- Email: [email protected]
Development of a positron trap for the laser cooling experiment of positronium
Applied Surface Science 149 Ž1999. 106–109
Development of a positron trap for the laser cooling experiment of positronium Tetsuro Kumita a,) , Hokuto Iijima a , Yoshiyuki Igura a , Mitsuhiro Irako a , Jun’ichi Kaneko a , Tachishige Hirose a , Nagendra Nath Mondal a , Katsuyuki Kobayashi b, Yasuhiro Okada c , Masatoshi Kajita d a
Department of Physics, Tokyo Metropolitan UniÕersity, 1-1 Minami-Ohsawa, Hachioji-shi, Tokyo 192-0397, Japan b Femtosecond Technology Research Association (FESTA), Japan c Sumitomo HeaÕy Industries, Japan d Communications Research Laboratory, Japan
Abstract A cloud of ortho-positronium produced at the room temperature can be cooled down to 0.6 K utilizing a long pulse Ž180 ns. laser with 243 nm wavelength. We developed a time bunching system of a slow positron beam to synchronize production of positronium atoms to the laser pulse. After a theoretical examination using a Monte Carlo simulation, an experiment was made for confirmation. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 03.75.F; 32.80.P; 36.10.D; 52.55.L Keywords: Laser cooling; Positronium; Bose–Einstein condensation; Positron trap
1. Introduction We have been carrying out theoretical and experimental studies on laser cooling of positronium ŽPs.. Doppler cooling of Ps can be realized utilizing an ultraviolet laser with 243 nm wavelength, which corresponds to the energy of 1s–2p transition w1x. After 28 cycles of 1s–2p stimulated absorption and 2p–1s spontaneous emission, a Ps cloud at room temperature are cooled down to its possible lowest
)
Corresponding author. Tel.: q81-426-77-1111 ext. 3328; Fax: q81-426-77-2483; E-mail: [email protected]
temperature 0.6 K, which corresponds to the one photon recoil limit. Since lifetime of the 2p–1s spontaneous emission is 3.2 ns and probability of a positron to share the 2p state is 1r2 for saturated intensity of laser light, the cooling process takes about 180 ns, which is as short as the lifetime of ortho-Ps Ž142 ns.. Rapid cooling of Ps may lead us to a lot of interesting physics, such as Bose–Einstein condensation of Ps w2x. In order to achieve laser cooling of ortho-Ps, we have developed a long pulse laser system whose repetition rate is 25 Hz. Production timing of Ps atoms has to be synchronized to the laser pulse, so that a pulsed positron beam with 25 Hz repetition is required. We developed a new bunching system for RI-based positron beams for this purpose.
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 8 2 - 8
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
2. Laser source The special laser system for the Ps cooling experiment is developed with Aculight w3x. The system consists of a 972 nm Cr:LiSAF ŽCr 3q-doped LiSrAlF6 . laser, a second harmonic generator, and a fourth harmonic generator as shown in Fig. 1. Specifications of the laser source are as follows: 1. Wavelength: 243 nm 2. Pulse length: 160 ns ŽFWHM. 3. Bandwidth: 140 pm ŽFWHM. 4. Power: 60 mJ 5. Repetition rate: 25 Hz The long pulse length is required from the cooling time Ž180 ns. and the wide bandwidth is to compensate the Doppler shift of the wavelength due to thermal motion of the Ps at the room temperature. Cr:LiSAF is chosen because it has long lifetime of the excited state Ž67 ms. and low gain, which result long laser pulse. The pulse length and the bandwidth of 972 nm light from the Cr:LiSAF are measured to be 250 ns ŽFWHM. and 1.2 nm ŽFWHM., respectively. The second and the fourth harmonic generators consist of six BBO Žb-Ba borate. crystals, respectively. Each of six crystals can be aligned independently to optimize intensity of the laser light. This
107
structure is to keep the wide bandwidth of the laser light after conversion from 972 to 486 nm Ž243 nm. by the second Žfourth. harmonic generator.
3. Positron trap We utilize the positron beam line TOPPS ŽTOkyo metropolitan university Polarized Positron System., whose details are described elsewhere w4x, to produce Ps atoms. TOPPS consists of a 100 mCi 22 Na RI source, a transmission moderator Ž6 mm polycrystal tungsten., and a magnetic beam transport system. TOPPS has a beam buncher developed by Sumitomo Heavy Industries w5x, which can form 1 ns bunches of positrons by velocity modulation, for lifetime measurement of ortho-Ps. It compresses positrons come during a time period of 50 ns into 1 ns, so that 0.005 positrons stay in a pulse if 10 5 positronsrs are injected to the buncher. For the experiment of laser cooling, positron pulses have to be synchronized with the laser beam, whose repetition rate is 25 Hz. The buncher mentioned above is not suitable for this purpose because intensity of positrons in a pulse is too low. Thus, we are developing a new pulsing system to produce about 10 ns pulses with the repetition rate of 25 Hz.
Fig. 1. A schematic view of the 243 nm laser system.
108
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
Basic idea of the pulsing system is shown in Fig. 2. Positrons are trapped in an electrostatic potential well and then extracted as a pulsed beam by opening the potential gate. Injected positrons are longitudinally confined by the electric potential and radially by the axial magnetic field. In order for the positrons to be trapped, they have to lose their kinetic energy in the potential well. A variety of energy-loss mechanisms, such as inelastic collision with gas molecules, are proposed and applied by other groups till now. Here, we propose a new mechanism adopting a transverse magnetic field as in Fig. 2. This apparatus can be smaller in size and lower in cost compared with other systems because of its simple structure. A positron crosses the transverse magnetic field changes its momentum direction and thus lose the longitudinal component of kinetic energy, so that it cannot pass the potential gate at the entrance again. To estimate the performance of this positron trap, a Monte Carlo simulation is performed using a computer code POEM ŽPOlarized beam simulator in Electric and Magnetic fields. w6x, in which the relativistic equation of motion in electromagnetic fields is solved by Runge–Kutta method. The electric and the magnetic fields are calculated by POISCR w7x, which solves Poisson equation numerically with the geometry of coils and electrodes, and fed into POEM. Conditions of the simulation are set as follows. The electric potential of the entrance Žexit. gate is 199 V
Fig. 2. A schematic view of the positron trap.
Fig. 3. Simulated positron trajectory in the positron trap.
Ž201 V.. Injected positrons are monochromatic with the kinetic energy of 200 eV and their directions are at the angle of 0.18 with respect to the beam axis. Fig. 3 shows a typical simulated trajectory of a positron. The simulation shows about 60% of incident positrons are trapped for 50 ms as in Fig. 4. A prototype of the pulsing system is tested using an electron beam with the energy of 200 eV. Here, electric potential at the entrance Žexit. gate is set to 198 V Ž202 V., the axial magnetic field is 80 Gauss, and the transverse field is 5 G. Trapped positrons are extracted by lowering the exit potential down to 192 V for 100 ns. Fig. 5 shows number of electrons in a 100 ns pulse is enhanced when the trapping system is working. It should be noted that the enhancement
Fig. 4. Number of positrons trapped in the Monte Carlo simulation. The number is normalized to one for 0 ns.
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
109
4. Summary We developed the experimental apparatus for laser cooling experiment, such as the long pulse 243 nm laser and the positron trap. Performance of the positron trap is examined using an electron beam. We will soon start the experimental study of Ps laser cooling.
Acknowledgements
Fig. 5. Number of electrons measured at the lower stream of the trap. Kinetic energy of electrons is 200 eV, electric potential of the entrance Žexit. gate is 198 eV Ž202 eV., and the axial and transverse magnetic fields are 80 and 5 G, respectively. Solid Ždashed. line is for the case of the trap turned on Žoff.. The exit potential is lowered down to 192 eV for 220-T - 320 ns. About 8% of trapped electrons are extracted while the exit gate is open.
is not large enough because the exit potential gate is open by 8 V and only about 8% of trapped electrons are expected to be extracted. The Monte Carlo simulation shows 90% of positrons can be trapped for 50 ms by optimizing shape of the electric potential and a pulsed beam with about 200 positrons per pulse can be formed from a DC beam of 10 5 positronsrs by increasing the extraction efficiency.
We would like to thank Dr. A. Endo and Mr. M. Yorozu of Sumitomo Heavy Industries, for their suggestions on long pulse laser. This research was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.
References w1x w2x w3x w4x
E.P. Liang, C.D. Dermer, Opt. Commun. 65 Ž1988. 419. P.M. Platzman, A.P. Mills Jr., Phys. Rev. B 49 Ž1994. 454. Aculight, http:rrwww.aculight.comr. M. Irako, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, T. Kumita, K. Matsuzawa, N.N. Mondal, Appl. Surf. Sci. 149 Ž1999. 16, these Proceedings. w5x T. Nakajyo, M. Hirose, Appl. Surf. Sci. 149 Ž1999. 34, these Proceedings. w6x T. Kumita, POEM, A Simulation Program for the Spin Motion, Preprint TMU-HEPrExp 97-1. w7x THE CERN-POISSON PROGRAM PACKAGE ŽPOISCR., CERN Program Library Long Writeup T604.
Development of a positron trap for the laser cooling experiment of positronium Tetsuro Kumita a,) , Hokuto Iijima a , Yoshiyuki Igura a , Mitsuhiro Irako a , Jun’ichi Kaneko a , Tachishige Hirose a , Nagendra Nath Mondal a , Katsuyuki Kobayashi b, Yasuhiro Okada c , Masatoshi Kajita d a
Department of Physics, Tokyo Metropolitan UniÕersity, 1-1 Minami-Ohsawa, Hachioji-shi, Tokyo 192-0397, Japan b Femtosecond Technology Research Association (FESTA), Japan c Sumitomo HeaÕy Industries, Japan d Communications Research Laboratory, Japan
Abstract A cloud of ortho-positronium produced at the room temperature can be cooled down to 0.6 K utilizing a long pulse Ž180 ns. laser with 243 nm wavelength. We developed a time bunching system of a slow positron beam to synchronize production of positronium atoms to the laser pulse. After a theoretical examination using a Monte Carlo simulation, an experiment was made for confirmation. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 03.75.F; 32.80.P; 36.10.D; 52.55.L Keywords: Laser cooling; Positronium; Bose–Einstein condensation; Positron trap
1. Introduction We have been carrying out theoretical and experimental studies on laser cooling of positronium ŽPs.. Doppler cooling of Ps can be realized utilizing an ultraviolet laser with 243 nm wavelength, which corresponds to the energy of 1s–2p transition w1x. After 28 cycles of 1s–2p stimulated absorption and 2p–1s spontaneous emission, a Ps cloud at room temperature are cooled down to its possible lowest
)
Corresponding author. Tel.: q81-426-77-1111 ext. 3328; Fax: q81-426-77-2483; E-mail: [email protected]
temperature 0.6 K, which corresponds to the one photon recoil limit. Since lifetime of the 2p–1s spontaneous emission is 3.2 ns and probability of a positron to share the 2p state is 1r2 for saturated intensity of laser light, the cooling process takes about 180 ns, which is as short as the lifetime of ortho-Ps Ž142 ns.. Rapid cooling of Ps may lead us to a lot of interesting physics, such as Bose–Einstein condensation of Ps w2x. In order to achieve laser cooling of ortho-Ps, we have developed a long pulse laser system whose repetition rate is 25 Hz. Production timing of Ps atoms has to be synchronized to the laser pulse, so that a pulsed positron beam with 25 Hz repetition is required. We developed a new bunching system for RI-based positron beams for this purpose.
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 8 2 - 8
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
2. Laser source The special laser system for the Ps cooling experiment is developed with Aculight w3x. The system consists of a 972 nm Cr:LiSAF ŽCr 3q-doped LiSrAlF6 . laser, a second harmonic generator, and a fourth harmonic generator as shown in Fig. 1. Specifications of the laser source are as follows: 1. Wavelength: 243 nm 2. Pulse length: 160 ns ŽFWHM. 3. Bandwidth: 140 pm ŽFWHM. 4. Power: 60 mJ 5. Repetition rate: 25 Hz The long pulse length is required from the cooling time Ž180 ns. and the wide bandwidth is to compensate the Doppler shift of the wavelength due to thermal motion of the Ps at the room temperature. Cr:LiSAF is chosen because it has long lifetime of the excited state Ž67 ms. and low gain, which result long laser pulse. The pulse length and the bandwidth of 972 nm light from the Cr:LiSAF are measured to be 250 ns ŽFWHM. and 1.2 nm ŽFWHM., respectively. The second and the fourth harmonic generators consist of six BBO Žb-Ba borate. crystals, respectively. Each of six crystals can be aligned independently to optimize intensity of the laser light. This
107
structure is to keep the wide bandwidth of the laser light after conversion from 972 to 486 nm Ž243 nm. by the second Žfourth. harmonic generator.
3. Positron trap We utilize the positron beam line TOPPS ŽTOkyo metropolitan university Polarized Positron System., whose details are described elsewhere w4x, to produce Ps atoms. TOPPS consists of a 100 mCi 22 Na RI source, a transmission moderator Ž6 mm polycrystal tungsten., and a magnetic beam transport system. TOPPS has a beam buncher developed by Sumitomo Heavy Industries w5x, which can form 1 ns bunches of positrons by velocity modulation, for lifetime measurement of ortho-Ps. It compresses positrons come during a time period of 50 ns into 1 ns, so that 0.005 positrons stay in a pulse if 10 5 positronsrs are injected to the buncher. For the experiment of laser cooling, positron pulses have to be synchronized with the laser beam, whose repetition rate is 25 Hz. The buncher mentioned above is not suitable for this purpose because intensity of positrons in a pulse is too low. Thus, we are developing a new pulsing system to produce about 10 ns pulses with the repetition rate of 25 Hz.
Fig. 1. A schematic view of the 243 nm laser system.
108
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
Basic idea of the pulsing system is shown in Fig. 2. Positrons are trapped in an electrostatic potential well and then extracted as a pulsed beam by opening the potential gate. Injected positrons are longitudinally confined by the electric potential and radially by the axial magnetic field. In order for the positrons to be trapped, they have to lose their kinetic energy in the potential well. A variety of energy-loss mechanisms, such as inelastic collision with gas molecules, are proposed and applied by other groups till now. Here, we propose a new mechanism adopting a transverse magnetic field as in Fig. 2. This apparatus can be smaller in size and lower in cost compared with other systems because of its simple structure. A positron crosses the transverse magnetic field changes its momentum direction and thus lose the longitudinal component of kinetic energy, so that it cannot pass the potential gate at the entrance again. To estimate the performance of this positron trap, a Monte Carlo simulation is performed using a computer code POEM ŽPOlarized beam simulator in Electric and Magnetic fields. w6x, in which the relativistic equation of motion in electromagnetic fields is solved by Runge–Kutta method. The electric and the magnetic fields are calculated by POISCR w7x, which solves Poisson equation numerically with the geometry of coils and electrodes, and fed into POEM. Conditions of the simulation are set as follows. The electric potential of the entrance Žexit. gate is 199 V
Fig. 2. A schematic view of the positron trap.
Fig. 3. Simulated positron trajectory in the positron trap.
Ž201 V.. Injected positrons are monochromatic with the kinetic energy of 200 eV and their directions are at the angle of 0.18 with respect to the beam axis. Fig. 3 shows a typical simulated trajectory of a positron. The simulation shows about 60% of incident positrons are trapped for 50 ms as in Fig. 4. A prototype of the pulsing system is tested using an electron beam with the energy of 200 eV. Here, electric potential at the entrance Žexit. gate is set to 198 V Ž202 V., the axial magnetic field is 80 Gauss, and the transverse field is 5 G. Trapped positrons are extracted by lowering the exit potential down to 192 V for 100 ns. Fig. 5 shows number of electrons in a 100 ns pulse is enhanced when the trapping system is working. It should be noted that the enhancement
Fig. 4. Number of positrons trapped in the Monte Carlo simulation. The number is normalized to one for 0 ns.
T. Kumita et al.r Applied Surface Science 149 (1999) 106–109
109
4. Summary We developed the experimental apparatus for laser cooling experiment, such as the long pulse 243 nm laser and the positron trap. Performance of the positron trap is examined using an electron beam. We will soon start the experimental study of Ps laser cooling.
Acknowledgements
Fig. 5. Number of electrons measured at the lower stream of the trap. Kinetic energy of electrons is 200 eV, electric potential of the entrance Žexit. gate is 198 eV Ž202 eV., and the axial and transverse magnetic fields are 80 and 5 G, respectively. Solid Ždashed. line is for the case of the trap turned on Žoff.. The exit potential is lowered down to 192 eV for 220-T - 320 ns. About 8% of trapped electrons are extracted while the exit gate is open.
is not large enough because the exit potential gate is open by 8 V and only about 8% of trapped electrons are expected to be extracted. The Monte Carlo simulation shows 90% of positrons can be trapped for 50 ms by optimizing shape of the electric potential and a pulsed beam with about 200 positrons per pulse can be formed from a DC beam of 10 5 positronsrs by increasing the extraction efficiency.
We would like to thank Dr. A. Endo and Mr. M. Yorozu of Sumitomo Heavy Industries, for their suggestions on long pulse laser. This research was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.
References w1x w2x w3x w4x
E.P. Liang, C.D. Dermer, Opt. Commun. 65 Ž1988. 419. P.M. Platzman, A.P. Mills Jr., Phys. Rev. B 49 Ž1994. 454. Aculight, http:rrwww.aculight.comr. M. Irako, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, T. Kumita, K. Matsuzawa, N.N. Mondal, Appl. Surf. Sci. 149 Ž1999. 16, these Proceedings. w5x T. Nakajyo, M. Hirose, Appl. Surf. Sci. 149 Ž1999. 34, these Proceedings. w6x T. Kumita, POEM, A Simulation Program for the Spin Motion, Preprint TMU-HEPrExp 97-1. w7x THE CERN-POISSON PROGRAM PACKAGE ŽPOISCR., CERN Program Library Long Writeup T604.