LIU meeting 1

LLRF FOR THE SPS 800 MHZ CAVITIES

P. Baudrenghien, G. Hagmann

4/4/2012

LIU meeting 2

Motivation for upgrade• The present system has only amplitude/phase control with low

bandwidth (measurement in the center of the beam batch). It cannot control transient beam loading

• The new system will include 1-T feedback, feedforward, longitudinal damper (dipole and quadrupole – if needed), longitudinal blow-up and built-in observation and post-mortem

• The design is much inspired by the LHC 400 MHz LLRF. It profits from synergy with the ongoing 352.2 MHz LLRF design for Linac4

• Before designing, we will develop a detailed model (including cavity response, transmitter nonlinearities,…) to predict the influence of technical specifications on beam stability. A similar exercise was done for the LHC by the LARP collaboration (SLAC). We wish to involve this collaboration in the SPS upgrade.

4/4/2012

LIU meeting 3

Retrospective• The first design to control transient beam loading (and improve the

threshold for longitudinal coupled-bunch instability) in a TWC cavity (SPS 200 MHz) was made by D. Boussard in 1985 with G. Lambert on charge of the electronics [1]. They named their design One-Turn Delay Feedback.

• Their system used the signals from combiners of all antennas of each cavity (with delays corresponding to the particle time of flight), summing on all four cavities and acting back on the drive of two cavities. We had two 4-sections and two 5-sections cavities. The drive was sent to one cavity of each type with delay/phase balancing. The bandwidth was limited to < 1 MHz.

4/4/2012

LIU meeting 4

• When preparing the SPS as injector into the LHC, improved impedance reduction was needed and I inherited the project of upgrading the design, with the following requirements• One system per cavity• Increased bandwidth• Addition of a feed-forward

• The LLRF was designed with more than 10 MHz BW… but first tests with the TXs in 2000 showed that the power limitations had not been properly included in the design [2]. The non-linear phase characteristic of the TX, out of band, and the non-linearity, actually localized in the Class A solid state drivers much reduced the usable BW. Others have done similar errors: The PEP II impedance control was effectively limited by the non-linearity of the klystron drivers.

4/4/2012

LIU meeting 5

• For the LHC we have attempted to integrate the LLRF design in its environment from the start

• Three questions are essential:• How much is the beam affected by the LLRF technical choices? Imperfections result

in poor transient beam loading compensation, longitudinal stability issues and RF noise driven emittance blow-up

• What is the effect of the High Level imperfections? The non-linearity and frequency response of the power chain must be considered from the start

• What is the importance of imperfections in the LLRF on the overall performances Typical imperfections are misalignments (slightly offset phase of an RF feedback for example) or noise figure of the various components.

• An answer can only come from a detailed model of the RF chain. That was done for the LHC in a LARP collaboration with SLAC. Key players had designed and operated the much similar PEPII cavity controllers (J. Fox et al.)

4/4/2012

LIU meeting 64/4/2012

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-120

-110

-100

-90

-80

Frequency (MHz)

Gai

n (d

B)

GAIN/PHASE Modulator Gain = -7 Modulator Phase = 80

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-600

-400

-200

0

200

Frequency (MHz)

Pha

se (d

egre

es)

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-120

-110

-100

-90

-80

Frequency (MHz)

Gai

n (d

B)

GAIN/PHASE Modulator Gain = -7 Modulator Phase = 85

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-600

-400

-200

0

200

Frequency (MHz)

Pha

se (d

egre

es)

• This resulted in identifying the few key elements in the LLRF that were critical to limit RF noise, and the sensitivity of beam stability to misalignment in the LLRF parameters [3]

• For example, it was found that a 5 degrees offset in the RF feedback phase severely distorts the flat response of the closed loop feedback, resulting in a four-fold increase in growth rate of the most unstable coupled-bunch mode driven by the LHC cavity impedance at the fundamental (opposite) [4]

LIU meeting 7

A detailed model for the 800 MHz• We wish to orient the LARP collaboration on the development of a

detailed model of the SPS 200-800 MHz RF• It would include a detailed description of the hardware: Non-linearity

(mainly High Level part), noise (both low and high level), misalignments

• Our SLAC colleagues are interested• This would serve as a guidance when making technical choices in the

LLRF design• We need backing for the LARP meeting.

4/4/2012

LIU meeting 8

Basic functionalities • The key features of the SPS 800 MHz Cavity Controller are

• A strong RF feedback to reduce impedance at the fundamental (transient beam loading and stability issue). Given the loop delay in the SPS installation (TX on the surface), this can only be a One-Turn Delay feedback

• A feedforward, generating a TX drive from a beam current measurement (PU) further reducing the beam induced voltage

• A local loop around the TX to make the outer feedback insensitive to the TX gain and phase changes, and to locally reduce the noise produced by the high Level (called klystron polar loop in the LHC and Linac4 designs)

• A channel to modulate the cavity field in phase and amplitude, as actuator for a longitudinal feedback (dipole and quadrupole mode respectively)

• A similar channel to implement longitudinal blow-up at 800 MHz (either via phase or amplitude excitation)

• A facility to remotely measure open-loop and closed-loop response for remote setting-up• Built-in observation memories for diagnostics and post-mortem

• Unique to the SPS 800 MHz design• No need for tuning (TWC cavity)• Requirement to keep phase alignment with the voltage measured in the sum of the four

200 MHz cavities. This system is considered as the master. The SPS RF is not democratic!

4/4/2012

LIU meeting 9

Block diagram4/4/2012

LIU meeting 10

Travelling wave cavity • The TX can influence the accelerating field through the main coupler only,

while the beam excites the field in each cell, thereby generating a travelling wave that propagates through the structure

• The Beam loading impedance is therefore different from the Generator Induced impedance and at some frequencies beam loading compensation is impossible [5]

4/4/2012

Left: Forward transfer impedance Zrf as a function of deviation from the centre frequency.Right: Beam transfer impedance Zb as a function of deviation from centre frequency (Top = real part, bottom = imaginary part). Above plots correspond to the 4 sections 200 MHz cavities-

v

v

v

L

jRLRZ

LV

g

g

1

sin2

2

2sin

82

2sin

2 2

2

22

20

LIU meeting 11

800 MHz TX-cavity chain• Cavity

parameters

• Cavity response first zeros at +-3.145 MHz

• TX response: - 3 dB at +- 1 MHz

• We aim at +- 6 MHz for the feedback.

4/4/2012

Centre freq. 800.888 MHz

Phase advance per cell p/2

Group velocity vg/c +0.035

Cell length 93.5 mm

Total length L (37 cells) 3.460 m

Series impedance R2 0.647 MW/m

LIU meeting 12

OTFB response [6]• The One-Turn feedback response has peaks at n f rev This response is easily

synthesized if the sampling clock is a multiple of the revolution frequency

• The sign inversions due to the cavity response occur at fixed frequencies (independent of frev). These 180 degrees phase inversions must be compensated in the RF feedback to avoid Close-Loop instability. This is done by inserting a filtering that mimics the cavity zeros, the synthesized cavity

• We use a sampling clock locked to the revolution frequency and must therefore shift the synthesized cavity response during the acceleration ramp (demodulation-filtering-modulation).

4/4/2012

LIU meeting 13

Planning• The cavities have been equipped with one antenna in every two cells• The summing network is being designed (help from D. Valuch). We

will have a prototype before end p run 2012• In the meantime the electronics is being designed with simplified

functionalities (no feed-forward). It will be tested with beam in second half 2012, using center probe only (and fixed frequency?)

• The design will be modified according to these first results and completed during LS1

4/4/2012

Current 800MHz System (NIM)

LL Cavity 1 LL Cavity 2LL Common

4/4/2012 LIU meeting 14

New 800MHz System (VME)

LL Cavity 1 LL Cavity 2

4/4/2012 LIU meeting 15

New 800MHz System (VME)

RFFGCMM Switch&LimitCavity Loops 200MHZ

Quadrupler

Clock Distributor

Linux FrontEnd CTRV

VM

E b

us

(A2

4D

16)

RF

Lo

wL

eve

l B

ac

kpla

ne

4/4/2012 LIU meeting 16

VME Cards

Switch & Limit Clock Distributor

RF design & FPGA (Controls, Acquisitions,…) on the same board

4/4/2012 LIU meeting 17

Needs for one 800MHz Cavity :

Module Name Status

Linux Front-end (CPU) Installed

CTRV (timing) Installed

CMM (Crate Management) Installed

WBS (Wide Band Switch) LHC Spares available, need new series production (only needed for ions operation)

RFFG (Function generator) PCB under design, V1 mid-June 2012

Switch&Limit Proto V2 under test

Clock Distributor Proto V2 under test

200MHz Quadrupler Proto in production, V1 mid-May 2012

Cavity Loops (RF Feedback) PCB Under design, V1 August 2012

Veto Sum Under specification

VME Cards

4/4/2012 LIU meeting 18

Digital filtering & IQ demodulationTWC 800 MHz pLHC Frequencies :

PLL : K=2, M=31, N=8Frf200 = 199.943MHz -> 200.395MHz∆Frf200 = 452KHz∆Frf800 ≈ 1.8MHzLO = M/N * Frf200 ≈ 775 MHzFs = Frf200 / K ≈ 100 MHzFif = Fs/4 ≈ 25 MHz

Fs

4/4/2012 LIU meeting 19

Fine delay(1 Turn)

RF Function Receiver(Voltage/Phase offset Setpoint)

Longitudinal Damper(From Beam Control, Optical Gbit link)

Ref Phase from 200MHz Cavity Σ

Comb filter(Beam synchronous

clock)

Cavity filter(Absolute Cavity

response)

Feed forward(Beam synch clock)

Polar Loop

In-situ Observation

&BBNANoise

(Blow-up)

RF Modulator(Single side band Transmitter)

4/4/2012 LIU meeting 20

TWC 200 MHz Phase Σφoffset

φ TWC200 Σ

φ TWC800

Bunch lengthening Bunch shortening

4/4/2012 LIU meeting 21

Comb filter (filter with beam synchronous clock)

GainFrev @ injection

Freq

GainFrev @ extraction

Freq

4/4/2012 LIU meeting 22E

xtra

ctio

nIn

ject

ion

Cavity filter

Cavity impedance is complex [sin(x)/x], sign changes @ the zeros=> Need absolute filter response (The cavity response does not change during the acceleration ramp!)

FreqFcav

800.888MHz

~3.2MHz

Frf flat top~801.6MHz

∆f=+0.71MHz

Frf flat botom~799.8MHz

∆f=-1.12MHz

Þ~12MHz (first two zeros) feedback bandwidth is requestedÞDigital filter clock is derived from the RF (Beam sync clock)

4/4/2012 LIU meeting 23

Cavity filter (in baseband)• As the Digital Cavity is beam synchronous clocked, its absolute transfer function changes with the RF frequency.• In order to compensate this effect, prior and after the filter, the feedback signal is modulated (digitally) with the beat frequency (∆f=Frf – Fcav)

∆f

F

down modulation with ∆f

F

FilteringBefore Filtering

F

Up modulation with ∆f

∆f

After Filtering

F

After Filtering without down/up

modulation

4/4/2012 LIU meeting 24

Cavity filter (implementation)

Beat frequency computation (from pre-defined function)

Down/up modulation

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References[1] D. Boussard et al., Controls of Strong Beam Loading, IEEE Transaction on Nuclear Sciences., 1985

[2] P. Baudrenghien et al., Control of strong beam loading. Results with beam, Chamonix 2001

[3] T. Mastoridis et al., RF system models for the CERN Large Hadron Collider with application to longitudinal dynamics, Phys. Rev. Sp. Topics. AB, 13, 102801, 2010

[4] P. Baudrenghien et al., The LHC RF System. Is it working well enough ? Chamonix 2011

[5] D. Boussard, Travelling-Wave structures, Joint US-Cern-Japan Intl School, Tsukuba, 1996

[6] P. Baudrenghien, CAS RF 2000 and 2011

4/4/2012