1Matthias LiepeAugust 2, 2007 LLRF for the ERL Matthias Liepe

2Matthias LiepeAugust 2, 2007 RF Field Control: Requirements The envisioned X-ray science will require a very energy-stable beam: –Bunch timing jitter  bunch length (100 fs) –Bunch to bunch energy spread  intra-bunch spread This translates into the following cavity field stability requirement: –Amplitude stability:  A / A  –Phase stability:    0.05 deg

3Matthias LiepeAugust 2, 2007 Challenges Field control with  f  bandwidth –Strong amplitude and phase perturbations! –Ponderomotive instabilities (from Lorentz-forces) –High Q L operation desirable to reduce RF power Beam current / phase fluctuations –Large currents need to compensate: 100 mA – 100 mA  small fluctuations cause large field perturbations! Solution: –Low microphonics levels (cryomodule design with vibration decoupling and damping, active frequency control) –Use fast control system to stabilize fields at high Q L  Run at very high loaded Q L  6.5  10 7  Use <5 kW of RF power to operate cavity

4Matthias LiepeAugust 2, 2007 ERL: Optimal Loaded Q and RF Power ERL:  No effective beam loading in main linac! (accelerated and decelerated beam compensate each other)  Only wall losses: some Watts  Run cavity at highest possible loaded Q But: The higher the loaded Q, the smaller the cavity bandwidth! Vibration Mode cavity field [arb. units] frequency [GHz] cavity field [arb. units] frequency [GHz] cavity field [arb. units] frequency [GHz] cavity field [arb. units] Frequency – 1.3 GHz [Hz] 13 Hz bandwidth cavity field [arb. units] frequency – 1.3 GHz [Hz] Lorentz-Force detuning:  f = K  E 2 = many bandwidths! cavity field [arb. units] frequency – 1.3 GHz [Hz] Add Microphonics !

5Matthias LiepeAugust 2, 2007 Need for low Microphonics Cavity and cryostat design for low microphonics Active frequency control (fast frequency tuner) What is a realistic estimate for the peak detuning?

6Matthias LiepeAugust 2, 2007 Measured Microphonics Levels Assume optimistic 10 Hz as typical detuning (< 20 Hz peak).  Q L =6.5  10 7 (adjustable coupler range: 2  10 7 to 1  10 8 )

7Matthias LiepeAugust 2, 2007 Peak Power (Q L =2· 10 7 … 1· 10 8 ) 5 kW gives sufficient overhead, and allows operation up to 20 MV/m (for  f<20 Hz) Required power [kW]

8Matthias LiepeAugust 2, 2007 Cavity Mechanical Frequencies Rring / Req=0.65 dof mode 1 mode 8 mode 5 mode 3 ring0.7*req0.4*req0.65*reqno ring ring-left0.65*req no ring ring-right0.75*req 0.65*reqno ring modefreq / Hz Courtesy E. Zaplatin

9Matthias LiepeAugust 2, 2007 Cavity /Module Design for low Microphonics Cavity design: –Low sensitivity to He- pressure changes –High mechanical vibration frequencies Module design: –High mechanical vibration frequencies –Decouple module from vibration sources 1 bar pressure Courtesy E. Zaplatin

10Matthias LiepeAugust 2, 2007 Main Linac Frequency Tuner Fast frequency tuning (piezo tuner) essential for active reduction of microphonics Selected blade tuner as baseline: –High stiffness –Piezos easy to integrate and can be places at ideal positions –Injector frequency tuner is prototype for main linac tuner –Microphonics compensation studies planned (horizontal test module) –Potential alternative: simplified blade tuner, Saclay III/IV

11Matthias LiepeAugust 2, 2007 RF Field Stabilization Measure cavity RF field. Derive new klystron drive signal to stabilize the cavity RF field. Derive new frequency control signal to keep cavity at design frequency. Frequency tuner

12Matthias LiepeAugust 2, 2007 One Cavity per IOT Plan on having one IOT per cavity: –Higher field stability –Vector sum control has risk of instability from Lorents-forces – Simpler high power RF distribution –Reliability (only would loose one cavity not several if IOT trips) –Higher efficiency –Can run each cavity at optimal gradient / flexibility

13Matthias LiepeAugust 2, 2007 Cornell’s RF Field Control System Fast digital components Low noise field detection Advanced and fast feedback and feedforward control loops Fast cavity frequency control (piezo cavity frequency tuner) Designed in house  Designed to deal with large amplitude and phase field perturbations  Prototype system operates in CESR since 2004 (first digital RF controls in a storage ring) Virtex II FPGA DSP

14Matthias LiepeAugust 2, 2007 Cornell high Q L Control Test at the TJNAF FEL JLab FEL Operated cavity at Q L =1.2·10 8 with 5 mA energy recovered beam. Had the following control loops active: PI loops for cavity field (I and Q component) Stepping motor feedback for frequency control Piezo tuner feedback for frequency control

15Matthias LiepeAugust 2, 2007 Q L Control Test: Cavity Ramp Up accelerating field [MV/m] time [sec]  150 Hz Lorentz-force detuning (compensated by piezo), cavity half bandwidth = 6 Hz ! 15 Start-up: Field Ramp at Q L = 1.2·10 8 With “old” JLAB system:  minutes time scale

16Matthias LiepeAugust 2, 2007 Q L Control Test: Cavity Ramp Up (II) time [sec] piezo drive signal [arb. units] Piezo drive signal to compensate Lorentz-force detuning cavity filling detuning [Hz] time [sec] Lorentz-force detuning without compensation: 150 Hz remaining microphonics cavity half bandwidth: 6 Hz

17Matthias LiepeAugust 2, phase [deg] time [sec] accelerating field [MV/m] time [sec] Without feedback: Highest Q L, highest Field Stability How high can one push Q L ?  Proof-of-principle experiment with ERL cavity Q L =1.2  10 8 (factor 6 above state of the art)  cavity bandwidth=12Hz (f=1.5GHz) Results:  Can operate SRF cavity at very high Q L and very good field stability at the same time!  Field stability surpasses Cornell ERL requirements  Very efficient cavity operation (some 100 W instead of kWs)

18Matthias LiepeAugust 2, 2007 Other Issues and R&D Items Radiation –Electronics located in SRF linac tunnel  gammas, neutrons from cavity field emission currents and beam loss Reliability –384 systems  need MTBF > year Cost –Reduction desirable

19Matthias LiepeAugust 2, 2007 Radiation Effects (1 Tunnel) Based on FLASH data: Can expect about 10 Gy = 1,000 rad per year in the tunnel from field emission currents at 16.2 MV/m in cw operation Beam loss will increase this further > 20 cm heavy concrete sufficient to shield LLRF electronics from gamma radiation Neutrons and resulting Single-Event Upsets (SEUs) are a potential problem and need further studies

20Matthias LiepeAugust 2, 2007 R&D Items For prove-of-principle: –Piezo R&D, demonstration of microphonics compensation: planned at HTC and injector module –Feedback with high beam current Final design –Strongly depends on digital technology available  finalize later –Reliability needs detailed studies, including radiation effects …