Performing Accurate Multi-Paction Measurements
- Sunday, 01 August 2010
Multi-paction effects can impact microwave components in high-power satellite communications (sat-com) systems. The nonlinear break-down-voltage phenomenon occurs in high-vacuum environments above a certain threshold voltage, and can degrade the performance of RF and microwave components or, in extreme cases, damage the components or the system. Although multi-paction effects are often difficult to predict and measure, properly equipped test systems with dedicated software can accurately identify microwave components that may multi-pact, effectively screening them to avoid damage in a deep-space application.
Performance degradation or damage due to multi-paction processes is associated with waveguide components used in deep-space applications, occurring at high power levels in a vacuum environment. Multi-paction phenomena can affect both waveguide and coaxial passive components in the transmit signal chain, such as diplexers, filters, multi-couplers, and antennas, as well as the coaxial cables and connectors used to link these components.
For a multi-paction discharge to take place, three ingredients are needed: an RF source, free electrons, and a vacuum. The process occurs when a charged particle (free electron) inside a gap (such as a waveguide component) in a vacuum environment, or one with relative low atmospheric pressure, oscillates under the influence of a large external electric field (the high-power RF source, such as a transmit amplifier). Every time these accelerated charged particles strike the gap wall, they release secondary charged particles. As the process is repeated millions of times per second, the number of electrons multiplies rapidly, leading to a multi-paction discharge. The discharge itself may absorb or release little power, but can cause increased outgassing in the component or system, which can lead to a gas discharge. A gas or corona discharge in a space system can release a large amount of power, thus causing component performance degradation or even system failure.1
Multi-paction occurs when the ambient pressure is sufficiently low that the electron mean free path is longer than the electron separation distance. Multi-paction effects have been observed at pressures less than 10-2 Torr. A vacuum environment is required for a multi-paction discharge since at atmospheric pressure, the charged particles are more likely to collide with air particles, reducing the velocity of the charged particles as well as their potential to release secondary charged particles. Waveguide components are often pressurized for this reason to avoid the effects of multi-paction at high RF power levels.
Mathematically, multi-paction events are understood as a function of the input power to a component, according to Larmor’s formula:
P = 2(e2a2)/[3(4πε0c3)]
where P = the power generated by an accelerated charge, e = the charge of an electron, ε0 = the dielectric constant, a = the acceleration of the charge, and c = the speed of light in a vacuum, or 3 × 108 m/s.
The role of a circuit’s dielectric constant, ε0, can also be seen in this formula, where higher values of dielectric constant will result in reduced power for the accelerated charge. Similarly, the multi-paction threshold voltage, V0, can be found from the equation:
V0 = (2πd/λ)2 [mec2/πe]
where V0 = the acceleration voltage between charged surfaces, me = the mass of an electron, λ = the wavelength, and d = the spacing between surfaces.
The dimensions of an RF circuit also play a considerable role in the occurrence of multi-paction effects. Since multi-paction effects are essentially arcing of electrons across a narrow gap, sharp or jagged edges within an RF/microwave component can provide the geometry for bunching of free electrons in high concentrations, leading to a discharge event.2 Conversely, rounded metal surfaces and thin dielectric coatings within a component can lessen the possibility of a multipaction discharge even within a high vacuum at high RF power levels.