LINAC 4 – Control System and Adaptive Feedforward Design

Anirban Krishna BhattacharyyaBE – RF – FB

Introduction

The Control Loops

Introduction

Total loop delay of 1100 ns.

Start up strategy

100 μsec are allowed for Low Level RF (LLRF) loops stabilization. The sequence leading to beam injection is as follows:

• Filling of cavity open-loop using feed forward set point (SPFF ) for 50 μsec. This set point can be computed from the saturation power of the klystron and is given by

For the first 10 μsec of the process the feed forward set point value is ramped from 0 to SPFF .•The feed back is then switched on and for 12 μsec the loop gains Kp and Ki are ramped to the desired values. The job of the controller is thus to follow the cavityvoltage set point by correcting for the error produced by the feed forward set point.•After 38 μsec the beam is injected.

PsatSPFF=

Vcav

1mW

PIMS Cavity: ZTT = 26e6 MΩ/m L = 1.79 m Q0 = 17000 QL = 7100 Φ = -20° Z0 = 50 Ω Psat = 1.4 MW

Beam current: 40 mACavity voltage: 7.00542 MVPower loss: 5%

Parameters

Controller structure: , where, , and, 1+aτs

KP

1+τsΤ =

KP

KI

a = 10

Smith-Predictor Design

Process model estimation:

1. Cavity is narrow band compared to all other loop components.

2. Cavity has only single resonance.3. Other loop components only contribute to gain.4. 10% error in knowledge of cavity parameters and delay.

Requirements:

1. Model for process/plant.2. Estimation of time-delay

What is this?

Results: Single Resonance Cavity

KP = 30KI = 2.73e6

Results: Single Resonance Cavity

0.5337°

Results: Single Resonance Cavity

PIMS Cavity Model (Experimental)

Notch Filter for PIMS Cavity

Effect of Notch

Nyquist Plot

KP = 30KI = 2.73e6

Comparison of Cavity models

Results: PIMS Model

0.5926°

Results: PIMS Model

Conclusions

• Error for beam injection (40 mA) is 1.4% in Voltage and 0.5926° in phase. At steady state they reduce to 0.1913% in voltage and 0.1333° in phase.

• Parasitic resonance of PIMS can be compensated by notch filter.

• This is feedback alone, with Smith-Predictor. The Adaptive Feedforward should further improve on this, particularly in the case of Klystron ripple, which is reproducible from shot to shot.

• Klystron is operating very close to saturation with 40 mA beam current, 7.0054 MV cavity voltage and QL = 7100.

Smith Predictor

Controller Plant

Delay

1+GCGe-sτd

GCGe-sτd

Transfer function =>