PEP-II LLRF Tuners Panel


Each RF cavity has a mechanical tuner to control the resonant frequency as temperature and loading conditions change.  The position of each tuner is controlled by a stepping motor.  A linear potentiometer mounted on the tuner assembly provides an absolute read back of the present tuner position.  Each tuner motor is driven by a Allen Bradley tuner motor controller (type 1746-HSTP1) driving a Superior Electric stepper motor translator (model SS2000MD4) housed in the tuner driver chassis.  This combination of hardware handles the basics such as limit switches and current limiting for the motors.  An EPICS driver was written for the motor controller, allowing a fully programmable tuner loop to be implemented.

The Tuners panel is accessed from the Main Station panel or the Feedback panel and provide controls for the following loops.

Main Tuner Loop

This loop compares the phase of the forward RF being delivered to the cavity with the phase of the RF measured via the cavity probe (after the appropriate phase offsets are taken into account) and moves the tuner to keep this phase error nulled.

Loading Angle Offset Loop

This loop adds a phase offset to the normal tuner loop phase error signal based on the voltage distribution observed on the cavities.  Each cavity is assigned a setpoint in percent, representing the portion of the total station gap voltage it is expected to produce.  Once the beam current reaches the preset minimum ("Min Beam Curr (mA)" - nominally 300 mA), the loop will slowly add a phase offset to the main tuner loop to achieve the desired cavity contribution.  This is an extremely useful feature, allowing the gap voltage on any cavity to be reduced to help prevent faults.  This loop also compensates for longitudinal misalignment of the cavities and can easily correct for 10 degree alignment errors.

Cavity Parking Loop

When the station is set to the PARK state, this loop will set the tuner resonant frequency, estimated by a polynomial, to the "Desired PARK Frequency (kHz)" located on the Inp&Consts (input and constants) Tuner panel.

Controls and Monitors for All Tuners in the Station

Controls and Monitors for Each Tuner