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

1Matthias Liepe August 2, 2007

LLRF for the ERLLLRF for the ERL

Matthias Liepe


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

-0.1 0 0.14995

5000

5005

Bunch timing jitter [psec]

Bun

ch E

nerg

y [M

eV]

•This translates into the following cavity field stability requirement:

– Amplitude stability: Amplitude stability: AA / A / A 10 10- 4- 4

– Phase stability: Phase stability: 0.05 deg 0.05 deg


Challenges• Field control with f bandwidth

– Strong amplitude and phase perturbations!– Ponderomotive instabilities (from Lorentz-forces)– High QL 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 QL

Run at very high loaded QL 6.5 107

Use <5 kW of RF power to operate cavity


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

0 0.2 0.4 0.6 0.8 1 1.2 1.40

0.2

0.4

0.6

0.8

1

cavi

ty f

ield

[ar

b. u

nit

s]

frequency [GHz]1.29 1.295 1.3 1.305 1.31

0

0.2

0.4

0.6

0.8

1

cavi

ty f

ield

[ar

b. u

nit

s]

frequency [GHz]1.2999 1.3 1.30010

0.2

0.4

0.6

0.8

1

cavi

ty f

ield

[ar

b. u

nit

s]

frequency [GHz]-1000 -500 0 500 1000

0

1

cavi

ty f

ield

[ar

b. u

nit

s]

Frequency – 1.3 GHz [Hz]

13 Hz bandwidth

0

1

-1000 -500 0 500 1000cavi

ty f

ield

[ar

b. u

nit

s]

frequency – 1.3 GHz [Hz]

Lorentz-Force detuning:f = KE2

= many bandwidths!

0

1

-1000 -500 0 500 1000

cavi

ty f

ield

[ar

b. u

nit

s]

frequency – 1.3 GHz [Hz]

Add Microphonics !


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?

106

107

108

1090

2

4

6

8

10R

F p

ower

[W]

QL

0 Hz10 Hz20 Hz30 Hz40 Hz


Measured Microphonics Levels

• Assume optimistic 10 Hz as typical detuning (< 20 Hz peak).

QL=6.5107 (adjustable coupler range: 2107 to 1108 )


Peak Power (QL=2· 10 7

… 1· 10 8)

• 5 kW gives sufficient overhead, and allows operation up to 20 MV/m (for f<20 Hz)

11

1

2

2

22

3

3

33

4

4

4

4

5

5

5

5

6

6

6

6

7

7

7

7

8

8

8

9

9

9

10

10

10

pe

ak d

etu

nin

g [

Hz]

gradient [MV/m]10 15 20 25

0

10

20

30

40

50

60Required

power [kW]


Cavity Mechanical Frequencies

Rring / Req=0.65

dof

mode 1

mode 8

mode 5

mode 3

ring 0.7*req 0.4*req 0.65*req no ringring-left 0.65*req 0.65*req 0.65*req no ringring-right 0.75*req 0.75*req 0.65*req no ring

mode freq / Hz freq / Hz freq / Hz freq / Hz1 131.03 85.34 115.15 54.622 131.04 85.33 115.15 54.623 315.52 191.3 268.39 133.344 315.52 191.3 268.39 133.345 409.83 250.67 344.89 195.906 459.51 294.12 456.26 226.607 549.51 294.13 456.27 226.608 549.51 394.77 456.85 319.34

Courtesy E. Zaplatin


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


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 Microphonics compensation studies planned

(horizontal test module)(horizontal test module)– Potential alternative: simplified blade tuner, Saclay

III/IV


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


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


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 houseDesigned 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 FPGADSP


Cornell high QL Control Test at the TJNAF FEL

JLab FELJLab FEL • Operated cavity at QL=1.2·108 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


QL Control Test: Cavity Ramp Up

0 0.2 0.4 0.6 0.8 10

5

10

acce

lera

ting

field

[MV

/m]

time [sec]

150 Hz Lorentz-force detuning (compensated by piezo), cavity half bandwidth = 6 Hz !

15Start-up: Field Ramp at QL = 1.2·108

With “old” JLAB system: minutes time scale


QL Control Test: Cavity Ramp Up (II)

0 0.2 0.4 0.6 0.8 1-5000

-4000

-3000

-2000

-1000

0

time [sec]

piez

o dr

ive

sign

al

[arb

. uni

ts]

Piezo drive signal to compensateLorentz-force detuning

cavity filling

0 0.2 0.4 0.6 0.8 1-30

-20

-10

0

10

20

detu

ning

[Hz]

time [sec]

Lorentz-force detuning without compensation: 150 Hz

remaining microphonicscavity half bandwidth: 6 Hz


0 1

-100

0

100

ph

ase

[d

eg]

time [sec]

0 10

5

10

ac

ce

lera

tin

g

fie

ld [

MV

/m]

time [sec]

Without feedback:

Highest QL, highest Field Stability

•How high can one push QL?

Proof-of-principle experiment with ERL cavity

•QL=1.2108 (factor 6 above state of the art) cavity bandwidth=12Hz (f=1.5GHz)

•Results:Can operate SRF cavity at

very high QL 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)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

time [sec]klys

tron

pow

er [

kW]

5.5 mA4 mA2.5 mA0 mA

5.0 mA recirculated beam beam takes 43 kW of

RF power and recovers 43 kW of

RF power!

0 112.1

12.2

12.3

12.4

time [sec]acc

ele

rati

ng

fi

eld

[M

V/m

]

0 199.5

1010.5

11

ph

ase

[d

eg]

time [sec]

A/A 1·10 - 4

0.02 deg


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


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


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 …

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

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