Knowledge Bank

In the Flygare-Balle Fourier transform spectrometer a microwave pulse is applied to a Fabry-Perot cavlty. synchronized with an expanding jet of a gas mixture from a pulsed supersonic $nozzle.1$ Measurement of the Stark effect in this type of spectrometer has been hampered by the difficulty of producing an adequately homogeneous electric field without degrading spectrometer performance. The common approach to the production of a homogeneous electric field is to use large. parallel metal plates, spaced as closely as possible. This method works poorly in the present case because the plates disturb both the microwave field and the gas expansion. We have built a device which generates a good electric field (line broadening of the Stark shift) over a volume of $-8\times B\times 10\prime \prime$, but does not disturb the gas expansion significantly. It consists of two square Plexiglass frames $(13\times 13\prime /\prime o.d..11\times 11\prime \prime i.d.$ and $1\prime \prime$ thick), joined at the corners by four aluminum rods 2*** long. forming a structure similar to a box kite. Twelve 24 gauge wires are stretched 1*** apart along each of the four long faces. At one end, each pair of adjacent wires is connected by a 5 Mohn resistor and the four corner wires to external terminals. The orientation of the electric field is selected by the connections of the terminals to the HV power supply. The microwave field is unaffected by the assembly at frequencies above 10 GHz. tolerable at 9 GHz, but unusable below 8 GHz. Examples of performance will be presented. Modifications to allow operation at lower frequencies are under consideration.