10.112 TFBDRIVER—Driver for a turn-by-turn feedback loop

Driver for a turn-by-turn feedback loop
Parallel capable? : yes
GPU capable? : no
Back-tracking capable? : no

Driver for a turn-by-turn feedback loop

Driver for a turn-by-turn feedback loop

This element is used together with the TFBPICKUP element to simulate a digital turn-by-turn feedback system. Each TFBDRIVER element must have a unique identification string assigned to it using the ID parameter. The same identifier must be used on a TFBPICKUP element. This is the pickup from which the driver gets its signal. Each pickup may feed more than one driver, but a driver can use only one pickup.

A 30-term FIR filter can be defined using the A0 through A29 parameters. The output of the filter is simply ∑ _i=0²⁹a_iP_i, where P_i is the pickup filter output from i * U turns ago, where U is the UPDATE_INTERVAL value specified for the pickup. The output of the filter is optionally delayed by the number of update intervals given by the DELAY parameter.

To some extent, the DELAY is redundant. For example, the filter a₀ = 0,a₁ = 1 with a delay of 0 is equivalent to a₀ = 1,a₁ = 0 with a delay of 1. However, for long delays or delays combined with many-term filters, the DELAY feature must be used.

The output of the filter is multiplied by the STRENGTH parameter to get the kick to apply to the beam. The KICK_LIMIT parameter provides a very basic way to simulate saturation of the kicker output.

The plane that the TFBDRIVER kicks is determined by the PLANE parameter on the corresponding TFBPICKUP element, and additionally by the LONGITUDINAL parameter, as described in Table 3

Note: The OUTPUT_FILE will produce a file with missing data at the end of the buffer if the OUTPUT_INTERVAL parameter is not a divisor of the number of passes.

The FREQUENCY and PHASE parameters may be used to specify the resonant frequency of the driving cavity and its phase relative to the center of the bunch. If the frequency is not specified, the kicker is assumed to kick all particles in a bunch by the same amount.

For longitudinal feedback only, a more sophicated approach is available using a circuit model developed by T. Berenc (APS) may be employed to simulate driving the cavity resonance. To invoke this, the user must provide the loaded Q of the cavity using the QLOADED parameter, the (R_a∕Q) using RAOVERQ, and the resonant frequency of the unloaded cavity using FREQUENCY. Optionally, the drive frequency may be specified using DRIVE_FREQUENCY; it defaults to the unloaded resonant frequency.

Typically one should choose the resonant frequency to be (n±)f_b, where f_b is the bunch frequency and n is an integer. This will ensure that the kick to one bunch from the residual voltage from the previous bunch (both beam-loading and generator terms), is approximately minimized. Checking the ResidualVoltage column in the output file to confirm this is advised.

In addition to the resonant and drive frequencies, one must specify a clock frequency with CLOCK_FREQUENCY and a clock offset with CLOCK_OFFSET. The clock used used to determine when the drive current changes, which happens at regular intervals. The clock offset is used to ensure that the change does not occur during passage of the bunch. If the clock offset is too small and the bunch length too long, this will happen and results in an error. The phase shift that results from the clock offset is automatically compensated.

Beam loading is not included in the model, but can be superimposed by inserting an RFMODE element with matching parameters.

See Section 7.2.14 of Handbook of Accelerator Physics and Engineering (Chao and Tigner, eds.) for a discussion of feedback systems.