arXiv subjects

S. Popa

Publications and source records attributed to S. Popa.

9 recordsLinked to original sources

arXiv2026

High-Repetition-Rate Laser-Driven Fast Neutron Generation from Planar Liquid Heavy Water Jets

We demonstrate a high-repetition-rate, directional pitcher-catcher neutron source employing a continuously flowing, ambient-temperature liquid jet pitcher and a beryllium catcher target. Proton or deuteron beams are accelerated at 0.5 Hz from water (H$_{2}$O) or heavy water (D$_{2}$O) sheet jets using the $2ω$ ALEPH laser at Colorado State University. Deuteron-induced neutron generation shows an average flux enhancement in the target normal direction of 13x compared to proton-induced neutron generation, achieving peak fluxes of $7.9\times10^{5}$ neutrons/sr. The experimentally predicted ion-to-neutron conversion efficiency of 1.5$\times 10^{-5}$ for protons and 1.1$\times 10^{-3}$ for deuterons indicates a significant yield contribution from the deuteron breakup reaction. High shot-to-shot stability neutron spectra up to 8.7~MeV ($^9$Be(d,n)$^{10}$B) are observed for more than 200 consecutive shots.

physics.plasm-ph↗

arXiv2026

Experimental evidence of production of directional muons from a laser-wakefield accelerator

We report on experimental evidence of the generation of directional muons from a laser-wakefield accelerator driven by a PW-class laser. The muons were generated following the interaction of a GeV-scale high-charge electron beam with a 2cm-thick Pb target and were detected using a Timepix3 detector placed behind a suitable shielding configuration. Data analysis indicates a $(99.1\pm0.5)$% confidence of muon detection over noise, in excellent agreement with numerical modelling. Extrapolation of the experimental setup to higher electron energies and charges suggests the potential to guide and separate from noise approximately $10^4$ muons/s onto cm$^2$-scale areas for applications using a 10 Hz PW laser. These results demonstrate the possibility of generating and transporting directional muon beams using high-power lasers and establish a foundation for the systematic application of laser-driven high-energy muon beams.

physics.acc-ph↗

arXiv2025

Slowing YbF molecules using radiation pressure

We report radiation pressure slowing of YbF molecules to low velocity. In YbF, laser slowing is hindered by leaks out of the optical cycle attributed to low-lying metastable electronic states arising from inner-shell excitation. We bring this population back into the optical cycle once it has decayed to the electronic ground state using microwaves to couple the relevant rotational levels. We measure the scattering rate and closure of the optical cycle as repumps are added, and study the destabilzation of dark states by a magnetic field and by polarization modulation, finding that both are helpful for maximizing the scattering rate. Starting from a beam with a mean speed of 80 m/s, and using frequency broadened slowing light, we reduce the mean speed of the beam and produce a substantial flux in the low velocity tail of the distribution. Slowing increases the fraction of molecules below 40 m/s from 0.4(1)% to 7.0(2)%, and the fraction below 30 m/s from zero to 3.2(1)%. The establishment of a nearly-closed optical cycle and the production of molecules at low velocity are important steps towards trapping YbF molecules for future measurements of the electron's electric dipole moment.

physics.atom-ph↗

arXiv2024

Slow molecular beams from a cryogenic buffer gas source

We study the properties of a cryogenic buffer gas source that uses a low temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s$^{-1}$ and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s$^{-1}$ and a flux of $9 \times 10^9$ ground state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s$^{-1}$. These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap.

physics.atom-ph↗

arXiv2024

Understanding inner-shell excitations in molecules through spectroscopy of the 4f hole states of YbF

Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention, but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multi-photon ionisation spectroscopy. We investigate the excited states of lowest energy, 8474, 9013 and 9090 cm$^{-1}$ above the ground state, all corresponding to the configuration 4f$^{13}$6s$^{2}$ ${}^{2}F_{7/2}$ of the Yb$^+$ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31050 cm$^{-1}$ that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study a state at 48729 cm$^{-1}$, which is above the ionization limit and features strong auto-ionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund's case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron's electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely-spaced levels of opposite parity.

physics.atom-ph↗

arXiv2023

The New Small Wheel electronics

The increase in luminosity, and consequent higher backgrounds, of the LHC upgrades require improved rejection of fake tracks in the forward region of the ATLAS Muon Spectrometer. The New Small Wheel upgrade of the Muon Spectrometer aims to reduce the large background of fake triggers from track segments that are not originated from the interaction point. The New Small Wheel employs two detector technologies, the resistive strip Micromegas detectors and the "small" Thin Gap Chambers, with a total of 2.45 Million electrodes to be sensed. The two technologies require the design of a complex electronics system given that it consists of two different detector technologies and is required to provide both precision readout and a fast trigger. It will operate in a high background radiation region up to about 20 kHz/cm$^{2}$ at the expected HL-LHC luminosity of $\mathcal{L}$=7.5$\times10^{34}$cm$^{-2}$s$^{-1}$. The architecture of the system is strongly defined by the GBTx data aggregation ASIC, the newly-introduced FELIX data router and the software based data handler of the ATLAS detector. The electronics complex of this new detector was designed and developed in the last ten years and consists of multiple radiation tolerant Application Specific Integrated Circuits, multiple front-end boards, dense boards with FPGA's and purpose-built Trigger Processor boards within the ATCA standard. The New Small Wheel has been installed in 2021 and is undergoing integration within ATLAS for LHC Run 3. It should operate through the end of Run 4 (December 2032). In this manuscript, the overall design of the New Small Wheel electronics is presented.

hep-ex↗

arXiv2007

Amalgamated Free Products of $w$-Rigid Factors and Calculation of their Symmetry Groups

We consider amalgamated free product II$_1$ factors $M = M_1 *_B M_2 *_B ...$ and use ``deformation/rigidity'' and ``intertwining'' techniques to prove that any relatively rigid von Neumann subalgebra $Q\subset M$ can be intertwined into one of the $M_i$'s. We apply this to the case $M_i$ are w-rigid II$_1$ factors, with $B$ equal to either $\Bbb C$, to a Cartan subalgebra $A$ in $M_i$, or to a regular hyperfinite II$_1$ subfactor $R$ in $M_i$, to obtain the following type of unique decomposition results, à la Bass-Serre: If $M = (N_1 *_C N_2 *_C ...)^t$, for some $t>0$ and some other similar inclusions of algebras $C\subset N_j$ then, after a permutation of indices, $(B\subset M_i)$ is inner conjugate to $(C\subset N_i)^t$, $\forall i$. Taking $B=\Bbb C$ and $M_i = (L(\Bbb Z^2 \rtimes \Bbb F_{2}))^{t_i}$, with $\{t_i\}_{i\geq 1}=S$ a given countable subgroup of $\Bbb R_+^*$, we obtain continuously many non stably isomorphic factors $M$ with fundamental group $\mycal F(M)$ equal to $S$. For $B=A$, we obtain a new class of factors $M$ with unique Cartan subalgebra decomposition, with a large subclass satisfying $\mycal F(M)=\{1\}$ and Out$(M)$ abelian and calculable. Taking $B=R$, we get examples of factors with $\mycal F(M)=\{1\}$, Out$(M)=K$, for any given separable compact abelian group $K$.

math.OA↗

arXiv2005

The Influence of Neutron Irradiation on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz Superconducting Phase: the Role of the Grain Edge

Using the transport and magnetization measurements the influence of neutron irradiation at a fluence of 5x10$^{17}$ n cm$^{-2}$ on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6x10$^{57}$ A/cm$^{2}$ (at 77.3K and in the applied field up to 160 kA m) and ΔMirr/ΔMnonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than 1 at high fields, correlated with the shape of the hystertic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.

cond-mat.supr-con↗

arXiv2005

Evidence for Irradiation Triggered Nonuniform Defect Distribution In Multiharmonic Magnetic Susceptibility of Neutron Irradiated YBa2Cu3O7-x

Multiharmonic ac-magnetic susceptibility \ch11,\chi2,chi3, of neutron irradiated Li-doped YBa2Cu3O7-x has revealed a nonmonotonic dependence of all harmonics on the neutron fluence. The irradiation has a strongly depressive influence on the intergrain connection suggesting an increase of the effective thickness of the intergranular Josephson junction at aneutron fluence of 0.98x10$^{17}$ cm$_{-2}$. Less damaged are the intragrain properties. A spectacular enhancement of the superconducting intragranular properties reflected in the characteristics of all harmonics was observed at highest fluence Φ= 9.98x10$^{17}$ cm$_{-2}$. We assume that this effect results from the development of a space inhomogeneous distribution with alternating defectless and defect rich regions.

cond-mat.supr-con↗