The harmonic gyroklystron' (HGK) is capable of generating kilowatts of average power at millimeter wavelengths. Operatingthe device at a harmonic (n > 1) of the cyclotron frequency reduces the required magnetic field to levels attainable by normalsolenoids. Size and weight of the device are reduced with a normal solenoid. An advanced centerpost (ACP) gun is usedto generate the hollow axis-encircling beam. Computer simulations predict very low velocity and guiding center spreads inthe beam. A 95 GHz fourth harmonic HGK design simulation predicts 30% overall efficiency and > 2kW average power.The electron gun will be tested in a beamstick in which the rf circuit is replaced by segmented capacitive probes.