COAXIAL CAVITIES

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OPTHOS coaxial cavities are capable of exciting microwave discharges in a wide variety of gases.  RF input is through a standard Type-N female connector.  Lamp tubing up to 13 mm in diameter can be accommodated.  A separate entrance tube is provided for air cooling of lamp and cavity.

Cavities are normally connected to a microwave power supply by a coaxial cable.  After RF power is applied, the discharge is initiated by a brief spark from a high frequency coil.  The cavity is then tuned to optimum operating conditions with the aid of a bi-directional RF power meter to monitor the forward and reflected power levels.  A power meter of this type is standard on all Opthos microwave generators. 

The selection of a cavity will depend on the type of discharge to be excited.  Opthos manufactures the following three types of cavities.

Evenson Cavity  As described by Fehsenfeld, Evenson, and Broida, Rev. Sci. Instrum. 36, 294 (1965), this cavity can excite discharges in both static and flowing gases at pressures ranging from a few milli-Torr to several hundred Torr.  Two tuning adjustments are provided: a threaded tuning stub projecting into the cavity from the top and a three-part tuning handle that slides along the center conductor of the RF input connector.  The central section of the tuning handle is made of ceramic to withstand high temperatures.  The discharge can be viewed through a slotted hole in a removable cap at the bottom of the cavity.  The removable cap allows the cavity to be installed on existing gas systems.

McCarroll Cavity  This is a modified version of the Evenson cavity incorporating tubular side arms that act as wave guides.  As described by McCarroll, Rev. Sci. Instrum. 41, 279(1970), the presence of these side arms improves the coupling of RF power to the discharge and reduces the tendency for arcing between the tuning stub and sliding handle at optimum tuning conditions.  This cavity is most commonly used in work with flowing gas systems.  Like the Evenson cavity, it has a removable cap with slotted viewing port for use on existing systems.

Cylindrical Cavity  Often referred to as a Broida cavity, this cavity provides a more uniform, less concentrated excitation than the Evenson or McCarroll cavities.  The design is also from Fehsenfeld, Evenson, and Broida.  It is excellent for general applications, such as the excitation of metallic-hallide lamps, mercury lamps, and rare-gas lamps.  There are no tuning adjustments.  After the discharge is initiated the lamp is moved around in the cavity to minimize the reflected power.  The discharge can be viewed through a circular/slotted port near the top of the cavity.

Frequency:  2450 MHz

Permissible lamp size:    Up to 13 mm in diameter

Input connector:  Type N; female

Materials:    Brass body, plated with silver and overcoated with rhodium

Tuning:   Evenson and McCarroll cavities - threaded tuning stub and sliding tuning handle

                   Cylindrical cavity - no tuning adjustments

Cooling:   Inlet tube for air cooling on all cavities

Body Diameter:    Evenson and McCarroll cavities - 25 mm

                                  Cylindrical cavity - 29 mm

Body length:    Evenson and McCarroll cavities - 56 mm (not including central tuning stub; about

                             80 mm with tuning stub)

                             Cylindrical cavity:    130 mm

McCarroll side arms:    149 mm end to end

Shipping Weight:    2 pounds