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Hisakado TH, Akiyama T, Carlstrom TN. Laboratory tests of CO 2 laser collective Thomson scattering for measurements of ion temperature in the divertor. Rev Sci Instrum 2022; 93:113547. [PMID: 36461511 DOI: 10.1063/5.0101407] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/31/2022] [Accepted: 10/13/2022] [Indexed: 06/17/2023]
Abstract
Collective Thomson scattering (CTS) is a diagnostic method that measures the ion velocity distribution of a plasma. CO2 laser CTS measurements are challenging because of the inherently small Doppler broadening and scattering signals that are difficult to detect. We implemented a heterodyne detection scheme to measure spectrum changes of less than a GHz. To maximize the collected light at small scattering angles, we designed a unique light collection approach consisting of a customized conical-shaped (axicon) lens with a hole in the center. The axicon lens is used to collect the scattered light emitted within an annular cross-section from the scattering volume while the probe beam is passed through the hole at the center of the lens. The performance of the heterodyne detection scheme and annular collection approach was demonstrated using a Transverse Excited Atmospheric pressure CO2 laser with a pulse energy of 160 mJ at λ = 10.59 µm.
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Affiliation(s)
- T H Hisakado
- General Atomics, San Diego, California 92186-5608, USA
| | - T Akiyama
- General Atomics, San Diego, California 92186-5608, USA
| | - T N Carlstrom
- General Atomics, San Diego, California 92186-5608, USA
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Jones ACL, Rutbeck-Goldman HJ, Hisakado TH, Piñeiro AM, Tom HWK, Mills AP, Barbiellini B, Kuriplach J. Angle-Resolved Spectroscopy of Positronium Emission from a Cu(110) Surface. Phys Rev Lett 2016; 117:216402. [PMID: 27911545 DOI: 10.1103/physrevlett.117.216402] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/11/2016] [Indexed: 06/06/2023]
Abstract
The affinity A_{Ps} of positronium (Ps) atoms for a metal is the negative of the maximum kinetic energy with which Ps is emitted into vacuum when thermalized positrons in a metal encounter the surface. When this quantity is measured by ground state Ps time of flight (TOF), the precision is severely limited by the short triplet state lifetime of 142 ns. By quickly converting the emitted Ps atoms into long-lived Rydberg states, we are able to dramatically increase the TOF to allow precision measurements of A_{Ps}. From our measurements made on a Cu(110) sample at T=128 K, we find A_{Ps}(128 K)=(-2.476±0.010_{stat}±0.013_{syst}) eV, compared with the result A_{Ps}(128 K)=(-2.545±0.010_{num}±0.010_{syst}) eV found using highly accurate generalized gradient approximations for both electrons and positrons within density functional theory. Such precision opens up opportunities in the quest for an improved density functional.
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Affiliation(s)
- A C L Jones
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - H J Rutbeck-Goldman
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - T H Hisakado
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - A M Piñeiro
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - H W K Tom
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - A P Mills
- Department of Physics and Astronomy, University of California Riverside, Riverside, California 92521, USA
| | - B Barbiellini
- Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA
| | - J Kuriplach
- Department of Low Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, CZ-180 00 Prague, Czech Republic
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Cassidy DB, Hisakado TH, Tom HWK, Mills AP. Positronium hyperfine interval measured via saturated absorption spectroscopy. Phys Rev Lett 2012; 109:073401. [PMID: 23006369 DOI: 10.1103/physrevlett.109.073401] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2012] [Indexed: 06/01/2023]
Abstract
We report Doppler-free measurements of the positronium (Ps) Lyman-α transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength λ(L) = 243.0218 ± 0.0005 nm, we also observed a crossover resonance at λ(C) = 243.0035 ± 0.0005 nm, arising from the excitation of 1(3)S(1) atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1(1)S(0) ground state. Since (λ(L)-λ(C)) is related to the Ps hyperfine interval E(hfs), this observation constitutes the first optical measurement of this quantity and yields E(hfs) = 198.4 ± 4.2 GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, 92521-0413, USA
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Abstract
We report optical spectroscopic measurements of molecular positronium (Ps(2)), performed via a previously unobserved L=1 excited state. Ps(2) molecules created in a porous silica film, and also in vacuum from an Al(111) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state. Future experiments making use of the photoionized vacuum L=1 Ps(2) could provide a source of Ps(+) ions, as well as other multipositronic systems, such as Ps(2)H(-) or Ps(2)O.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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Abstract
We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2(3)P state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2(3)P atoms to Rydberg levels occurs very efficiently (~90%) and that the ~25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (3)S-2(3)P transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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Abstract
We have observed that the amount of positronium (Ps) emitted from the surface of p-Si(100) is substantially increased if the sample is irradiated with 532 nm laser light just prior to the implantation of positrons. The energy of the emitted Ps has a constant value of ∼0.16 eV and is independent of the Si temperature and the applied laser fluence, while the photoemission yield depends on both of these parameters. These observations are consistent with Ps production via a previously observed excitonlike positron surface state that is populated in response to the production of electron-hole pairs in the Si. Possible applications of Ps photoemission include probing surface electron dynamics on Si, the generation of ultrashort Ps or positron pulses using ps lasers, and efficient production of Ps in cryogenic environments.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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Cassidy DB, Hisakado TH, Tom HWK, Mills AP. Laser excitation of positronium in the Paschen-Back regime. Phys Rev Lett 2011; 106:173401. [PMID: 21635035 DOI: 10.1103/physrevlett.106.173401] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2011] [Indexed: 05/30/2023]
Abstract
Zeeman mixing of singlet and triplet 2P states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the n=2 states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field >0.01 T and is essentially eliminated above ∼2 T. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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Cassidy DB, Hisakado TH, Tom HWK, Mills AP. New mechanism for positronium formation on a silicon surface. Phys Rev Lett 2011; 106:133401. [PMID: 21517381 DOI: 10.1103/physrevlett.106.133401] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2011] [Indexed: 05/30/2023]
Abstract
We describe experiments in which positronium (Ps) is emitted from the surface of p-doped Si(100), following positron implantation. The observed emission rate is proportional to a Boltzmann factor exp{-E(A)/kT}, which is dependent on the temperature T of the sample and a characteristic energy E(A)=(0.253±0.004) eV. Surprisingly, however, the Ps emission energy has a constant value of ∼0.16 eV, much greater than kT. This observation suggests the spontaneous emission of energetic Ps from a short-lived metastable state that becomes thermally accessible to available surface electrons once the positron is present. A likely candidate for this entity is an electron-positron state analogous to the surface exciton observed on p-Si(100) c(4×2) by Weinelt et al. [Phys. Rev. Lett. 92, 126801 (2004)].
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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Cassidy DB, Bromley MWJ, Cota LC, Hisakado TH, Tom HWK, Mills AP. Cavity induced shift and narrowing of the positronium Lyman-α transition. Phys Rev Lett 2011; 106:023401. [PMID: 21405227 DOI: 10.1103/physrevlett.106.023401] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/08/2010] [Indexed: 05/30/2023]
Abstract
We report experiments in which the line shape of the Lyman-alpha (1S-2P) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval ΔE for confined atoms was observed to be larger than that of free Ps by 1.26±0.06 meV. A configuration interaction calculation yields results that are consistent with our ∼5 nm sample, and suggests that ΔE decreases dramatically for larger cavity diameters. The linewidth of the transition, (0.066±0.004) nm (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.
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Affiliation(s)
- D B Cassidy
- Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
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