PEP-II LLRF Tuners Panel

Overview

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

Loop Control - Turns the tuner loop ON/OFF. Before the tuner loop can be enabled, the tuners must be configured as instructed in the Station Configuration Procedure. Normally ON.
Loop Interval - Using a right mouse click, the tuner loop sample rate can be changed. The loop is presently configured for a 2 second sample rate. Altering the rate would most likely require changing the tuner loop gains and is not recommended (or deemed necessary) at this time. Nominal sample rate is 2 seconds.
Klys Frwd Pwr Limit (kW) - Sets the minimum klystron output power that the tuner loop will operate. Below this power level, the tuners will not move. Nominal value = 1 kW.
Min Beam Curr (mA) - Sets the minimum beam current required before the loading angle offset loop will operate. Nominal value = 300 mA.
Loading Angle Offset Control - Turns the loading angle offset loop option ON/OFF. Normally ON.
Park state: Go Home - Forces ALL the station's tuners to move to their park home positions. The position is defined by the "PARK Home" entry located on the Inp&Consts Tuner panel.
Park state: Set Home - Updates the park home position for each cavity in the station. This is done after a cavity or tuner assembly is replaced. Before activating, the station must be placed in PARK state with the correct "Desired PARK Frequency (kHz)" value entered for each cavity.
TUNE/ON state: Go Home - Forces ALL the station's tuners to move to their TUNE/ON home position. The position is defined by the "TUNE/ON Home" entry located on the Inp&Consts Tuner panel.
TUNE/ON state: Set Home - Updates the TUNE/ON home position for each cavity in the station. This is done automatically during tuner configuration which is a part of station configuration but can be done manually when there is no beam.
Avg Freq Offs (kHz) - The average of the individual cavity resonant frequencies estimated by the tuner polynomials. It is used to determine the phase offset required to keep the direct loop properly phased as the cavities detune. This feature is enabled by the "Freq Offs Tracking" ON/OFF button on the Direct Loop or Feedback panel. The average frequency offset is also used to direct the tuners to their park positions when the station is placed in the PARK state.

Controls and Monitors for Each Tuner

Fixed Offs (Deg) - Allows the user to place a constant phase offset into the tuner loop. Intended to be used as a diagnostic but not currently used. Normally 0.00 Deg.
Setpoint (%) - Set point for the loading angle offset loop. Determines the percent of the total station gap voltage a single cavity will contribute (once the "Min Beam Curr" limit has been reached). For example, in a 4-cavity station, each cavity will normally be set to 25%. If one cavity is prone to RF breakdown, its set point can be reduced to 22% and the remaining three raised to 26%. Important note: The sum of all the station cavity set points must equal 100%.
Position Control (mm) - Each slider indicates the present value of the tuner according to the software in the EPICS tuner driver. This value is not the potentiometer read back. User can turn OFF the tuner loop, and use a left mouse click or up/down keys to manually move the tuner. A right mouse click will display more options. To the left of each slider is the position read back from the linear potentiometer. It is not important that the two agree but will be calibrated during tuner configuration.
Temperatures - The present value of the temperature for the fixed tuner, movable tuner, and movable tuner bellows are displayed in degrees C.
Stepper Motor - Opens the Step Motor panel. On this panel, the stepper motor controller can be reinitialized by left mouse button click in the "Stop & Init" button. This also forces the controller software task control position to match the present potentiometer read back value.
Loop Status - Should normally read GOOD. Important - If it ever reads "DRV_LIMT", it is likely that the controller is instructing the tuner to move but nothing is happening. This condition can be caused by the stepper motor translator faulting. We have observed this condition on several occasions.
If the translator has faulted, a red LED fault light will appear on the unit which is located in the Tuner Driver Chassis. The translator can be reset (even while the station is operating) by removing the large round MS type connector on the rear of the Tuner Driver Chassis (this removes the 24 power to the unit) and then reconnecting. If this does not solve the "DRV_LIMT" error condition, the tuner assembly might have a broken shaft pin or some other mechanical problem. Before committing to an access, the station's crate processor should be rebooted once to eliminate the possibility of an obscure software glitch causing the problem.
Ld Angle Error (Deg) - Current value of the tuner loop phase error. Will turn yellow if the value exceeds the "Ld Angle Err Limit (Deg)" entry on the Inp&Consts Tuner panel. This should normally remain below the "Error Deadband (Deg)" parameter on the Inp&Consts Tuner panel. Nominal value for the deadband = 0.25 degrees.
Ld Angle Offs (Deg) - Current value of the phase offset created by the loading angle offset loop. If all cavities are set for equal "Setpoint (%)" (25% in a 4-cavity station) and there is sufficient beam in the ring, this value can be used to determined if the adjustable waveguide bellows are properly adjusted. The offset is reset to zero when the station goes to the OFF state.
Cav Strength (%) - Percent contribution the cavity is making to the total station gap voltage.
Freq Offset (kHz) - Estimated cavity resonant frequency based on the tuner polynomials. Typically the polynomials are accurate to ~10 kHz which has proven to be sufficient.
Diag Plots - Brings up an EPICS plot which displays the loop phase error, tuner position, and loading angle offset. A right mouse click in the plot window will allow auto scaling the plot to the current data.
Inp&Consts - Opens up the Inp&Consts Tuner panel where most of the detailed tuner parameters including the tuner polynomial coefficients are stored.