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The front end of Los Alamos Neutron Science Center (LANSCE) linearaccelerator uses four 201.25-MHz Drift-Tube Linacs (DTLs) to accelerate the H+and H- beams to 100 MeV. Three of the 201.25-MHz DTLs are powered by diacrodesand the first DTL is powered by a tetrode. A 20-kW solid-state power amplifier(SSPA) is used to provide ~15 kW drive power to the tetrode. The SSPA iswater-cooled and consists of 24 push-pull LDMOS transistors operating at 45% oftheir power saturation capability, providing ample power headroom and excellentlinearity. However, the phase of the SSPA is perturbed at +/-20 degrees over afew ten minutes partially caused by the temperature dependent phase variationof the air-cooled SSPA driver circulator. This phase variation consumes most ofthe phase control margin of the cavity field feedback controller. In order tomitigate the effect of the SSPA phase variation on the cavity field, adisturbance observer controller (DOBC) has been designed and implemented on thecavity field control FPGA, which functions in parallel with the cavity fieldfeedback controller. In this paper, the DOBC design and its function as well asits short- and long-term performance are addressed.

Disturbance Observer Application for the Compensation of the Phase Drift of the LANSCE DTL LINAC Solid State Power Amplifier