Got Coax

I got the cable today but I have TOF work tomorrow and won’t be up until Wednesday.  We can run another test then.  With the fixed N-to-Probe, new coax and limited run times of 2 minutes, we should see if the cavity works.

Test Results from Run Number One

Hi Kevin, I thought you would be interested in the results from our first test run:

We ran the experiment, (We being Rambabu, one of his students named Gary and myself) on Wednesday and much to our amazement the cavity moved.  However, after a longer test run, it was apparent the movement was caused by a slowly melting coax cable.  During the last test run, shortly before the two minute mark, there was a puff of smoke and I hit the stop button.  It turns out, not only was the coax heating, getting to 51deg C on the plastic casing, but the dielectric inside the probe melted causing the smoke.

The picture shows a number of interesting things, first the cavity is twisted off center and continued to do so over the entire run.  Second, there is the smoke which came from the dielectric melting around the probe.

There was one encouraging sign which is that after the cavity was turned off it did swing back towards neutral.  Unfortunately because of the coax twisting due to heat, it is uncertain if the movement was our expected result or the coax cooling.

Two adjustments are necessary, the first is the removal of.coax which means we will have to place the entire apparatus including the circulator, power supplies, fans, etc at the end of a pendulum.  Essentially the entire shelf shown in the picture below will be hung at the end of a pendulum.

The second adjustment is that we need a probe that has a dielectric with a much higher melting strength but the same 50 ohm impedance.

The good news is that it does not look like arcing occurred inside the cavity between the probe and walls.  Copper rocks!

Overall, it was a successful test run in that we learned three things:

  • Need a high melting strength dielectric in our probe assembly (or better design) – Interestingly enough, the probe in the waveguide-to-N at the top of the pendulum, did not have problems (no smoke, lol), maybe because the signals were attenuated by the melting coax, hehe.
  • The coax has to be removed.  The problem with going with a higher power cable is that they start to have significant stiffness.
  • From what I could tell, there was no arcing inside the cavity, i.e. between the probe and wall, and although the magnetron did get warm, 34c max, meaning it was working, the circulator is working great.  The water used as the load in the circulator, even after all our test runs totaling probably four minutes, only got to 22c, three degrees over ambient.  There is about four litres of water total in the cooling system.

It will probably take me a while to find a better probe, but I should be up at least once next week to wire everything up.