- Machinist training starting well, simulation results and power supply breakthroughs.
The first machinist class, covering the theoretical material, ended on December 7th and getting %96 in the class means I had fun! For example, we learned how to calculate tapers which will be immensely useful for future cavities. The practical part of the class starts in January and I look forward to it.
Neil has also been busy in Kelowna and came up with some great finds:
- He emailed the manufacturer of the Signal Hound and, among other tweaks, learned of “peak hold mode” making it capable of seeing the magnetron’s jumpy signal. This is great news!
- If you remember October’s update, we had contacted John Gerling at 2450mhz.com in order to get some specs and schematics for the ASTeX magnetron head which his company repairs. Given he had licensed that technology to ASTeX, he responded that he could not help us, but, Neil managed to find a service bulletin off his website that showed us something interesting – the ASTeX magnetron head uses an electromagnetic!!
Woohoo! We had suspected as much looking at the wires that led into the magnet, but having a diagram means we don’t have to guess.
On the cavity side, I have also been simulating like crazy in order to learn more about how modes are created. I have been working primarily with two types of cavities, a larger circular cavity at 1.5Ghz and another at 5.8Ghz.
Back when I started this research, I was looking for other people who had been working with the TE0,1 mode and found a paper, Energy Storage Cavities at the ALS, (actually a website at the time) written by two summer students at the Lawrence Berkeley National Laboratory. They built a simple round resonating chamber made from some scrap aluminum pipe with a diameter of 33cm and a length of 72cm where the TE0,1,5 mode resonated at 1.5Ghz. They were testing, completely unrelated to the EMdrive, to see if they could use the cavity as a type of energy storage mechanism for tuning purposes on a particle accelerator. Their results are useful because they published the size of cavity and the frequency the got the TE mode at, which I can duplicate with CST.
I have also learned more about CST Microwave Studio, for example, that I can use the Eigenmode solver to find modes! The Eigenmode solver is like a simpler version of the Frequency Domain solver I have been using before, but is perfectly suited for resonating structures. My old work flow use to be:
- Build a cavity with ports -> Solved with the Frequency solver over a large frequency range requiring a lot of samples (20 to 40 or more) -> Looked for the resonances on the S parameter chart -> Added field monitors at all the resonances -> Simulated -> Looked for the TE mode -> (and if necessary, re-simulate).
My work flow is now:
- Build the cavity with probes but without ports – > Simulate with the Eigenmode solver to find what the resonate frequencies are -> Find the TE modes and their frequencies -> Then add ports -> With a narrow frequency band centred on the TE mode , simulate with the Frequency Domain solver. By using a narrow frequency band, the Frequency domain solver takes much less time and it reduces the guess work, as there can be up to ten or more resonances in a single gigahertz spread.
The Eigenmode solver also shows me what a pure TE0,1 should look like and at what frequency, for example:
But I learned something else, thanks to “ifmicrowavescouldtalk.com” [link], a probe can act as a disturbance and split a mode:
“Now we break the symmetry of the structure with a little cylindrical indentation as shown below. Such a perturbation “splits” the eigenfrequencies of modes 1 and 2 from the original 16.83 GHz to 16.5 GHz and 16.87 GHz, as found by the eigensolver tool.”
Even if the Eigenmode solver shows that a cavity can resonate in a TE0,1 mode, the trick is to add a probe that will actually excite that mode.
I also learned from a powerpoint slide entitled “Introduction to RF Cavities for Accelerators” [PPT Link] by Dr. G Burt at the Lancaster University. When using a loop couple to the B-Field (Magnetic field) probe, as we are, the larger (and deeper) the loop, the lower the coupling. Interestingly, when using a straight probe, the higher the penetration, the stronger the coupling. I had run into this exact situation because when I reduced the size of the cavity to resonate at 5.8Ghz, I didn’t reduce the probe loop size and couldn’t figure out why the TE mode was so messed up. After reducing the size of the probe size in half, it works like a charm (Q of 32K).
I have also been studying the magnetic field configuration around the probe to verify that the reason a shorter loop probe has better coupling is because it properly encircles the magnetic field.
I have also been trying a variety of probe shapes and locations to increase the Q value:
I ordered up more probes mounts for the upcoming future test cavities and got ones that are threaded to make mounting easier:
These are SMA style connectors which are only for measurement of the test probe cavities. I will move back to N-style probe connectors when an actual high powered cavity is built.
Some of the latest discussions on the EMDrive are here which starts in March of 2011 and ended in December 3rd, 2011. Not much of note, except a link (that still works) to the Chinese paper.