Microwave cavity-The Daily Ledger

A microwave cavity or radio frequency cavity (RF cavity) is a special type of resonator, consisting of a closed (or largely closed) metal structure that confines electromagnetic fields in the microwave or RF region of the spectrum. The structure is either hollow or filled with dielectric material. The microwaves bounce back and forth between the walls of the cavity. At the cavity's resonant frequencies they reinforce to form standing waves in the cavity. Therefore, the cavity functions similarly to an organ pipe or sound box in a musical instrument, oscillating preferentially at a series of frequencies, its resonant frequencies. Thus it can act as a bandpass filter, allowing microwaves of a particular frequency to pass while blocking microwaves at nearby frequencies.

A microwave cavity acts similarly to a resonant circuit with extremely low loss at its frequency of operation, resulting in quality factors (Q factors) up to the order of 106, for copper cavities, compared to 102 for circuits made with separate inductors and capacitors at the same frequency. For superconducting cavities, quality factors up to the order of 1010 are possible. They are used in place of resonant circuits at microwave frequencies, since at these frequencies discrete resonant circuits cannot be built because the values of inductance and capacitance needed are too low. They are used in oscillators and transmitters to create microwave signals, and as filters to separate a signal at a given frequency from other signals, in equipment such as radar equipment, microwave relay stations, satellite communications, and microwave ovens.

Microwave cavity

RF cavities can also manipulate charged particles passing through them by application of acceleration voltage and are thus used in particle accelerators and microwave vacuum tubes such as klystrons and magnetrons.

Microwave cavity

Theory of operation

Most resonant cavities are made from closed (or short-circuited) sections of waveguide or high-permittivity dielectric material (see dielectric resonator). Electric and magnetic energy is stored in the cavity and the only losses are due to finite conductivity of cavity walls and dielectric losses of material filling the cavity. Every cavity has numerous resonant frequencies that correspond to electromagnetic field modes satisfying necessary boundary conditions on the walls of the cavity. Because of these boundary conditions that must be satisfied at resonance (tangential electric fields must be zero at cavity walls), at resonance, cavity dimensions must satisfy particular values. Depending on the resonance transverse mode, transverse cavity dimensions may be constrained to expressions related to geometric functions, or to zeros of Bessel functions or their derivatives (see below), depending on the symmetry properties of the cavity's shape. Alternately it follows that cavity length must be an integer multiple of half-wavelength at resonance. In this case, a resonant cavity can be thought of as a waveguide equivalent of short circuited half-wavelength transmission line resonator.

Microwave cavity

The external dimensions of a cavity can be made considerably smaller at its lowest frequency mode by loading the cavity with either capacitive or inductive elements. Loaded cavities usually have lower symmetries and compromise certain performance indicators, such as the best Q factor. As examples, the reentrant cavity and helix resonator are capacitive and inductive loaded cavities, respectively.

Loaded microwave cavities

A microwave cavity has a fundamental mode, which exhibits the lowest resonant frequency of all possible resonant modes. For example, the fundamental mode of a cylindrical cavity is the TM010 mode. For certain applications, there is motivation to reduce the dimensions of the cavity. This can be done by using a loaded cavity, where a capacitive or an inductive load is integrated in the cavity's structure.

The precise resonant frequency of a loaded cavity must be calculated using finite element methods for Maxwell's equations with boundary conditions.

Loaded cavities are particularly suited for accelerating low velocity charged particles. This application for many types of loaded cavities, Some common types are listed below.