Helix current faults are not necessarily true indications of high helix current. You must first understand what helix current is and how it’s measured.
True helix current is a function of how well the TWT electron beam is focused. The helix (or slow wave structure) is a long coil of wire or wire ribbon that goes from the RF Input of the TWT to the RF Output. The Cathode to Helix Voltage determines the velocity of the electron beam. As the electron beam traverses the length of the tube and through the middle of the Helix, the electron beam migrates from a straight path down the center of the tube and helix toward the Helix on its way to the Collector. The electrons see the Helix as a High Plus (+) Potential. As electrons “brush” the center of the wire coil, they find their way back to Cathode Supply Plus (+) via the chassis as the helix is at chassis ground potential. HPA manufacturers will put a resistor in series with this path between Chassis Ground and Power Supply Return (+). [For those of you that don’t know...electrons come from the Negative (-) end of a D.C. (direct current) power supply (or battery).] When the electrons pass through the series resistor between chassis and Power Supply Return (+), a Voltage is developed across the Load Resistor. This voltage can be used for metering and the Helix Current Fault Circuit. If the Current goes up to high, the voltage across the “Sense Resistor” goes up as well. This voltage, if it goes too high will trigger a “Helix I” fault. Some HPAs call it a Helix Run Fault and still others may refer to it as a Body I fault.
Any time electrons go from Cathode Supply Negative (-) to Cathode Supply Positive (+) via the Chassis, the HPA will register Helix Current. False Helix Current Trips may be the result of an Arc between the Cathode Negative and Chassis Ground. In some circumstances, Components or wiring in the High Voltage Power Supply Circuit may fail and the the result will be a Helix Current Fault. The Fault Circuit doesn’t know where the electrons are coming from...just that the voltage across the sense resistor has gone too high!
