Measurements of adiabatic dual rf capture in the SIS 18

O. Chorniy

2

Contents

• Introduction

• Dual harmonic rf capture, scheme 1

• Dual harmonic rf capture, scheme 2

• Single rf capture

• Summary and Outlook

3

Introduction

In dual harmonic rf bucket (in comparison with single harmonic rf bucket):

• The flat voltage leads to a longer bunch. RF bucket area is larger then for single harmonic. It reduces maximum density in the bucket center.

• Bunch in a dual rf bucket is more stable. Nonlinearity created by dual harmonic rf bucket increases Landau damping.

-3 -2 -1 0 1 2 30.0

0.5

1.0

Lon

gitu

dina

l sig

nal,

arb.

u.

distance from the bucket center , rad

Longitudinal density

rfV

t

0

)2sin(2

)sin( 00 t

VtVV rfrfrf

2V1V

Dual harmonic rf signal

Dual harmonic rf capture, scheme 1

50.0 0.2 0.4 0.6 0.8 1.0

Vin

RF

volt

age

ampl

itud

e, V

time, s

h=4 h=8

V0

V0/2

RF amplitude ramps for differentharmonics starts with time delay

V200inV

V60000 V

Dual harmonic rf capture, scheme 1

Features of the measurements:

Constant (injection) energy

Ion

Intensity, part.

18Ar109 10210

Pickup RF cavity (h=4)

SIS

RF cavity (h=8)

Fixed rf capture for different intensities

msT hcapt 2004 msT hcapt 1008

6

Diagnostics in time and frequency domain

-3 -2 -1 0 1 2 30.0

0.5

1.0 dual (measured) dual (fitted)

Lon

gitu

dina

l sig

nal,

arb.

u.

distance from the bucket center , rad

single (measured) single (fitted)

-3000 -2000 -1000 0 1000 2000 30000.000

0.001

0.002

0.003

0.004

d

c

scho

ttky

ampl

itude

, arb

.u.

f-57frev

, Hz

single dual

s

c

Longitudinal beam profile measurements

Longitudinal schottky measurements

7

Longitudinal bunch profiles in a single and dual rf buckets

-3 -2 -1 0 1 2 30.0

0.5

1.0 dual (measured) dual (fitted)

Lon

gitu

dina

l sig

nal,

arb.

u.

distance from the bucket center , rad

single (measured) single (fitted)

][0 rfYExp

)2cos1(2

)cos1( rfY

Maxwell-Boltzmann beam profile

RF potential in general form

2

1Dual harmonics rf

Single harmonic rf 0rms

p

p

0

~ -represent rms bunch length for short bunches

8

Longitudinal bunch spectrum in a single and dual rf buckets

-3000 -2000 -1000 0 1000 2000 30000.000

0.001

0.002

0.003

0.004

d

c

scho

ttky

ampl

itude

, arb

.u.

f-57frev

, Hz

single dual

s

c431926),(10 s

sc

ss F

From bunch spectra in a single rf bucket

361884),(20 ddc

ds F

From bunch spectra in a dual rf bucket

1719080 sSynchrotron frequencies should be equal

expected 18160 s

9

Longitudinal bunch profiles at different intensities

-2 0 20.0

0.2

0.4

0.6

0.8

1.0

long

itudi

nal s

igna

l, ar

b.u.

0.5109

6109

11010

21010

, rad

single rf bucket

-2 0 20.0

0.2

0.4

0.6

0.8

1.0

long

itud

inal

sig

nal,

arb.

u.

0.5109

6109

11010

21010

, rad

dual rf bucket

One of the reasons for the bunch lengthening can be increase of momentum spread at injection. Another reason can be the mismatch of rf frequency.

10

Momentum spread of the injected coasting beam at different intensities

-4 -2 0 2 40

2

scho

ttky

am

plit

ude,

arb

.u.

dp/p, 10-3-4 -2 0 2 4

0

2

scho

ttky

am

plit

ude,

arb

.u.

dp/p, 10-3

Low intensity momentum spread High intensity momentum spread

Thus the emittance (and as a result, bunch length) may grow due to filamentation of the part of momentum spread which is not matched with

synchronous energy

11

Dilution factor

Longitudinal phase space before and after rf capture

coastS0p

dp

bunchS

rf bucket

Dilution factorcoast

bunch

S

S

rms

coast p

dpS

522

-2 0 20.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Lon

gitu

dina

l sig

nal,

arb.

u.

distance from the bucket center , rad

measured bunch profile theoretical fit ()

-4 -2 0 2 40.0

0.4

0.8

1.2

Scho

ttky

am

plit

ude,

arb

. u.

dp/p, 10-3

mesured momentum Gaussian fit

(rms momentum spread 2.8*10-4)

m

m

rfsbunch YS

)(2

52 2

0

dilution factor (experiments)

dilution factor (ESME result)

26.1

3.1

Bunch profile(dual rf bucket)Schottky spectra

The dilution factor was calculated for lowest intensity

Dual harmonic rf capture, scheme 2

0.0 0.2 0.4 0.6 0.8 1.0

Vin

RF

volt

age

ampl

itud

e, V

time, s

h=4 h=8

V0

V0/2

Amplitude ramps for different harmonics starts simultaneously

-4 -2 0 2 40

2

4

6

scho

ttky

am

plit

ude,

arb

. u.

dp/p, 10-3

measurements Gaus. fit

Pickup RF cavity (h=4)

SIS

RF cavity (h=8)

Features of the measurements:

Constant intensity

Capture time

Dual harmonic rf capture, scheme 2

10101

msms 20010 Momentum spread:

Different rf capture times for fixed intensity

4103

14

Capture efficiency at different capture times

0 50 100 150 2002.0

2.2

2.4

Dil

utio

n fa

ctor

, arb

. u.

Tcapt

, s

Short capturing time influences noise in the stationary bunch form. Possible explanation: mismatch of rf frequency.

Emittance increase by factor of 2 even for “adiabatic” times.

Measured Dilution factor=coast

bunch

S

S

200ms 50ms 10ms

15

Comparison with simulation results

Simulations (ESME):Experiment (SIS) vs simulations (ESME)

0 50 100 150 2001.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Dil

utio

n fa

ctor

, arb

. u.

Tcapt

, s

0.1 0.2 0.3 0.4 0.5 0.6 0.70.5

1.0

1.5

2.0

2.5

3.0

Vin=400 V

Vin=200 V

(dp/p)rms

of coasting beam before rf capture

Dil

utio

n fa

ctor

msTcapt 100

0 100 200 300 400 500 600

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Dil

utio

n fa

ctor

, arb

. u.

mismatch in rf frequency, Hz

Tramp=10 ms Tramp=20 ms Tramp=50 ms Tramp=100ms

A significant deviation of the simulation results from the measurements was found

The difference cannot be explained by the error in the value of the initial voltage

Simulation results provide us the value of frequency mismatch of 500 Hz

3

0

103.0

rmsp

dp

3

0

103.0

rmsp

dp

VVin 200

VVin 200

16

Dilution factor vs initial conditions

)(2

1

00 insrmsVp

dpc

R

h

0.0 0.1 0.2 0.3 0.4 0.51.0

1.5

2.0

2.5

3.0

3.5

4.0

Dil

utio

n fa

ctor

Ar18+

U73+

0p

dp

0

Phase space, coasting beam

initial rf bucket

max00/

pdp

pdp

rms

Simulation results for different ions (done by T. Shukla)

In process:Analytic description of the curve Dilution factor( )

Initial conditions can be described by:

msTcapt 100

17

Comparison between the different dual harmonics rf capture schemes

0 100 200 300 400 500 600

0 .0

0 .5

1 .0

1 .5

2 .0

2 .5

0 100 200 300 400 500 600

0 .0

0 .5

1 .0

1 .5

2 .0

2 .5

Bunch length 218 deg

Bunch waterfall plot in the case of rf capture with delay between harmonics

Bunch length 206 deg

Bunch waterfall plot in the case of rf capture without delaybetween harmonics

Measurements at high intensity :Long capture time : 100-200 ms

910

Single rf capture results

19

RF capture in single rf bucket

0 50 100 150 2001.0

1.2

1.4

1.6

1.8

2.0

bunc

h le

ngth

, rad

Tcapt

, s

0 10 0 20 0 30 0 40 0 50 0 60 0

5 .5

6 .0

6 .5

7 .0

7 .5

8 .0

8 .5

0 100 200 300 400 500 600

5 .5

6 .0

6 .5

7 .0

7 .5

8 .0

8 .5

200 ms 100 ms 10 ms

Almost constant bunch lengthfor all capture times.

Capturing time has influence on the presence of persistent longitudinal oscillations (in dual harmonics rf scheme only small noise).

0 100 200 300 400 500 6005 .5

6 .0

6 .5

7 .0

7 .5

8 .0

8 .5

20

Spectra of the bunch in single rf bucket

-2000 0 20000.00

0.01

s0

1

ampl

itud

e, a

rb. u

.

f-57frev

, Hz

c

0 50 100 150 2000

Inte

nsity

of d

ipol

e m

ode,

arb

. u.

Tramp

, s

Do we expect similar strong longitudinal oscillations in dual rf bucket at higher intensities? Will it lead to instabilities?(Future work)

Beam spectra

c

1

0s Synchrotron frequency

Dipole frequency

Frequency of persistent oscillations

Oscillations cannot be damped by dipole feedback system.

21

Conclusions

• To save the machine cycle time both harmonics can start at the same time.

• Dilution factor can be reduced by rf capture at different rf

frequencies.

• For the intensity of 10^10 ramping time can be down to 20-50 ms (only for dual rf harmonics regime)

• Dual rf capture experimental studies will be continued for higher intensities