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LINAC 4 – Control System and Adaptive Feedforward Design

LINAC 4 – Control System and Adaptive Feedforward Design. Anirban Krishna Bhattacharyya BE – RF – FB. Introduction. The Control Loops. Introduction. Total loop delay of 1100 ns. Start up strategy.

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LINAC 4 – Control System and Adaptive Feedforward Design

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  1. LINAC 4 – Control System and Adaptive Feedforward Design Anirban Krishna Bhattacharyya BE – RF – FB

  2. Introduction The Control Loops

  3. Introduction Total loop delay of 1100 ns.

  4. 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 μsecthe loop gains Kp and Ki are ramped to the desired values. The job of the controller is thus to follow the cavity • voltage set point by correcting for the error produced by the feed forward set point. • After 38 μsec the beam is injected. Vcav SPFF= Psat 1mW

  5. Parameters 1+τs 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 mA Cavity voltage: 7.00542 MV Power loss: 5% KP 1+aτs KP Τ = KI Controller structure: , where, , and, a = 10

  6. Smith-Predictor Design What is this? Requirements: Model for process/plant. Estimation of time-delay Process model estimation: Cavity is narrow band compared to all other loop components. Cavity has only single resonance. Other loop components only contribute to gain. 10% error in knowledge of cavity parameters and delay.

  7. Results: Single Resonance Cavity KP = 30 KI = 2.73e6

  8. Results: Single Resonance Cavity 0.5337°

  9. Results: Single Resonance Cavity

  10. PIMS Cavity Model (Experimental)

  11. Notch Filter for PIMS Cavity

  12. Effect of Notch

  13. Nyquist Plot KP = 30 KI = 2.73e6

  14. Comparison of Cavity models

  15. Results: PIMS Model 0.5926°

  16. Results: PIMS Model

  17. 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.

  18. Smith Predictor Delay τd τd 1+GCGe-s GCGe-s Transfer function => Controller Plant

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