Microwave Tuning Apparatus
Christopher Ploch
Completed Tuning Apparatus
Tuning device and plasma.
Summary
This project addresses the problem of designing and fabricating a cheap, replaceable microwave
tuning apparatus in order to continue Dr. Remillard’s research of studying the microwave
breakdown of gases. The purpose of this research is to study and model how microwave induced
electric fields cause gases to break down into plasma; a small cavity containing a functional rod
and tuner is necessary to provide an adjustable gap for the gas to break down in. The plasma is
generated in the gap between the end of the rod and the circular plate attached to the tuner. The
cavity that has been used in the past now needs to be replaced because the tuner has seized up
inside of its bushing, which, in turn, cannot be removed from the cavity. The focus of this project
is designing the microwave tuning device inside the cavity.
One need for the new tuner is that there must be a good way of removing it so that it can be
replaced in the future. It must be cheap to fabricate and be less expensive than the custom made
tuner originally used. The tuning sensitivity should be low, and should be comparable to that of
the original tuner. It is important that the tuner is adjustable so that gap size can be changed and
it must allow small changes to be made accurately. In order to make the research as repeatable as
possible, it will also be important to have an accurate way of setting the tuner to the desired gap
size every time. Other needs for the device include that it have a way of locking a specific gap
size in place, that it is able to keep the surfaces of the tuner and the rod parallel to each other, and
that it is durable so that the problem of the tuner seizing up does not occur for a long time.
Several different concepts were considered to solve this problem. In the first concept, the tuner
consisted of a finely threaded rod resting inside of a bushing, which can easily be removed from
the cavity due to its coarser threading. In the second concept, the rod is threaded instead of the
tuner, and it sits in a bushing as well. In the third concept, the entire cavity is placed on an
optical mount that allows it to translate forward and backward. The tuner is fixed. In the fourth
concept, the tuner is attached to a dial indicator, which allows the gap to be easily measured. In
the fifth concept, the gap is measured by using a micrometer. A portion of the micrometer would
be removed when experiments were taking place. In the sixth concept, the tuner is attached to a
rotary motion vacuum feedthrough. The gap is controlled using micrometer controls located
outside of the vacuum chamber. The seventh concept uses a pneumatic rotary motion vacuum
feedthrough. In this concept, the gap can be adjusted using a controller.
The first concept was chosen as the final design for this project due to simplicity, low cost, and
reliability. A very finely threaded cap screw with a circular plate attached at the end was used as
the tuner. By rotating this screw inside of the bushing, forward and backward translation was
possible. The gap consists of the space between the tuner plate and the stationary rod.
Once fabricated, this design performed adequately and met the requirements. The most basic and
important requirement was that the device be able to achieve microwave breakdown and
generate a plasma discharge, and it was able to do this. It provided the gap size range desired and
was able to be used easily and accurately. It was also much cheaper than the custom made tuner.
The tuning sensitivity of the new tuner was compared to that of the old tuner in order to
determine if they had similar functioning. While not quite as low as the tuning sensitivity of the
old tuner, the sensitivity of the new tuner was still good enough to be used in actual research.