Axel Neumann, TTC CW-SRF - PowerPoint Presentation

Slide1

Axel Neumann, TTC CW-SRF meetingU Cornell, 06/11-06/14/2013Ithaca, NY

Experience with high loaded Q cavity operation at HZBSlide2

Test set up: Horizontal test facility HoBiCaTTesting fully equipped cavities includinghelium vessel, motor- and piezo tuner,CW modified TTF couplers, magneticshielding, etc.Temperature range down to 1.5 K, typically 1.8 K with 100 W@ 1.8 K: 16 mbar ±30 mbar

rmsCoupling variable, installations downto bc

=1 possibleRF set up: 19 kW IOT, 400 W solid stateamplifier driven by PLL or Cornell’sLLRF system

Two cavities tested in parallel orsample studiesGun cavity tested with diagnostic beam-line

Page

2Slide3

High QL Measurements done/planned at BESSY/HZB Page

3

Cavity

typeRF requirements or

achievements

Design/operated

Q

LProject

Eacc=15-20 MV/m,Q0

=1-3

.1010

, s

f=0.02

deg.,

s

A

/A=1.10-43.107 Operated:5.106-2.108BESSY FEL, SC CW,low beam loading Epeak=12-25 MV/mQ0=2-7.109sf=0.02 deg., sA/A=1.5.10-4Operated: 3.106-1.5.107Lead cathode, all SC gun for low current FELsEacc=20 MV/m,Q0=1-2.1010 , sf≤0.05 deg. 5.107BERLinPro Linac, zero beam-loadingE0=30 MV/m,Q0≤4.1093.6.106BERLinPro Gun prototype, 4mAEacc=10 MV/msee Cornell’s presentation1.105BERLinPro Injector,100 mA, low QL

Tested in

HoBiCaT

Tested in

HoBiCaT

Expected 2014

Expected 2014

TESLA

1.6 cell

Pb

/

Nb

hybrid, J.

Sekutowicz

Cornell booster

HZB

1.4 cellSlide4

CW operation: Field stability determined by Microphonics Page 4

2.5 mm

Niobium

walls

Deterministic

,

narrow

-band

sources:

Vacuum pumps

Stochastic

background

noise Field amplitude variation: Dynamic Lorentz force, Df/DEacc² = 1Hz/(MV/m)²G. Bissofi222 Hz151 Hz Response of the Cavity-Helium vessel-Tuner system:(FEM simulations, e.g.:Devanz et al. EPAC 2002)

Mechanical

oscillations

of

the Cavity:Microphonics

Helium

pressure

fluctuations

D

f

/

D

p

= 50-60 Hz/

mbar

,

Gun:100 Hz/

mbar

Heat

transport

dynamics

16

mbarSlide5

Measurements at high loaded Q Page 5 CW operation: Microphonics, peak events? Field stability at the presence of microphonics

High cw gradient: E.g. 20 MV/m 

Ponderomotive

instabilities by LF detuning

High cryogenic dynamic losses,

helium bath stability

Beam transients

during ramping to 100 mA, how to handle? (ERL) Residual beam-loading due to beam losses  non-

perfect recovery, time jitter?

Combine microphonics compensation with

LLRF at high QL for a multi-cell cavity

X

X

Low beam-loading CW SRF

linacs

allow operation at high QL narrow bandwidth (order of 10s Hz)Slide6

Power requirements and parameter spacePage 6

Q

L

Studied within this work

But:

Effective detuning is a

convolution of the detuning

spectrum with the

cavity response (

bandwidth

)

Cavity transfer function itself

altered by controller settings

(feedback gains)

Pros:

High QL: low forward power fora given field level, reduction of thermal stress for RF transmissionline/coupler7-cell cavity, 20 MV/m, Ib=0Slide7

Detuning spectrum versus bandwidth (TESLA cavity)Page 7For two different tuning schemes (Saclay I andINFN Blade) open loop measurements of

microphonics vs. QL were performedBoth tuners showed to have different transfer

functions and thus detuning spectraQ

L,Saclay: 3.107-4

.

10

8

QL,Blade: 7.

105-2.107

Saclay

: Excitation of 1

st

mechanical

eigenmode

sets in

Blade: Mechanical eigenmodeat 300 Hz, vacuum pump freq.Slide8

Detuning characterization of a TESLA cavity, short-termPage 8

8

s

f

= 1.56 Hz

S

FFT

FFT

HoBiCaT

:

s

f

= 1 - 5 Hz (

rms

)

 2-13° phase error (Aim: 10-2 °) „open loop“ „closed loop“ He pressure variations: fmod < 1 Hz Cavity specific: Lines at 30 or 41 Hz Excited EigenmodeHe pressure-variationsDetuning (Hz)Detuning (Hz)Time (s)

Characterisation: Measurement resultsSlide9

Longterm stability: Peak eventsPage 9

Microphonics

recorded

at

HoBiCaT

with

TESLA

cavity

for

48

hours

9RMS Values around 1-5 Hz  Determines field stability and thermal loading of RF system (5 kW)Peak values extend out to 17 σ!  Determines RF power installation (15 kW)Peak events occur 10-20 times a day!(This was partly improved by changes tothe

control

settings

of

the

under-press.pumps.)

Expected field

stability: 0.02 - 0.1°

For

„comfort“

want to

reduce the

microphonics

Gaussian sub-range

0.8 Hz

rmsSlide10

Time-frequency analysis by WaveletsPage 10

10

y

^

t/s

0

y

s

.

w

Morlet

Variation up to

D

f =10 Hz

on a ~100 ms time scale

Spectrum of He-pressure variations of stochastic nature Adaptive, „learning“ (dynamic) compensation mandatory Need for classic feedback controlDf (Hz)Slide11

Detuning compensation: Characterize the tunercavity actionPage 11

Mechanical

resonances

Turbo pump

TESLA Cavity

1.6 cell gun Cavity

Mechanical

resonance

Helium

Activity?Slide12

Model based controller: Fit of the transfer functionPage 12

12

Relevant for tuning

Fit: Parallel

acting

2

nd

order

systems

Evaluate

response

of higher modes at lower frequencies >20 modes needed for fit Systems complexity complicates use of model based feedbacks (e.g. Kalman filter) Transfer function aslook-up tableKalman approach testedwithin a Master thesis(P. Lauinger), test in prep.TESLA CavitySlide13

A tested scheme: Least-mean-square based adaptive feedforwardPage 13

13

FFT

∙H

-1

IFFT

t (s)

D

U (V)

Compensating

signal

D

l (nm)

t (s)

External mechanical

oscillations

t (s)Df (Hz)Detuningof the cavityCalculationof optimalFIR filterparameters

For white noise excitationThe FIR filter would beH-1

piezoD

fSlide14

Compensation resultsPage 14

SFFT

Open

loop

s

f

= 2.52 Hz

Feedback only

s

f

= 0.89 Hz

Feedback

and Feed-

forward

s

f = 0.36 HzResults:QL=6.4.107Multi-resonance control:Piezo resolution seemsto limit control ofneighboring modesSingle-resonance control:Slide15

LLRF studies with U Cornell: Limits of QLPage 15

QL=5.

107, f1/2=13 Hz

9 cell TESLA

cavity

E

acc

= 10-12 MV/mTbath= 1.8 KPI

piezo loop8/9-p filter optimizedQ

L=1.

108, f1/2=6.5 Hz

Q

L=2.108 , f1/2

=3.25 Hz

LF

detuning  IOT beam instableCavity field triplog(sf)Best results:5.107 0.008°1.108 0.0093°2.108 0.0236°Areas with sf>0.1 were blanked out Slide16

LLRF studiesPage 16QL

sf (Hz)

sf (deg)

sA/AP

f

(kW)

5

.107

9.50.0081.10-41.1061

.108

7.90.009

2.10-4

0.5952.

10

8

4.2

0.0243.10-40.324Slide17

Gun cavity LLRF and microphonics studiesPage 17@25 MV/m, quenchwould occur after few minutes

Insufficient

cooling of cathode

A lot of power dissipated in LHe bath

 effect on microphonics?

More about Gun Cavity in talk by

A.

BurrillSlide18

Results with the SC Gun CavityPage 18

QL

: 1.4.107, due to ponderomotive instability changed to 6.6

.106 At 25 MV/m the cavity losses increase

due to bad thermal contact of cathode plug and back wall via indium seal

Microphonics increase by factor of three, mechanical resonance excited

Strong line at 35 Hz appears:Eigenmode of the Helium bath?Slide19

On-going studies: Thesis P. LauingerPage 19Cavity driven by LLRF at E

peak=15 MV/mPiezo compensation in PI loop mode with low-pass filteringAdditional power dissipated in

LHe bathby heater within liquidMicrophonics recorded while heater is powered

Heater power (W)

Microphonics (Hz)

T

LHe

(K)

Q

crit

(W/cm²)

P

heater

(

D

fmax) (W) T. Peterson, TESLA-Report 1994-18Further studies with TESLA cavity planned as well as microphonicscompensation using Kalman filter approach Slide20

SummaryPage 20 Microphonics as main error source for field stability extensively characterized for TESLA and 1.6 cell Gun Cavities at various QL Microphonics compensation demonstrated with TESLA cavity at high QL, an order of magnitude feasible

Needs to be implemented within operating LLRF system LLRF studies showed a stable operation at up to Q

L=2.108,

still needs to be demonstrated for fields larger than Eacc>12 MV/m

Experiments to correlated microphonics and helium heat transport dynamics were started,

more results hopefully this summer

New microphonics compensation schemes will be tested soon

For both cavity types studied a field stability of at least sf≤0.02 deg and

s

A/A≤1.10

-4 was demonstrated

Thanks to, people involved:

S.

Belomestnykh

*,

J. Dobbins, R. Kaplan, M. Liepe, C. Strohman (Cornell, *now BNL) for the LLRF system J. Sekutowicz (DESY), P. Kneisel (JLab) for the 1.6 cell Gun CavityW. Anders, A. Burrill, R. Goergen, J. Knobloch, O. Kugeler, P.Lauinger + HoBiCaT personell (HZB)