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

Size: px

Start display at page:

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

Kimberly Cole
6 years ago
Views:

1 Workshop on Beam Cooling and Related Topics COOL September 011, 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

2 Contents Introduction: The LEPTA facility 1. Surko trap and rotating wall technique. Experimental results of particle storage in trap with rotating wall (RW).1. Examination of storage and lifetime of charge bunch in the trap.. 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.. 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

3 Introduction: The LEPTA facility cooling section kicker Low Energy Particle Toroidal Accumulator 10E6 100sec=10E8 e+ positron trap Helical quadrupole 10E4 o-ps per sec Ps detector Na 10E6 e+ per sec 3

4 Introduction: The LEPTA facility Trap & Injector 4

5 1. Surko trap and rotating wall technique 000 mm eu Area 1 Area Area 3 Buffer gas, Pressure, Torr е + N N z 5

6 Surko trap and rotating wall technique Rotating wall (RW) technique: rotating electric field at the trap entrance RF generator Phase shifter (а) B R~0.1 1 cm -V(x) (b) (c) L~30 40 cm 6

7 Surko trap and rotating wall technique Principles of electron or positron storage in the Surko trap In application to LEPTA facility I II III IV V VI VII VIII V e

8 3. Experimental results of particle storage in trap with rotating wall (RW) 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 (000). T.J. Murphy and C.M. Shurko, Phys. Plasmas, 8, 1878 (001). J. R. Danielson, C. M. Surko, and T. M. O Neil PRL., 99, (007). With antiprotons (Hbar production, ALPHA, CERN) J. R. Danielson, et al., PRL. 100, (008). G. B. Andresen, et al., Nature, 468, 673 (010). 8

9 xperimental results of particle storage in trap with rotating wall (RW) Our experimental results of electron storage Зависимость числа накопленных электронов от времени накопления 9,00E+07 Stored electron number vs time N 8,00E+07 7,00E+07 6,00E+07 5,00E+07 4,00E+07 3,00E+07,00E+07 1,00E+07 0,00E t, сек Optimal f rotating = 600 khz, Amplitude = 1 V, ε = 0.4, τ life = 80 s 1 3 Pressure and potential distributions are optimized Same + transverse correction field is optimized Same + rotating field is ON and optimized 9

10 First experiments on slow positron accumulation in the positron trap have been performed. June 010 The trapped positron number (arb. units) PMT signal (proportional to the trapped positrons number) vs accumulation time Upt, mv t, s 10

11 eu, В 38 N=3*10 36 the potential allows 8 us number in the bunch ~ 8 U, V 6 4 Particle storage and the space charge limit Dynamical control of to increase the particle N=6*10 7 N=0 N=*10 8 by times! L, мм I L, mm II III 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. IV V VI VII VIII V e Dependence of depth of the potential well on number of the stored up electrons 11

12 Experimental results of particle storage in trap with rotating wall (RW) experimental results of electron storage, RW role RW resonance, frequency Stored electron number was measured by pulsed extraction of electrons to the collector RW resonance voltage 1

13 Experimental results of particle storage in trap with rotating wall (RW) RW off,on B=1300G,ton off=80s 8.00E+07 RW on RW off after 80 sec RW on after 80 sec 6.00E+07 N 4.00E+07.00E E t,s RWoff-RWon RWon-RWoff RW on 1 3

14 Experimental results of particle storage in trap with rotating wall (RW) Transverse bunch profile, CCD camera MCP + Luminescent screen + CCD camera *) 1 cm *) Courtesy of Budker INP colleagues 1 4

15 Experimental results of particle storage in trap with rotating wall (RW) Dynamics of extracted bunch profile 0 sec RW on 5 sec RW on 10 sec RW on 15 sec RW on 0 sec RW on 5 sec RW on 30 sec RW on 30 sec RW off 30 sec RW opposite drive 1 5

Experimental results of particle storage in trap with rotating wall (RW) RW influence on electron storage, RW on Injection and accumulation (30 s)

16 Experimental results of particle storage in trap with rotating wall (RW) RW influence on electron storage, RW on Injection and accumulation (30 s) injection off delay (confinement) extraction We see as the RWfield during time confinement works. The effect is the bunch transverse compression. 1 6

17 perimental results of particle storage in trap with rotating wall (RW) Injection and accumulation RW on (30 s) injection off delay (confinement) RW off extraction We see as the RWfield OFF works. The effect is the bunch transverse expansion. 1 7

18 perimental results of particle storage in trap with rotating wall (RW) Injection and accumulation RW off (30 sec) injection off delay (confinement) RW on extraction We see as the RWfield ON works. The effect is the bunch transverse compression. 1 8

Gaussian distribution of the density Fitting of transverse plane CCD photo ( )

19 perimental results of particle storage in trap with rotating wall (RW) Electron bunch compression. Gaussian distribution of the density Fitting of transverse plane CCD photo ( ) = ( ) ( ) nr nga 0 exp rσ / 1/ ( ) = ( π ) exp[ ] G A A A σ = ( ) ( ) A et / Vc Vex ϕ r λ D fcn C B A cm x A B C 1 9

20 perimental results of particle storage in trap with rotating wall (RW) Measurements of the bunch size and density distribution by collector current Beam profile was scanned with movable collector at the trap exit Error function Fcn аааа ж мм жжжжж ммм жж а мм а м ж аа ма ма ж мж а ж м 3 а м ам жа 1 м жа м м м ж а а 1 ж ам жжж а м ж а жа м x f cn Gaussian distribution 1 Normal distribution electron current at various positions of the collector 1. RW on 30 sec. RW on 30 sec, off 5 sec 3. RW on 30 sec, off 10 sec The result of fitting with Gaussin function. Bunch expansion at RW-field off x 0

21 Theory and numerical simulations of particle motion in the tr Transverse motion of the trapped particle E ω,v/cm 0.05 e + f RW, khz 600 n e,cm ω p, c -1 B, Gauss ω B, c -1 p N, Torr R, cm L, cm Why the direction of rotation of the RW field is important? 1 1. Why the characteristic frequencies such different?. Why there is the resonance on frequency of the RW field?

22 Theory and numerical simulations of particle motion in the trap Longitudinal motion of the trapped particle B -U(x) RW electrodes B long E long The particle injection in the storage region trap with initial energy part Then the particle in inelastic collisions with molecules of buffer gas is cool cooled to a room temperature E part 0.05 ev p N, Torr L~30 40 cm, T = L/ υ long long f res RW 1 T long 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? E 1 ev

Theory and numerical simulations of particle motion in the trap 8.00E+07 RW off,on B=1300G,ton off=80s Other questions: N 6.00E+07 4.00E+07.00E+07 0.00E+00 0 50 100 150 00 50 t,s 6.

23 Theory and numerical simulations of particle motion in the trap 8.00E+07 RW off,on B=1300G,ton off=80s Other questions: N 6.00E E+07.00E E t,s 6. Why the RW field increments quantity of the confinement RWoff-RWon RWon-RWoff RW 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? * J. R. Danielson and C.M. Shurko, Phys. Plasmas, 13, (006). 3

24 Theory and numerical simulations of particle motion in the trap Solution of equations of particle bunch dynamics in the trap Transverse positron motion in the crossed B-field and RW E-field and E-field of the bunch space charge && ξ + iω & ξ ηξ = ε B i e ω RW t e E 0 m ω eb 4π en ω B = ω p = ε = mc m B e 0 e n ω p η = ER = π = m m ω ω ω p B p ωrw = 1± 1 1 [] 1 ω ω B p nec ω ω << ω B RW = = π B ωb B In the solution there is the resonance on frequency of the RW field. ω [ ] RW = ω B Frequency and direction of rotation of the RW field coincides with frequency and direction of this drift! The motion in the traversal plan is drift in crossed a longitudinal magnetic field and an electric field of a space charge. RW nec = fdrift = = 600 khz B f 4

25 Theory and numerical simulations of particle motion in the trap Dependence of particle rotation velocity on RW frequency Rotation in the direction opposite to the drift 5000V,[cm/s] Optimal RW frequency Rotation in the drift direction The resonant RW-field increments orbital velocity of particles in the trap! ґ ґ кГц 0 1. ґ ґ 10 7 f RW,Hz 5

26 Theory and numerical simulations of particle motion in the trap 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! 6

27 Theory and numerical simulations of particle motion in the trap Torque balanced steady states of single component plasmas RW on, transverse plan RW rotation in particle drift direction The resonant RW field in addition torque up the bunch. The bunch has the dipole moment relative trap centre. It reduces thermal drift by walls of the trap and increments the lifetime, stability and number of particles storage in the bunch! The maximum torque is transmitted the bunch in case of perpendicularity of a RW-field and the dipole moment: M θ RW RW M θ is min is max M RW θ dpθ = = d E dt ω 7

Theory and numerical simulations of particle motion in the trap Role of buffer gas and longitudinal motion of particles Effect of particle collisions with buffer gas molecules: drift energy losses in

28 Theory and numerical simulations of particle motion in the trap Role of buffer gas and longitudinal motion of particles Effect of particle collisions with buffer gas molecules: drift energy losses in inelastic collisions. T T 1 3 μs Effect segmented electrodes should occupy not all storage region and the scenario particle storage: 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 RWelectrodes. 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. long 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. RW B RW electrodes 8

29 4. Methods of positron lifetime increase and bunch compression The 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;. Compression and stabilization of the stored bunch by RW-field application: - achievable bunch life time > 100 sec, - achievable stored particle number > 10 9 (electrons), 10 7? 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. 9

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

30 5. Nearest plans Basic vacuum in the trap is critical parameter at positron accumulation The annihilation rate λ = πrcz n 0 eff He Z eff =4, H Z eff =14.6 N Z eff =30, O Z eff =36 C 10 H 8 Z eff = G. F. Gribakin, J. A. Young, C. M. Surko // Review of modern physics. 010, Vol. 8 P. 57. 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) ) Transverse e + bunch profile measurement with CCD camera; 3) Positron injection into LEPTA storage ring and positronium generation. e + + e - = ops pps 3 0

31 Summary and Outlook 1. The Rotating Wall method was studied experimentally at LEPTA injector and a high efficiency of particle storage with RW application has been obtained. 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 3 1

32 Thank you for attention!

Development of the Positron Injector for LEPTA Facility

Development of the Positron Injector for LEPTA Facility V.Bykovsky, M.Eseev *, A.Kobets, I.Meshkov, V.Pavlov, R. Pivin, A.Rudakov, G.Trubnikov, S.Yakovenko * - Lomonosov Pomor State Universitet, Arkhangelsk