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The two Voyager spacecraft use traveling wave tube amplifiers to send data back to us. These have 2 power settings: 12W and 18W. The power supply on board the Voyagers (an RTG) produces less power each year.
I'm trying to figure out what will happen when the available power drops below the level where the RTG can supply 12 W to the transmitter. Could this type of amplifier be driven at lower power levels, or is this an all-or-nothing proposition?
A TWTA uses a helix wound around the electron beam. The microwave signal applied to the helix interacts with with the electron beam and modulates the electron density. For good interaction the traveling wave on the helix and the electrons within the beam should have similar speed. If the acceleration voltage of the beam is too low, the electrons are too slow for the interaction with traveling wave.
So if there is too few power available to accelerate the electron beam, the amplification of the tube decreases to zero.
