RF-Gun beam based alignment at PITZ/FLASH

RF-Gun beam based alignment at PITZ/FLASH

M.Krasilnikov, DESY Zeuthen

LCLS Injector Commissioning Workshop (ICW)

October 9-11, 2006

M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH” 2

Outline

• Beam-Based Alignment (BBA) of RF gun:– Fields and geometry– BBA motivation

• Cathode laser BBA:– PITZ– FLASH– Limitations of laser BBA

• Main solenoid BBA:– Solenoid micromover system– Main problems and possible solutions– Limitations of solenoid BBA

• Conclusions


PITZ Solenoides for L=276 mm with cathode at z=0 - 16 Jun 05 - MK -

N:\4GROUPS\ZN_PITZ\SIM\KRAS\FIELDS\SOL_PITZ\SF\SOLENOID1.AM 3-09-2006 17:14:04

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60

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-20 -10 0 10 20 30 40 50 60 70

RF-Gun: fields and geometry (PITZ and FLASH)

•sol.mech.axis ≠mag.axis•sol.mech.axis ≠cavity el.axis•solenoid tilt angles•fields overlapping


Solenoid tilt effect

For Ecath=25MV/m solenoid tilt angle of ~3mrad is equivalent to

1 mm transverse offset

RF-Gun Alignment Motivation

Emittance growth (@z=15m) due to RF -gun misalignment

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

0 0.5 1 1.5 2x-offset,mm

norm

.em

it.,

mm

mra

d

-10

0

10

20

30

40

50

60

70

emit.

grow

th,

%

EmX (vs.las.offset)EmY (vs.las.offset)EmX (vs.sol.offset)EmY (vs.sol.offset)EmTr (vs.las.offset)EmTr (vs.sol.offset)

-4

-3

-2

-1

0

1

2

3

4

-3 -2 -1 0 1 2 3 4 5X,mm

Y,mm

solenoidoffsetX=1mm

solenoidangleX=3mrad

Imain=0A Imain=400A

Imain=400A

5% emittance growth (lower limit estimations):laser R<800um or solenoid R<500um or solenoid angle<1.5mrad (0.086deg)

No effect in the matching section has been considered!

ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV)


Cathode Laser Alignment (FLASH)

Laser spot positioning at the photo cathode:

+Transverse displacement and angle of the laser beam can be independently changed

-•Iris position has to be adjusted for every mirror movement•No VC and e-beam simultaneously


Cathode Laser Alignment (PITZ)


BBA of Laser on Cathodez=0 z=0.276m

z=0.935m

z=0.778m

Basic measurement:Mean position of electron beam at

LOW.Screen1 (DoubleDiagCross at z=0.778m)

vs.

RF gun launch phase (SP Phase)

Conditions:•Main and bucking solenoids off•All steerers off•Dipole (even it is ~0.2m after the screen) degaussed and off•Bunch charge ~10pC, pulse train 10-50 laser pulses•Moderate RF power in the gun: 0.8-1.5MW (exclude dark current)→Pz~2.6-3.3MeV/c

Preliminay (rough) laser alignment:•Using scintillating cathode•Centering in dark current images


Beam spot at Diag.Cross

screen(z=0.78m)

“Normal” phases

“Low” phases,rf focused beam

Measurements&SimulationsSimulations

-4

-2

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-40 -20 0 20 40 60 80 100 120RF Phase,deg

Xoff (mm)

Yoff (mm)

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-40 -20 0 20 40 60 80 100 120RF Phase,deg0

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Me

V/c

Charge (simulated) XYrms (simulated), mm

Pmean, MeV/c

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2

4

6

8

-40 -20 0 20 40 60 80 100 120RF Phase,deg

Be

am

ch

arg

e, p

C

Charge (measured)

Charge (simulated)

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1

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3

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5

6

7

-40 -20 0 20 40 60 80 100 120RF Phase, deg

rms

size

, mm

Xrms (measured) Yrms (measured)

Xrms (simulated) Yrms (simulated)

Beam charge, Transverse rms Size,Mean Momentum Vs. RF Phase

Beam Offset at z=0.78m Vs. RF Phase (0.5 mm vertical laser offset on the cathode has been assumed)

Measured and Simulated beam charge at z=0.78m Vs. RF Phase

Measured and Simulated beam rms size at z=0.78m Vs. RF Phase

BBA of Laser on Cathode


Cathode Laser BBA. Test Movement

yxlaser ;

Step 1: Laser test movement

One of results:(X0,Y0) – preliminary coordinates of the center at the screen

Measurement: Beam offset vs. rf phase

111 cybxa

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y_sc

ree

n1

, mm

SPPhase=121deg

SPPhase=147deg

222 cybxa Measurement: Beam offset vs. rf phase

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y_sc

ree

n1

, mm

before

after laser test movement

SPPhase=121deg

SPPhase=147deg

1221

112

1221

112

2

2

1221

11

baba

baaV

baba

babV

a

b

baba

ba

V

VV

yxy

yxx

yx

y

x

Step 2: Laser alignment

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21 22 23 24 25 26 27 28x_screen1, mm

y_sc

ree

n1

, mm

beforeafter laser test movementafter laser alignment

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20.5

21

21.5

22

22.5

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20 21 22 23 24 25 26 27 28x_screen1, mm

y_sc

reen

1, m

m

before

after laser testmovement

after laseralignment

SPPhase=121deg

SPPhase=147deg

calcuated initial laser

positioncalcuated laser position

after x=-1mm

electrical axis


Cathode Laser BBA. Difficulties

•Laser intensity non-homogeneity

•Laser position jitter

•Possible damage of YAG screen homogeneity

•Earth magnetic field (x,y,z)~(0.02mT,-0.03mT,-0.009mT)

Laser position jitter

3.15

3.2

3.25

3.3

3.35

0 50 100 150 200 250 300time, sec

<X

>, m

m

1.8

1.85

1.9

1.95

2

<Y

>, m

m

X_Mean

Y_Mean

stdev<X>=13um

stdev<Y>=11um

Extreme example of a damaged screen


Main Solenoid BBA. Micromover systemmain solenoid

beam axis


Main Solenoid BBAz=0 z=0.276m

z=0.935m

z=0.778m

Basic measurement:Mean position of electron beam at

LOW.Screen1 (DoubleDiagCross at z=0.778m)

vs.

Main solenoid current

Conditions:•Bucking solenoid off•All steerers off (or consider in simulations)•Dipole (even it is ~0.2m after the screen) degaussed and off•Bunch charge ~10pC, pulse train 10-50 laser pulses – to be tuned for Imain•Moderate RF power in the gun: 0.8-1.5MW (exclude dark current )→Pz~2.6-3.3MeV/c•RF launch phase*•Laser spot size possibly small


Main Solenoid BBABeam position (<X>,<Y>) at

LOW.Scr1(z=0.778m) as a function Imain E-beam displacement with a main solenoid

sweep. Possible reasons:

•Solenoid transverse offset (Xsol,Ysol)

•Solenoid tilt angles (AngleX, AngleY)

•Laser (small) offset (Xlas,Ylas) from the center

Other factors to be considered:

•RF gun launch phase and gradient

•Small offset (from X0,Y0 obtained after laser BBA)

•Solenoid calibrationFrom the laser BBA:X0=13.0mm Y0=14.8mm

Imain=0A

Imain=320A


Main Solenoid BBA. RF-gun launch phase and gradient

(21.4MV/m;22deg)

2

2.1

2.2

2.3

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2.7

2.8

0 10 20 30 40 50 60 70F0-Gun SPPhase, deg

Pm

ean,

MeV

/c

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Prm

s, k

eV/c

Pzmean, MeV/c

PZrms, keV/c

Cathode laser temporal profile

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inte

nsi

ty, a

.u.

streak-camera measurement

Bunch charge (LOW.FC1) vs. gun SPPhase

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bu

nch

ch

arg

e, p

C

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<Pz>

, MeV

/c

Charge, pC

PZmean

Bunch charge (LOW.FC1) vs. gun SPPhase

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e, p

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Integr(Prof), shif ted

Charge, pC

F0=-18deg


Simulation ToolAlignment Utility of the V-code - fast tracking code based on the

method of moments of particle distribution function

min,,,,,,,,, ** F offoffrfcathsolsolsolsollaslas YXEAngleYAngleXYXYX

,...,...,

,...,...,

320

0

22

320

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22

YsolsolXsolsol

A

AIoff

measn

simulnynoff

measn

simulnxn

solsol

A

AIoff

measn

simulnynoff

measn

simulnxn

YXYYYwXXXw

YXYYYwXXXw

main

main

F


Main Solenoid BBA. Test movementSimultaneous simulations of beam position before

and after test movement (Xsol=0.4mm)Advantages:

•Same RF gun launch phase and gradient

•Same offset X0*,Y0*

•Same solenoid calibration

Laser_Beam_CenterX 0.20 mm

Laser_Beam_CenterY -0.26 mm

XSolMainCenter 0.85 mm

YSolMainCenter 1.12 mm

AngleXSolMain -0.0808 deg

AngleYSolMain 0.0580 deg

Ez_Field_At_Cathode 22.2 MV/m

Initial_Phase -147.2 deg

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X, mm

Y, m

m

measured before TM

measured after TM

simulated

simulated

MainSol test movement


Main Solenoid BBA

Before BBA After BBA

Laser_Beam_CenterX 0.20 mm 0.11 mm

Laser_Beam_CenterY -0.26 mm -0.27 mm

XSolMainCenter 0.85 mm 0.041 mm

YSolMainCenter 1.12mm 0.063 mm

AngleXSolMain -0.0808 deg -0.0158 deg

AngleYSolMain 0.0580 deg 0.0057 deg

Ez_Field_At_Cathode 22.2 MV/m 22.4 MV/m

Initial_Phase -147.2deg -148.5 deg

-3

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0

0.5

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-101234 x, mm

y, m

m

Before BBA

After BBA


Main Solenoid BBA. Difficulties

•Laser BBA difficulties:

•Laser intensity non-homogeneity

•Laser position jitter

•Damaged YAG screen

+

•RF phase and gradient jitter

•Some uncertainty in the solenoid micromover (especially angles), z-position

Laser position jitter

3.15

3.2

3.25

3.3

3.35

0 50 100 150 200 250 300time, sec

<X

>, m

m

1.8

1.85

1.9

1.95

2

<Y

>, m

m

X_Mean

Y_Mean


RF-Gun BBA. Conclusions• BBA of Laser on Cathode:

– routine procedure at PITZ (FLASH)– based on a measurement of beam position vs. gun launch phase– test movement of the laser beam allows to determine a displacement vector

for the laser beam centering

• Main solenoid BBA:– multi-parameter task– based on beam position simulation vs. main solenoid current– test movements of the main solenoid and/or cathode laser allows to reduce

uncertainty in main solenoid misalignment

• Possible improvements:– implement earth magnetic field in BBA procedures– use BPM (LOW.BPM1) for solenoid BBA – more details on RF field profile– solenoid relative displacement online measurement



Reference RF Phase. Beam Size measurements

Transverse Beam Size at Screen 3 as a Function of RF Launch Phase for Various Main Solenoid Currents

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-130 -110 -90 -70 -50 -30

SP Phase / deg

mm

Rrms(240A) / mm

Rrms(250A) / mm

Rrms(260A) / mm

Rrms(270A) / mm

Rrms(280A) / mm

Rrms(290A) / mm

Rrms(300A) / mm

Pmean (a.u)

Fmax


PITZ1 Benchmark Problem: Fields

-0.2

-0.15

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z, m

Bz,

T

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Ez,

MV

/m

Main solenoid (Imain=320A)

Bucking solenoid (Ibuck=24A)

Ez (balanced)

cathode plane

Main Solenoid Calibration

0

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0 100 200 300 400 500Imain, A

|Bz_

pe

ak|

me

asu

red

, T

|Bz_peak(mT)| = 0.5871*Imain(A) + 0.3641

• Field balance in the rf gun cavity

• Solenoid calibration

• MF compensation

Ibuck=0.074847*Imain


Rough laser alignment using dark current symmetry. TTF

Dark current at TTF at screen Gun3 (z = 1.27m)

Without beam With beam

• Dark current rings originate from the edge of the Cs2Te coating and plug spring region

• A laser spot being aligned on the cathode center results in an electron beam centered with dark current rings

BBA: Step 0 – Rough Laser Alignment on the Cathode


Rough laser alignment using dark current symmetry. PITZ

Without beam With beam

Screen Diag. Cross (z = 0.87m)

Screen_PP(z = 2.62m)

BBA: Step 0 – Rough Laser Alignment on the Cathode

RF Gun Alignment: Satellites observation

@PP Screen @Dispersive Arm

@Diag.Cross, “low phase”

xy

xPz

@Diag.Cross

xy

Vacuummirror


SP phase = -130SP phase = -125SP phase = -120SP phase = -116SP phase = -114SP phase = -112SP phase = -110SP phase = -106SP phase = -100SP phase = -90SP phase = -70SP phase = -50SP phase = -30

BBA of laser on cathode

SP phase = -20SP phase = -16precision of mirror adjustment:better than 20 µm


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mm

mra

d

EmitX(aligned)

EmitY(aligned)

EmitX (laser X-offset=1mm)

EmitY(laser X-offset=1mm)

EmitX (solenoid X-offset=1mm)

EmitY (solenoid X-offset=1mm)

-8

-6

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-2

0

2

4

6

0 2 4 6 8 10 12 14 16 18z, mmm

<x> (aligned) <y> (aligned)<x> (laser X-offset=1mm) <y> (laser X-offset=1mm)<x> (solenoid X-offset=1mm) <y> (solenoid X-offset=1mm)

RF-Gun Alignment MotivationASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV)

trajectory

1

1.2

1.4

1.6

1.8

2

2.2

2.4

12 12.5 13 13.5 14 14.5 z, m

mm

mra

d

1

1.5

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2.5

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3.5

4

4.5

5

0 1 2 3 4z, m

mm

mra

d

emittance

Emittance degradation in the matching section is not included

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RF-Gun beam based alignment at PITZ/FLASH

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