Download
1 / 8
80 likes | 265 Views
Roadmap for Achieving a BEC of Positronium * Edison Liang, Rice University Motivation Positronium (Ps) is the lightest, simplest, weakly interacting, purely leptonic atom with many unique physical properties. A Bose-Einstein condensate (BEC) of Ps, if achievable,
E N D
Roadmap for Achieving a BEC of Positronium* Edison Liang, Rice University Motivation Positronium (Ps) is the lightest, simplest, weakly interacting, purely leptonic atom with many unique physical properties. A Bose-Einstein condensate (BEC) of Ps, if achievable, represents a new quantum regime of matter with many exotic fundamental properties and potentially transformative technological applications, from Doppler free spectroscopy, tests of QED, to annihilation gamma-ray laser (GRASAR). Recent advances in laser pair production demonstrate that the high density of positrons needed to achieve a BEC of Ps may soon be reachable in the laboratory. *research supported by DOE DE-SC0001481.
Advantages of using laser-created positron beams 1. Short-pulse (~ps). High current (≥ 1021e+/s) 3. High initial density (≥1017 e+/cc) 4. Narrow beam size (~100 microns) 5. Moderate energies (MeV’s instead of GeV’s) 6. Energy efficiency (~ few % of laser energy can be converted into positrons)
Major Technical Challenges 1. Positron yield ≥ 1013 per pulse 2. Positron density ≥ 1018/cc (=BEC critical density at cryogenic temperatures) 3. Slow/Cool positrons from MeV’s to ~10eV in short times (< ns) 4. Trap positrons with density ≥ 1018/cc in a volume ~ mm x 0.1mm x0.1mm.
Strategies to achieve High Positron Yield To optimize positron yield for a given laser pulse energy, we propose to use hotter incident electrons and thicker targets. We need to explore the regime with kTehot > 10 MeV, and thickness ≥ 5 mm. 2. We need to revisit the use of double-sided irradiation and longer pulses. 3. We need to optimize target shape to allow more positrons to escape.
Strategies for Collimation and Cooling 1. Use axial B (> 10 MG) to collimate the pair jet (rgyro = 2 micron /B7). Such B can be created using Helmholtz coils driven by long-pulse lasers. 2. Use intense IR lasers to cool hot electrons via resonant Compton scattering. Since resonant cross section >> Thomson cross section the cooling efficiency is enhanced. Preliminary estimates suggest <100 kJ of >10m laser energy is sufficient to cool a region ~ 0.1mm across. Such cooling may be achieved in < 1 ns.
Strategy for Ps and BEC formation Once the positrons are cooled to ~10 eV they can be dumped out electrostatically as a slow beam and injected into a cryogenically cooled porous silica matrix or aerogel Ps converter. 2. Formation and themalization rate of Ps need to be modeled carefully to predict the number and fraction of Ps that will condense into the p=0 state, and the rate of global BEC formation 3. Techniques for detecting and measuring the BEC need to be developed.
Strategy for making a GRASAR 1. The stimulated annihilation cross-section of pPs with only natural broadening is 10-20cm-2. Hence a Ps column density of 1021cm-2 is needed for gL=10 amplification. For a 1-m wide column we need a total of 1013 Ps. 2. To limit the loss of spontaneous annihilation, we will start with a long narrow column of oPs and flip the oPs into pPs with a 204 GHz microwave pulse sweeping in only one direction. 3. Detail physics of the GRASAR must be modeled carefully before experiments.
References 1. Cassidy, D. & Mills, A. 2007, Phys. Stat. Solidi A 4 3419. 2. Chen, H. et al 2009 PRL 102, 105001. 3. Charlton, M. & Humberston, J. 2001 Positron Physics (Cambridge, UK). 4. Cowan, T. et al 1999, Laser Part. Beams 17, 773. 5. Liang, E. & Dermer, C. 1988, Opt. Comm. 65, 419. 6. Liang, E., Wilks, S. & Tabak, M.1998, PRL 81,4887. 7. Liang, E. 2002, AIP Conf. Proc. 611 p.369(AIP, NY) 8. Nakashima, K. & Takabe, H. 2002, Phys. of Plasmas 9, 1505. 9. Surko, C. & Greaves, R. 2004, Phys. of Plasmas 11, 2333.
