Workshop on Beam Cooling and Related Topics COOL 11 12 – 16 September 2011, Alushta, Ukraine

Workshop on Beam Cooling and Related Topics

COOL 11 12 – 16 September 2011, Alushta, Ukraine

Methods for optimization of the dynamics of positrons storage in the Surko trap

*М. Есеев, A. Kobets, I. Meshkov, A. Rudakov, S. Yakovenko

JINR, Dubna*Lomonosov Northern Federal University, Arkhangelsk

Contents

Introduction: The LEPTA facility

1. Surko trap and “rotating wall” technique

2. Experimental results of particle storage in trap with “rotating wall” (RW)

2.1. Examination of storage and lifetime of charge bunch in the trap

2.2. Measurements of the transverse size of a bunch dynamics at

accumulation

3. Theory and numerical simulations of particle motion in the trap

3.1. Transverse motion of the trapped particle

3.2. Longitudinal particle dynamics, collisions with buffer gas molecules

3.3. Torque balanced steady states of charge bunch

4. Methods of positron lifetime increase and bunch compression

5. Nearest plans

Summary and Outlook

2 COOL 11 , M. Eseev, 14 September 2011


3

Low Energy Particle Toroidal Accumulatorcooling section

positron trap

kicker

10E6100sec=10E8 e+Helical quadrupole

Ps detector

10E4 o-Ps per sec 22Na10E6 e+ per sec

COOL 11 , M. Eseev, 14 September 2011

4


Trap & Injector


5

Area 1 Area 2 Area 3 eU

z

10-3

Buffer gas, Pressure, Torr

N2 N2 е+

10-4 10-6

2000 mm



6

1800

00 900

2700

Phase shifter

B

-V(x)

(а)

(b)

(c)

RF generator

L~30÷40 cm

R~0.1÷1 cm

“Rotating wall” (RW) technique: rotating electric field at the trap entrance



7

e-

0

-30

-60

I IIIII IV V VI VII VIII

-100 0

-29.3 -36.1-40.2-46.7

-50.3 V

-90

Principles of electron or positron storage in the Surko trap In application to LEPTA facility




8

The RW effect (compression of particle bunch) has been studied experimentally

• With ions: Mg+ and other ions (Laboratory University of

California at San Diego, Prof. Clifford Surko) X-P. Huang et al., PRL, 78, 875 (1997).

• With electrons:

Anderegg, E. M. Hollmann, and C. F. Driscoll, PRL, 81, 4875 (1998).

• With positrons: R. G. Greaves and C. M. Surko , PRL, 85, 1883 (2000).

T.J. Murphy and C.M. Shurko, Phys. Plasmas, 8, 1878 (2001).

J. R. Danielson, C. M. Surko, and T. M. O’Neil PRL., 99, 135005 (2007).

• With antiprotons (Hbar production, ALPHA, CERN) J. R. Danielson, et al., PRL. 100, 203401 (2008) .

G. B. Andresen , et al., Nature, 468, 673 (2010).



9

Зависимость числа накопленных электронов от времени накопления

0,00E+00

1,00E+07

2,00E+07

3,00E+07

4,00E+07

5,00E+07

6,00E+07

7,00E+07

8,00E+07

9,00E+07

0 10 20 30 40 50 60 70 80

t, сек

N

1 2 3

Stored electron number vs time

Same + transverse correction field is optimized

Same + rotating field is ON and optimized

Pressure and potential distributions are optimized

Optimal frotating = 600 kHz,

Amplitude = 1 V,

ε = 0.4, life = 80 s

Our experimental results of electron storage


10

First experiments on slow positron accumulation in the positron trap have been performed.

June 2010

The trapped positron number (arb. units)

150

170

190

210

230

250

270

0 10 20 30 40

t, s

Up

t, m

V

PMT signal (proportional to the trapped positrons number) vs accumulation time



11

22

24

26

28

30

32

34

36

38

0 100 200 300 400 500

L, мм

eU, В

N=0

N=6*107

N=2*108

N=3*108

Dependence of depth of the potential well on number of the stored up electrons

e-

I IIIII IV V VI VII VIII

-100 0

-29.3 -36.1-40.2-46.7

-50.3 V

Particle storage and “the space charge limit”

“Dynamical control” regime is gradual magnification of

locking potentials of the trap for the

purpose of maintenance of depth of the potential well

at storage.

U,

V

L, mm

“Dynamical control” of the potential

allows us to increase the particle number in the bunch ~ by 2

times!

12

Our experimental results of electron storage, RW roleRW resonance,

frequency

RW resonance

voltage


Stored electron number was measured by pulsed extraction of electrons to the

collector


13


RW off,on B=1300G,ton off=80s

0.00E+00

2.00E+07

4.00E+07

6.00E+07

8.00E+07

0 50 100 150 200 250

t,s

N

RWoff-RWon RWon-RWoff RW on

RW on RW off after 80 sec RW on after 80 sec


14

MCP + Luminescent screen + CCD camera*)

1 cm

Transverse bunch profile, CCD camera

*) Courtesy of Budker INP colleagues



15

Dynamics of extracted bunch profile


0 sec RW on 5 sec RW on 10 sec RW on

15 sec RW on 20 sec RW on 25 sec RW on

30 sec RW on 30 sec RW off30 sec

RW opposite drive


Injection and accumulation (30 s)→ injection off →delay (confinement) →extraction

RW influence on electron storage, RW on2. Experimental results of particle storage in trap with “rotating wall” (RW)

16


We see as the RW-field during time

confinement works. The effect is the

bunch transverse compression.


17

Injection and accumulation RW on (30 s)→ injection off →delay (confinement) RW off →extraction


We see as the RW-field OFF works. The effect is the bunch

transverse expansion.


18

Injection and accumulation RW off (30 sec)→ injection off →delay (confinement) RW on →extraction


We see as the RW-field ON works. The effect is the bunch

transverse compression.

Electron bunch compression. Gaussian distribution of the density

2 D

1/ 2expG A A A

/ c exA e T V V r

2

0 exp /n r n G A r

19

Fitting of transverse plane CCD photo

2 1 0 1 2cm x

0 .5

1 .0

1 .5

2 .0

fcn

A

A

B

B

C

C



Normal distribution electron current at various positions of the collector

1. RW on 30 sec2. RW on 30 sec, off 5 sec3. RW on 30 sec, off 10 sec

1

Measurements of the bunch size and density distribution by collector current

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5

x

0 .2

0 .4

0 .6

0 .8

1 .0

F cn

12

3

0 .0 0 .5 1 .0 1 .5 2 .0x

0 .5

1 .0

1 .5

fcn

The result of fitting with Gaussin function.Bunch expansion at RW-field off

1

2

3

Gaussian distribution

Error function


20

Beam profile was scanned with movable collector at the trap exit


Transverse motion of the trapped particle

21

e+

E, V/cm 0.05

fRW, kHz 600

ne, cm-3 107÷ 108

ωp, c-1 3.5 ∙ 107÷ 2 ∙ 108

B, Gauss 1200

ωB, c-1 2.1 ∙ 1010

pN2 , Torr 2.25∙10-6

R , cm 0.1÷2

L , cm 30÷40

21


1. Why the characteristic frequencies such different?

2. Why there is the resonance on frequency of the RW field?

3. Why the direction of rotation of the RW field is important?

Longitudinal motion of the trapped particle3. Theory and numerical simulations of particle motion in the trap

22

1

0.025coolpart

resRW long

E eV

f T

L~30 40 cm, 2 /long longT L

1partE eV


Then the particle in inelastic collisions with molecules of buffer gas is cooled to a room temperature

The particle injection in the storage region trap with initial energy

pN2 , 2.25∙10-6 Torr

4. Why frequency of the longitudinal motion will be compounded with frequency of the RW field?

5. What role of buffer gas intro storage region?

B

-U(x) RW electrodes

Blong || Elong


23


*J. R. Danielson and C.M. Shurko, Phys. Plasmas, 13, 055706 (2006).

RW off,on B=1300G,ton off=80s

0.00E+00

2.00E+07

4.00E+07

6.00E+07

8.00E+07

0 50 100 150 200 250

t,s

N

RWoff-RWon RWon-RWoff RW on6. Why the RW field increments quantity of the confinement up particles and the bunch lifetime?

B

RW electrodes

7. Why the segmented RW-electrodes should occupy not all storage region? (less than half of part of length of region storage*, in our

experiments - the one third part)

8. Why the RW field compression the bunch?

Other questions:


24

Transverse positron motion in the crossed B-field and RW E-field and E-field of the bunch space charge

B

eB

mc

RWi tBi e

0eE

m 24

p

e n

m

22

0 22p

R

e e nE

m m

24

B

Solution of equations of particle bunch dynamics in the trap


600 kHzRW drift

necf f

B

The motion in the traversal plan is drift in crossed a longitudinal magnetic field and an electric field of a space charge.

2

2

21 1

2pB

RWB

2RW B

21

22

2pB

RWB

nec

B

1p

B

In the solution there is the resonance on frequency of the RW field.

Frequency and direction of rotation of the RW field coincides with frequency and direction of

this drift!

2 . 107 1 . 107 0 1 . 107 2 . 107

205 000

210 000

215 000

220 000

225 000

600кГц

V,[cm/s]

Rotation in the direction opposite to the drift

Dependence of particle rotation velocityon RW frequency

Rotation in the drift direction


25

fRW,Hz

Optimal RW frequency


The resonant RW-field increments orbital velocity of particles in the trap!


26

Numerical simulation of particle motion in the trap• Particle collective motion, drift in the space charge and B fields• Gaussian distribution of the particle density• Longitudinal motion of the particle in the trap• “The overstep” method


RW off, transverse plan

Center of the trap

Bunch

RW electrodes

We see positron motion in the crossed B-field and E-field of the bunch space charge.

The orbital drift motion of particles retains them from thermal drift on the trap walls.

But inhomogeneities of the magnetic field and dispersion on residual gas lead to losses of orbital velocity of particles and bunch expansion and

decay!


27


RW rotation in particle drift direction

RW on, transverse plan The resonant RW field in addition torque up the bunch.

It reduces thermal drift by walls of the trap and increments the lifetime, stability and number of

particles storage in the bunch!

The bunch has the dipole moment relative trap centre.

The maximum torque is transmitted the bunch in case of perpendicularity of a RW-field and

the dipole moment:

Torque balanced steady states of single component plasmas

RW dPM

dt

d E

is minRWM is maxRWM

28



Effect of particle collisions with buffer gas molecules:

energy losses in inelastic collisions.

Role of buffer gas and longitudinal motion of particles

1 3driftlong RWT T s

Effect segmented electrodes should occupy not all storage region and the scenario particle storage:

•For magnification of torque of the bunch the moments injection and a particle start in RW-electrodes should be synchronised with orientation of the rotating field.

•The longitudinal periodic motion leads to insert of RW field during injection of the particle in RW-electrodes and to cutout its exit of the particle from RW-electrodes.

•Therefore RW-electrodes occupy not all storage region and periodicity of the longitudinal motion is compounded with frequency of the rotating field.

So we manage to explain stabilisation of the bunch and lifetime magnification, but the mechanism of compression of the bunch,

observed in experiment, is not clear yet.

B

RW electrodes

4. Methods of positron lifetime increase and bunch compressionThe main results:

1. Optimal parameters of the Surko trap at LEPTA have been found:- magnetic field value B>1000 G- base vacuum ~ 10-9 Torr- buffer gas pressure in storage region 10-6 Torr,- RW amplitude = 0.5 V and frequency 600 kHz,- RW rotation direction along the particle drift;

2. Compression and stabilization of the stored bunch by RW-field application: - achievable bunch life time > 100 sec,

- achievable stored particle number > 109 (electrons), 107 ? positrons

- achievable bunch transverse size < 1 cm;3. Bunch intensity increase by the controlled storage regime:

- dynamic magnification of frequency of the RW field and depth of the potential well with growth of number of the

storage up particles.

29 COOL 11 , M. Eseev, 14 September 2011

30

Plans of the new experiments with the Surko trap

1) Storage of positrons in the Surko trap and extraction of monochromatic bunch

for positron annihilation spectroscopy (transfer channel is under design)

2) Transverse e+ bunch profile measurement with CCD camera;

3) Positron injection into LEPTA storage ring and positronium generation.

5. Nearest plans

Basic vacuum in the trap is critical parameter at positronaccumulation

20 effr cZ n The annihilation rate

G. F. Gribakin, J. A. Young , C. M. Surko // Review of modern physics. 2010, Vol. 82 – P. 2257.

He Zeff=4, H2 Zeff=14.6N2 Zeff=30, O2 Zeff=36

C10H8 Zeff=1240000

pPs

oPse++ e- =


Summary and Outlook

31

1. “The Rotating Wall” method was studied experimentally at LEPTA

injector and a high efficiency of particle storage with RW application

has been obtained

2. Optimal Surko trap parameters have been found

3. It was found that the RW mechanisms were discussed at the LEPTA

Trap parameters

4. Methods of optimization of the particle storage and bunch

compression in the Surko trap has been obtained

5. First experimental results of positron storage in the LEPTA trap have

been presented


Thank you for attention!

Documents

Workshop on Beam Cooling and Related Topics COOL 11 12 – 16 September 2011, Alushta, Ukraine