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### High Frequency Beam Effects at the ESRF

J. Jacob

NSLS II Beam Stability Workshop

BNL, April 18th - 20th, 2007

High Frequency Beam Effects at the ESRF

High Frequency effects affecting beam stability at ESRF (6 GeV)

Multibunch total current,essentially narrow band impedances

Longitudinal:

HOM driven instabilities

Transverse

Dominated by resistive wall instabilities (screening HOM effect)

Ions

Single bunch current per bunch,broad band impedances

Longitudinal:

Microwave instability

Transverse:

Mode coupling instability - TMCI

Head Tail instability

Side effects:

High peak signals distortion of BPM readings

Heating (bellow shielding, special vessels, …)

Pressure burst, lifetime accidents, beam losses, …

RF Phase noise

Countermeasures

Constructive measures

Minimization of impedances

Vacuum chamber material

Discontinuities, bellow shielding,…

Cavity design

Passive damping (HOMs)

Active damping (Feedbacks)

Reduction of RF Phase noise

Operation parameters

Partial filling of the storage ring

Positive chromaticity

RF Voltage,…

Effect of Harmonic Cavities

Lifetime increase by bunch lengthening

Landau damping of LCBI

Effect on other beam dynamics

Outline

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

ESRF–SR: 6 five-cell cavities

- lowest LCBI thresholds: 40 mA
- stabilized by Landau damping from transient beam loading in fractional SR filling 200 mA in non symmetric 1/3, later 2/3 filling
- 1998: new cavity temperature regulation to ± 0.05ºC, for precise control of HOM frequencies

stable at 200 mA in uniform and symmetric 2 x 1/3 filling

Not possible to exceed 250 mA

- Dec 2006: longitudinal bunch-by-bunch feedback - LFB with 1 ms damping time

300 mA in uniform

- Limited b VRF: 9 11 MV against Robinson instability
- No further beam increase
- Window power at maximum
- Robinson even higher VRF
- Maximum 300 mA with existing cavities

R/Q = 139 /cell Qo = 38500

Rs = 26.8 M (5 cells) frf = 352.2 MHz

Vnom = 1.4 … 2.5 MV (Booster: 4 MV pulsed)

2 couplers: bmax = 4.4

Max 170 kW/coupler

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

1/3 fill

Ithreshold

[mA]

tbunch

Only one part of the bunch train participates to coherent motion

0.5 ns

Increasinggap

SR Portion filled

Streak camera

2 ms

Landau damping from fractional filling of storage ring

Streak camera image of a LCBI

[O. Naumann & J. Jacob]

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

ESRF Cavity Temperature regulation system (cav. 1 & 2)

T = Tset± 0.05 ºC

200 mA in uniform filling

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

Phase detection at 1.4 GHz

200 MHz BW low pass

FPGA processor

S

ADC in

RF clock

DAC out

RF clock

1.2 GHz to 1.4GHz

BW cavity

QPSK

modulator

1.4 GHz power amplifier

4 x 50 W = 200 W

December 2006: 300 mA reached thanks to LFB

(LFB = longitudinal digital bunch-by-bunch feedback)

- 300 mA delivery to users planned for mid 2008

Bandwidth: fRF/2 = 176 MHz

[inspired from PEP II, ALS, DAFNE,…design ]

[E. Plouviez, G. Naylor, G. Gautier, J.-M. Koch, F. Epaud, V. Serrière, J.-L. Revol, J. Jacob, …]

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

Dimensioning of LFB:

- ESRF natural damping time ts = 3.6 ms
- DSP algorithm minimum active damping timetdamp = 0.5 ms ts /7 (loop delay)

Gain:

- So, without safety margin: dt 1 fs / turn (Kicker provides 500 V)

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

2.5ps rms

0.4ps rms

LFB Spurious phase signals: mode 0 signals level

1V/ RF degree

7ps / RFdegree

Will not saturate the ADC(<8ps), nevertheless: high pass filter in analogue front end

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

LFB Spurious phase signals: beam loading transients

- 2 x 1/3 filling
- Would put +/-15 V at the input of the ADC
- Must be reduced by 30dB in the analog front end:

Beam Transient Suppression (BTS) in front end

Actually simple HP filter suffices

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

16 TAP FIR:16 x 31 ms = 0.5 ms = Tsynchrotron

- BP filter at fs
- Differentiation (Vkick jt): phase shift by 90°
- Total averaging 176, sensitivity: 1fs -> 0.08 fs

Factor 11 decimation:

11 T0 = 31 ms

FIR: (a,b,c,1,c,b,a,0,-a,-b,-c,-1,-c,-b,-a,0)

Further Mode 0 removal

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

1st aluminum prototype at ESRF RF lab

Improved design

Cut off 749 MHz

4 ridges

Cut off 460 MHz

New 352 MHz Cavities for ESRF

- Unconditional stability & higher current: 400…500 mA
- SC cavities (e.g. SOLEIL type): Beam power 2 couplers/cell
- NC single cell HOM damped cavities / 1 coupler/cell preferred solution
- R&D based one BESSY design with ferrite loaded ridge waveguides for selective HOM damping

[E. Weihreter, F. Marhauser]

Cut off 435 MHz

[N. Guillotin, V. Serrière, P. Roussely, J. Jacob]

High Frequency Beam Effects at the ESRF

Multibunch – HOM driven LCBI

measured on 1st Al prototype

GdfidL simulation of 1st Al prototype

GdfidL simulation of Improved design

Tolerated Longitudinal HOM impedance for 18 installed cavities

High Frequency Beam Effects at the ESRF

Multibunch –TCBI /Resist. Wall & Ions

- CBI from Transverse HOM impedance never observed: screened by Resistive Wall Instability (RWI)
- Since commissioning installation of smaller & smaller ID gaps:
- 8 mm inner height, 5 m long vessels NEG coated extruded Al
- Al: high conductivity maximize RWI thresholds
- NEG: efficient distributed pumping minimize Bremsstrahlung & ion instabilities
- 6 mm in-vaccum undulators: Ni-Cu foil
- Slightly positive normalized chromaticities to damp resistive wall and ion instability
- Goal: keep emittances ex = 4 nm rd ez = 25 pm rd
- For 200 mA, setting: xx = 0.2 xz = 0.6
- Vertical Broad Band Resonator (BBR) to be added to RW model to explain thresholds, BBR has a damping effect on narrow band TCBI (fres = 22 GHz, Rb/Q = 6.8 MW, Q = 1)
- First successful tests with transverse bunch-by-bunch feedback - TFB (developed in parallel with LFB) : allows operation with x = 0 for more dynamical aperture & longer lifetime
- Sytematic conditioning at restart shifts after vacuum opening during shut downs
- Experience at 300 mA
- Successful use of TFBto damp vertical ion instability

[P. Kernel, R. Nagaoka, J.-L. Revol]

High Frequency Beam Effects at the ESRF

Multibunch –TCBI /Resist. Wall & Ions

Uniform 124 mA Vertical

xx = 0.2 xz =0.19

991

Resistive wall

986

Ion signature around 5 fo

990

RWI threshold as a function of xv

Vertical spectrum near threshold in xv

[P. Kernel, R. Nagaoka, J.-L. Revol]

High Frequency Beam Effects at the ESRF

Single Bunch – Bunch Lengthening

4 Bunch

16 Bunch

Single Hybrid

Single 7/8

USM Range

4.5 mA

ps rms

I per bunch [mA]

[J.-L. Revol]

High Frequency Beam Effects at the ESRF

Single Bunch – Microwave instability

0.1 %

sE/E

0.2 %

0.4 %

sE/E

Energy spread measured at ESRF

keV

Tracking simulations fit of longitudinal BBR:

fres = 30 GHz, Rs = 42 kW, Q=1 ( Z/p = j 0.5 W)

I per bunch [mA]

High Frequency Beam Effects at the ESRF

Single Bunch – Vertical Instabilities

Vertical TMCI instability at zero chromaticity

TMCI threshold

Vertical Transverse Mode Coupling Instability at 0.67 mA (TMCI) forxv = 0

Vertical Head Tail Instability

[P. Kernel, R. Nagaoka, J.-L. Revol]

High Frequency Beam Effects at the ESRF

Single Bunch – Horizontal Head Tail

Gap Open

Injection saturation

February 2001

Decrease of the horizontal instability threshold

Mode +2

Zero current tune=0.44

September

2001

Overcome by pushing the chromaticity (or reducing the single bunch current in hybrid)

October

2002

Mode +1

The single bunch experiences an increase of the horizontal incoherent tune shift, coming from the asymmetry of the vacuum chamber

Hybrid/16Bunch

Working x

Single Bunch

Working x

Mode -1

Overcome by correcting “on line” the half integer resonance and decreasing the zero current tune.

Increasing difficulties in single bunch mode (also in 16 bunch and hybrid)

[P. Kernel, R. Nagaoka, J.-L. Revol]

High Frequency Beam Effects at the ESRF

Existing klystron transmitters:

dF/d(HV) 7 ° per % HV

Phase noise up to -50 dBc at multiples of 300 Hz / HVPS ripples

Beam sensitive (fsynchrotron = 1.2 to 2 kHz)

Fast phase loop → -70 dBc

Unstable behaviour

Multipactor / input cavity

Mod-Anode breakdowns

Many auxiliaries, trips

Risk of Klystron obsolescence

ESRF RF upgrade project:

Solid State Amplifiers - SSA,based on SOLEIL design

Intrinsicly redundant

Switched power supplies at 100 kHz (far from fsynchrotron)

Negligible phase noise

Overall 50 % efficiency

RF Phase noise352 MHz 1.3 MW klystron

Thales TH 2089

352 MHz–190 kW Solid State Amplifiers (2 units)

682 transistor modules + 42 in standby

[P. Marchand, T. Ruan et al.]

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

Bunch length

Bunch length

Vacc [MV]

V [MV]

Harmonic 3

Vacc (f)

Uloss/e

Uloss/e

Vhc (f)

f[rad]

f[rad]

Vm (f)

df/dt

df/dt

f

f

sL 4sL

tTouschek 4tTouschek

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

- Interest in a third harmonic RF system for the ESRF ?

200 mA uniformtlife = 60 h NO

90 mA in 16 bunchtlife = 10 h YES

up to 20 mA single bunchtlife = 5 h YES

- Interaction with BBR, accelerating and higher order modes ??

Multibunchsingle particlemodel:

Transient beam loading effects with a harmonic RF system

Single bunchmultiparticlemodel:

BESAC: Potential Well and Microwave Instability

[G.Besnier, C.Limborg, T.Günzel]

[J.Byrd, S.De Santis, J.Jacob, V,Serriere]

ALS, ESRF

ESRF

Multibunch multiparticlemodel

Harmonic cavity, Potential well & Microwave Instability,

AC and DC Robinson instabilities, Landau damping of LCBI

[V.Serriere, J. Jacob]

see also [R. Bosch]

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

Potential well

@ 5.5 mA

m-wave instab.

@ 20 mA

Harmonic cavity

Further factor 4

Harmonic cavity

Further factor 3

sL

sL

3sL

6 sL

12 sL

18 sL

m-wave instab.

@ 20 mA

Harmonic cavity

Reduces E-spread

sE

2 sE

1.7 sE

Main Results

- Potential well distortion (from BBR, i.e. Z/p = j 0.5 W):
- e.g. at ESRF in 16 bunch for I/bunch = 5.5 mA

- Microwave instability (from BBR above 5 mA, 30 GHz, 42 kW, Q=1):
- e.g. at ESRF in single bunch at 20 mA

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

Tracking code, confirmed by numerical resolution of Haissinski equation:

Total bunch lengthening = Potential well effect X Elongation from harmonic voltage

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

- Microwave Instability&bunch lengthening by harmonic voltage

At 25 mA: still bunchlength increase factor of 2.7

High Frequency Beam Effects at the ESRF

Harmonic Cavities – theoretical study

Super-3HC cavity pair:

3rd harmonic cavity for: SLS & Elettra

Scaling of the SC SOLEIL Cavity

Construction: CEA & CERN

Rs/Q= 90 W

Quality factor: Q0= 2.108

fres,hc = 1.5 GHz

Superconducting Module with a pair of cavities

W

ESRF :

Scaling of Super-3HC to fres,hc = 1056.6 MHz

Harmonic cavity technology for ESRF ? low total intensity modes !

- Passive NC Cu cavities: Nmin = 150 unrealistic
- Active NC Cu cavities: Nmin = 12 still not practical
- Passive SC cavity pair: Nmin = 4 imposed by AC Robinson
- Active SC cavity pair: N = 1 Only practical solution with 80 … 100 kW generator

Low R/Q of SC cavities less phase transients net gain in tlife less affected by gap in fill

High Frequency Beam Effects at the ESRF

HOM driven LCBI at MAX II:

Without harmonic cavity:

Ithreshold ≈ 10 mA

With harmonic cavity: stable at250 mA due to Landau damping

LCBI Prediction for the ESRF:

LCBI thresholds only slightly increased by Landau damping on a higher energy machine like ESRF

Harmonic Cavities – theoretical studyIbeam = 250 mA

sE/E

measurements

[Å. Anderson & al.]

Tracking

code

Tracking simulation

Linear model

sE, nat

Harmonic Voltage [kV]

High Frequency Beam Effects at the ESRF

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