Abstract
A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of ∼100 µs. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section—achieved with a tapered impedance transformer—placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or ∼40 kW.
Highlights
The multipactor phenomenon may occur when the mean free path of an electron within a structure is larger than the distance between two surfaces, in this case the top and bottom of a rectangular waveguide operated in its fundamental mode, and the movement of electrons is synchronized with the RF signal.1,2 Conditions for the multipactor effect are fairly specific, but space based systems are susceptible.3 In addition to physical damage to RF circuits,4 significant signal degradation is a possible result of multipactor
An RF source capable of at least 2 kW in the S-band was described with considerations necessary for reliability and protection of the relatively delicate Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT)
Offering high power density and efficiency compared to traditional vacuum tube systems and higher output power than Laterally Diffused Metal Oxide Semiconductor (LDMOS) or Gallium Arsenide (GaAs), the pitfalls of depletion mode HEMTs—i.e., biasing/sequencing requirements and avalanche susceptibility—are evident
Summary
The multipactor phenomenon may occur when the mean free path of an electron within a structure is larger than the distance between two surfaces, in this case the top and bottom of a rectangular waveguide operated in its fundamental mode, and the movement of electrons is synchronized with the RF signal. Conditions for the multipactor effect are fairly specific, but space based systems are susceptible. In addition to physical damage to RF circuits, significant signal degradation is a possible result of multipactor. The multipactor phenomenon may occur when the mean free path of an electron within a structure is larger than the distance between two surfaces, in this case the top and bottom of a rectangular waveguide operated in its fundamental mode, and the movement of electrons is synchronized with the RF signal.. Through simulation and the literature, it was found to require a certain level of power, starting at ∼2 kW for a 2 mm gap, to initiate the multipactor event through secondary electron emission in copper with more power required for larger gaps. This is primarily driven by the secondary electron yield (SEY) and where the first crossover point (SEY > 1) occurs. The balance of field and phase must be correct in order to achieve multipactor, generally of an odd order, N
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