Edison Liang, Rice University

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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1

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
.

2

Advantages of using laser
-
created positron beams


1.

Short
-
pulse (~ps).

2.
High current (≥ 10
21
e+/s)

3. High initial density (≥10
17

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)

3

Major Technical Challenges



1. Positron yield ≥ 10
13

per pulse



2. Positron density ≥ 10
18
/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 ≥ 10
18
/cc in a


volume ~ mm x 0.1mm x0.1mm.


4

Strategies to achieve High Positron Yield


1.
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


kT
ehot

> 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
.



5

Strategies for Collimation and Cooling


1. Use axial B (> 10 MG) to collimate the pair jet


(r
gyro

= 2 micron

/B
7
). 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

>10

m laser


energy is sufficient to cool a region ~ 0.1mm


across. Such cooling may be achieved in < 1 ns.



6

Strategy for Ps and BEC formation


1.
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.


7

Strategy for making a GRASAR


1.

The stimulated annihilation cross
-
section of


pPs with only natural broadening is 10
-
20
cm
-
2
.


Hence a Ps column density of 10
21
cm
-
2

is


needed for gL=10 amplification. For a 1
-

m


wide column we need a total of 10
13

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.

8

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.