you change the excitation frequency.

I understand that one can make a coaxial line and create a nice annular
spark, which has some interesting "death ray" and "ball lightning"
applications if you combine your annular gap with a ring  vortex generator.

However it is VERY different from how a standard low frequency tesla coil
works, which is more of a coupled lumped tuned circuit (there... that'll
start the quarterwave vs lumped flame thread going!)

Indeed, this is how many early linear accelerators worked.  To accelerate a
particle you want a big voltage across a gap (because that adds that many
volts to the energy of the particle).  An easy way to make really high
voltages is to run an unterminated 1/4 wave transmission line fed by a low
impedance source. Gang up a bunch of them in series, each one giving it's
little onward push to the particles, and you can get pretty high energies
fairly easily.  Two schemes were popular: A series of drift tubes, with
each stage getting longer and longer (to allow for the increasing speed of
the particles) with alternate tubes driven 180 degrees out of phase. (no
transmission lines really necessary here...) or a later scheme from Alvarez
where the accelerator is also a transmission line where the propagation
velocity is carefully chosen to match the velocity of the particles going
down the pipe.

If you can minimize the loss in the TL (i.e. by making it out of a
superconductor (no ohmic loss) and running it in a vacuum (no dielectric
loss), you can make this work pretty well. (the Q will be quite high,
though, and tuning can be a bear).

If you have a high power UHF or microwave source, you too could build
one... (Hmmmm, another use for those magnetrons you scavenged when getting
the MOTs for your tesla coil)...

Tesla list wrote:
> 
> Original poster: "35045 by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<free0076-at-flinders.edu.au>
> 
> I haven't looked on the web yet, but I thought I would mention something I
> saw with my eyes a couple of days ago.
> 
> At the Australian National University, Research School of Physical
> Sciences and Engineering, there are some glass cases with old creations
> that aren't needed anymore. One of the cabinets has a superconducting
> conical rod with the smaller end fitted to a toroidal shape but with the
> hole axis at right angle to the rod unlike tesla coils where the hole axis
> is in line with the coil.
> 
> The superconductor is fed it's 1/4 wavelength at the base and it is
> claimed that less than 10 W of power is needed to produce over a megavolt
> of RF at the toroidal bit. When a charged particle enters the toroid's
> hole at the right time it is accelerated to the middle and then repelled
> away when the voltage is reversed (needs to enter at the right phase).
> 
> Several of these resonators formed a large particle accelerator and they
> were probably used in many other particle accelerators too.
> 
> If you want to check out their website I can't guarantee that it's there
> since I haven't got the time to look, but try www.anu.edu.au and look for
> RSPhys.
> 
> Darren