POSITRON ACCUMULATOR SCHEME for AEGIS

Transcription

1 POSITRON ACCUMULATOR SCHEME for AEGIS A. S. Belov, S. N. Gninenko INR RAS, Moscow 1

2 What positron beam is requiered for AEGIS? Number of antihydrogen atoms produced with AEGIS scheme: N Hbar ~ ce n H- L i N e+e- / e+e- N e+e- = N e+ where N e+ - number of incident positrons, - is efficiency of cold orthopositronium production Number of antihydrogen atoms produced is proportional to number of positrons firing a positronium production target Expected ~ 10 8 positrons per period of cold antiprotons production ( ~200 sec) 2

3 Positron beam time structure Time structure of positron pulses is determined mainly by lasers pulses duration: 4 ns Another process which requires pulsed mode of operation is acceleration of antihydrogen atoms formed in the intersection region Positron beam has to be accumulated during ~ 200 sec, final duration ~ ns Time compression ratio should be ~10 10!!! A trap system have to use nonconservative interactions to avoid restrictions of Louiville theorem 3

4 Types of accumulators Penning trap: axial magnetic field for radial confinement and electric field for axial confinement of charged particles Nonconservative: capture due to collisions with buffer gas (Surko-type trap) collisions with ions with electrons Conservative: ramping of well depth magnetic mirrors. 4

5 Positrons are captured into trap due to energy loss in collision with N 2 molecules Positrons energy is chosen to ~ 9 ev in first stage to produce electronic excitation of N 2 molecules and minimise positronium production in the collisions Surko-type positron trap: operation principle 5

6 Surko-type positron trap: operation principle Buffer gas pressure in section 1 is ~10-2 mbar to have high probability for inelastic collision of positron during its one passage through the trap ( ~10-16 cm 2 ) 6

7 Surko-type positron trap -operation principle During ~1ms a trapped positrons undergo second and third inelastic collision and are stored finally in section 3 of the trap Stored positrons are cooled during 0.1 sec to surrounding gas temperature due to inelastic collisions with gas molecules (excitation of rotational and vibrational states) Buffer gas pressure in section 3 is ~ mbar to minimize annihilation losses 7

8 Surko-type positron trap: trapping efficiency Efficiency of trapping (ratio of number of accumulated positrons to number of injected positrons) reaches ~20-30%. It is determined by ratio of cross-sections of electronic excitation of gas molecules to crosssection of positronium production. In practice, pressures and electrodes potentials are tuned to maximize the efficiency. 8

9 Surko-type positron trap: trapping efficiency The accumulation process is not linear in time Saturation of total number of stored positrons is observed The saturation is due to annihilation of stored positrons and depends on gas pressure in 3rd stage of the accumulator Highest initial trapping efficiency (~20-30%) was obtained with pressure of!0-6 mbar, highest total number of stroed positrons is obtained with pressure of!0-7 mbar in 3rd section of a trap Total number of stored positrons 10 8 is obtained with 22 Na source of ~60 mci during ~ 200 sec. 9

10 Limitations for number of accumulated positrons Annihilation losses: n n eff p = mbar = 60 sec Space-charge: = Ne+/L (1+2 log e (R W /R p )) =10 V, R W /R p =5, L=20 cm, Ne+ = How to overcome these limitations? 10

11 Proposed accumulator scheme: Main Ideas: Use an additional trap on-line with Surko trap in the same magnetic field of ~ 0.15 T with good vacuum and better geometry for space-charge limit Surko trap can work with highest trapping efficiency with vacuum in 3rd stage of ~ 10-6 mbar. Positrons are stored in the Surko trap during ~ 20 sec. Then positrons are transferred to electrodynamical trap with vacuum of mbar, where positrons are accumulated during ~ 200 sec 11

12 Scheme of the proposed new positron trap 12

13 New positron trap Positrons are stored in the Surko-type trap during ~ 20 sec. Then positrons are extracted from the Surko-trap and are injected into the electrodynamical trap. Capture of positrons in the electrodynamical trap occurs due to increase of depth of a potential well during injection of positrons (ramping of the potential) 13

14 New positron trap Condition for the capture: /e 14

15 New positron trap advantages: Higher total trapping efficiency which is determined by operation of Surko-type trap with accumulation time of ~ 20 sec Vacuum in final stage of the accumulator can be two orders of magnitude better in comparison with the Surko-trap reduce annihilation losses of positrons in the trap two orders of magnitude better vacuum will lead to smaller gas load to main magnet during injection of positrons space -charge limit can be shifted away in some degree 15

16 Summary Combination of Surko-type trap and electrodynamical trap of positrons is proposed for AEGIS The new scheme efficiency is expected to be 20-30% for full accumulation time of 200 sec Residual vacuum in the output of the accumulator can be a factor 10 2 better than in final stage of Surko-type trap. Better vacuum will lead to reduce of annihilation rate of positrons by a factor of 10 2 Space - charge constraints can be relaxed due to proper choice of geometrical parameters of the electrodynamical trap expected number of accumulated positrons ~ ( 22 Na source of ~50 mci, moderation efficiency~ , trapping efficiency ~ 25%, 200 sec) 16