Cesium Vapor Research Articles

Status of SPIDER beam source after the first 3.5 years of operation

SPIDER is the negative ion source testbed for ITER neutral beam injector. It is currently the largest negative ion source ever built, equipped with a 100 keV accelerator, aiming at producing a negative ion beam with an extracted current density in hydrogen of 355 A/m2, beam-on time of one hour. During the first 3.5 years of operation several improvements on the SPIDER source and its operating conditions were implemented. This paper focuses on the description of materials degradation phenomena in SPIDER and on the lessons learnt that were derived from components inspection during the already planned shutdown in 2022. The aim of these inspections was to improve the design of some components and to repair or refurbish damaged components. In particular, this paper describes the characterisation of damaged molybdenum coating, testing of surface cleaning techniques and experimental evidence of caesium vapour uneven distribution inside the plasma chamber. Sputtering due to beam particles was also investigated experimentally to observe its effect on materials used in SPIDER.

Cavity-enhanced optical bistability of Rydberg atoms.

Optical bistability (OB) of Rydberg atoms provides a new, to the best of our knowledge, platform for studying nonequilibrium physics and a potential resource for precision metrology. To date, the observation of Rydberg OB has been limited in free space. Here, we explore cavity-enhanced Rydberg OB with a thermal cesium vapor cell. The signal of Rydberg OB in a cavity is enhanced by more than one order of magnitude compared with that in free space. The slope of the phase transition signal at the critical point is enhanced more than 10 times that without the cavity, implying an enhancement of two orders of magnitude in the sensitivity for Rydberg-based sensing and metrology.

Analytical model of a diode-pumped cesium laser for investigation of upper-state mixing and quenching reactions.

A one-dimensional analytical model of a diode-pumped cesium vapor laser was constructed, and equations were derived to explain the dependence of the laser power on the hydrocarbon gas partial pressure. By varying the hydrocarbon gas partial pressure over a wide range and measuring the laser power, the corresponding mixing and quenching rate constants were validated. A gas-flow Cs diode-pumped alkali laser (DPAL) was operated with methane, ethane, and propane as buffer gases, with the partial pressures varied from 0 to 2 atmospheres. The experimental results were found to be in good agreement with the analytical solutions, confirming the validity of our proposed method. Separate 3-D numerical simulations were performed, and the output power effectively reproduced the experimental results over the entire buffer gas pressure range.

Magneto-optic spectrometer and filter based upon tandem vapor cell dispersion in atomic cesium

A magneto-optic spectrometer based upon dispersion associated with the 852 nm absorption line in atomic cesium vapor is demonstrated with high frequency resolution appropriate for measuring motion (Doppler shift). The spectrometer measures frequency without depletion of signal from atomic absorption, and has an 11% total signal extinction from the optic elements of which it is constructed. Also demonstrated is an optical filter with a high transmission region that coincides with the operating range of the spectrometer useful for blocking background light. Possible applications may include ground based LIDAR for long range targets such as space debris and free space optical communication.

Multiplexed random-access optical memory in warm cesium vapor.

The ability to store large amounts of photonic quantum states is regarded as substantial for future optical quantum computation and communication technologies. However, research for multiplexed quantum memories has been focused on systems that show good performance only after an elaborate preparation of the storage media. This makes it generally more difficult to apply outside a laboratory environment. In this work, we demonstrate a multiplexed random-access memory to store up to four optical pulses using electromagnetically induced transparency in warm cesium vapor. Using a Λ-System on the hyperfine transitions of the Cs D1 line, we achieve a mean internal storage efficiency of 36% and a 1/e lifetime of 3.2 µs. In combination with future improvements, this work facilitates the implementation of multiplexed memories in future quantum communication and computation infrastructures.

First characterization of the SPIDER beam AC component with the Beamlet Current Monitor

SPIDER is the full scale prototype for the ITER Heating Neutral Beam source, hosted at the Neutral Beam Test Facility in Padova, Italy. The behavior of the beam must be thoroughly investigated to bring the machine’s performance in line with ITER’s requirements. In particular, ripples in the beam can affect its divergence. Measuring the AC component of the beam current can therefore help to understand the impact that the oscillations caused by the source power supplies or RF generators have on the beam optics.To minimize the occurrence of electrical discharges, SPIDER was recently operated with a mask designed to close a large number of extraction apertures. As a consequence, the space between each beamlet was substantially increased, allowing for the installation of the Beamlet Current Monitor (BCM) which enables a non-invasive direct measurement of the DC and AC components of the currents of 5 individual beamlets.A first assessment of the beamlet currents’ AC components (up to 10 MHz) was performed during SPIDER’s first campaign with cesium evaporation. Recurring oscillations were identified in various frequency ranges, with amplitudes reaching up to ∼7% of the beamlet current DC value. When possible, the ripples were correlated to oscillations due to the RF oscillators and the beatings caused by their mutual coupling, or to SPIDER’s power supply systems.

Geophysical Investigations within the Latus Dextrum of Porolissum Fort, Northwestern Romania—The Layout of a Roman Edifice

This paper summarizes the results of a recent geophysical investigation carried out at Porolissum, which is considered to be one of the most significant Roman sites in Romania. The geophysical survey was carried out within the latus dextrum of the fort, which is the same location that had been the subject of earlier geophysical surveys as well as older archaeological excavations (1970s) that had uncovered a multiroom building. A cesium vapor total field magnetometer and a multi-electrode resistivity meter for a dense Electrical Resistivity Tomography (ERT) survey were used. Eighty parallel ERT profiles in combination with the emerging total field magnetic data and an antecedent magnetic survey allowed us to complete a more precise interpretation regarding the building that once existed on the left side of Porolissum’s Principia (the commander’s house). In contrast to the magnetic survey, which only reveals a part of the building’s architecture, the ERT survey provides a comprehensive view of the structure’s layout. More than 20 rooms could be positively identified, and the existence of further rooms might be deduced from the data. The ERT scan revealed the existence of the building’s northern external wall as well, which is not reflected on the magnetic map. Because some parts of the building are not visible on the magnetic map, we can assume that the building was constructed with at least two types of rocks (magmatic and sedimentary). In addition to the archaeological interpretation of the geophysical anomalies, a number of discussions concerning the connection between our survey and the geology of the area were held. The complementarity of the magnetic and resistivity results prompted us to conceive a preliminary 3D reconstruction of the building. Even if the building function is unknown in the absence of reliable archaeological data, it could have been a storage building, a second praetorium, a valetudinarium (hospital), or an armamentarium (weapons storage building). The illustrative reconstruction was completed taking into consideration that the building was a Roman military hospital, which, based on the available data, may be considered a credible assumption.

Kinetic simulations of ignited mode cesium vapor thermionic converters

Cesium vapor thermionic converters are an attractive method of converting high-temperature heat directly to electricity, but theoretical descriptions of the systems have been difficult due to the multi-step ionization of Cs through inelastic electron–neutral collisions. This work presents particle-in-cell simulations of these converters, using a direct simulation Monte Carlo collision model to track 52 excited states of Cs. These simulations show the dominant role of multi-step ionization, which also varies significantly based on both the applied voltage bias and pressure. The electron energy distribution functions are shown to be highly non-Maxwellian in the cases analyzed here. A comparison with previous approaches is presented, and large differences are found in ionization rates due especially to the fact that previous approaches have assumed Maxwellian electron distributions. Finally, an open question regarding the nature of the plasma sheaths in the obstructed regime is discussed. The one-dimensional simulations did not produce stable obstructed regime operation and thereby do not support the double-sheath hypothesis.

Verification of an Abandoned Oil Well Head - Penka Farm

The exact geographic location and depth to the top of an abandoned cased oil well was successfully targeted using a cesium vapor magnetometer. The ground survey was conducted in Rush County, Kansas northwest of La Crosse, Kansas.

Development of a Cesium Vapor MEMS Cell for Differential Measurement of Microwave Electromagnetically Induced Transparency

In this paper, we develop a cesium vapor MEMS cell with dual-cavity to measure the electromagnetically induced transparency (EIT) effect. The vapor cell was shaped by a deep-etching silicon wafer and two BF33 glass covers with a two-step anodic bonding process. By using a near-infrared spectroscopy laser with the wavelength of 1455 nm, the cesium dispenser sealed in the vapor cell was dispensed. The Rydberg EIT spectra of the fabricated vapor MEMS cell were characterized experimentally and enhanced effectively by introducing two paths probe laser (852 nm). This proposal provides a sensitive and miniaturized sensing method for the measurement of the EIT effect and has a prospect for the applications of integrated quantum microwave field intensity meter.

A 509 nm pulsed laser system for Rydberg excitation of cesium atoms

<sec>Single photon source is a non-classical light field with anti-bunching effect, which has a potential applications in the research of fundamental physics problems, quantum precision measurement, quantum communication, quantum computing, etc. The strong interaction between highly excited Rydberg atoms presents an excitation blockade effect. In a dense Rydberg atomic ensemble, the excitation of more than one Rydberg atom within a blockade volume is suppressed, where the interactions of Rydberg atoms shift the atomic states out of resonance with an excitation laser.</sec><sec>We consider here the generation of single photon source by using a four-wave mixing scheme in a room-temperature atomic vapor cell. In a homemade micrometer-sized atomic vapor cell, one-dimensional size is smaller than the radius of Rydberg blockade and the other two-dimensional size is limited by the size of focused laser beam. The blockade radius is on the order of a few micrometers, depending on the Rydberg atom states. An excitation blockade effect can be used to realize single photon source in thermal cesium vapor microcells. The micron cesium-cell is used to spatially localize atomic groups, which results in the atomic decoherence time on the order of microseconds or even nanoseconds. This requires a high-power pulsed laser to prepare the Rydberg atomic state at a nanosecond scale.</sec><sec>Four-photon excitation schemes with narrow linewidth lasers are also used experimentally. The cesium-Rydberg state can usually be excited by the lasers with optical wavelengths 852 and 509 nm, respectively. The laser system is well-stabilized so that the detuning is small compared with the spontaneous linewidth of Rydberg state, while the laser power and temporal mode need to be specified for ns-time coherence in thermal cesium vapor microcells. The 852 nm laser can be achieved by modulating the continuous laser beam with the help of an electro-optic intensity modulator (EOIM). While this remains a technical challenge for 509 nm laser with ns-laser pulse. There is no EOIM to generate the ns-laser pulse with high power.</sec><sec>We demonstrate a novel generation method of 509 nm laser system. In our experiments, a 1018 nm fiber laser is used to produce a continuous laser with a typical linewidth of ～8 kHz and power of 10 mW. The nanosecond pulse is generated with the help of an electro-optic intensity modulator (EOIM) by modifying the continuous laser beam. The peak power of modulated optical pulse is amplified to 4600 W by using a homemade fiber amplifier. The output beam of 1018 nm is then injected into a periodically poled lithium-niobate (PPLN) to generate the second harmonics laser of 509 nm. The typical peak power of 509 nm reaches 173 W by optimizing PPLN phase matching parameters. The pulse repetition frequency of the 509 nm laser can be continuously tuned in a range of 300 kHz–100 MHz, and the pulse width can be continuously tuned in a range of 1–100 ns. Peak power fluctuation of the pulses is about 1.3%. The power 509 nm laser with optimized pulse parameters can be used to excite the cesium atom with GHz bandwidth. Meanwhile the two seed source lasers is well established experimentally, which allows alternating pulses with a different wavelength. This is an essential capability for realizing a single photon source through four-wave mixing.</sec>

Passive mode-locked cesium diode-pumped alkali laser.

A first, to the best of our knowledge, demonstration of passive mode-locking in diode-pumped alkali laser (DPAL) is reported in this paper. An intracavity cesium vapor cell, buffered by atmospheric pressure methane, is used to passively mode lock a continuously pumped cesium DPAL with a static gain medium. A train of short pulses with duration less than 460 ps was observed using a 2.0-GHz bandwidth detector that limited the real time duration measurements. The calculated minimum duration for these pulses is 57 ps.

Wafer-level vapor cells filled with laser-actuated hermetic seals for integrated atomic devices

Atomic devices such as atomic clocks and optically-pumped magnetometers rely on the interrogation of atoms contained in a cell whose inner content has to meet high standards of purity and accuracy. Glass-blowing techniques and craftsmanship have evolved over many decades to achieve such standards in macroscopic vapor cells. With the emergence of chip-scale atomic devices, the need for miniaturization and mass fabrication has led to the adoption of microfabrication techniques to make millimeter-scale vapor cells. However, many shortcomings remain and no process has been able to match the quality and versatility of glass-blown cells. Here, we introduce a novel approach to structure, fill and seal microfabricated vapor cells inspired from the century-old approach of glass-blowing, through opening and closing single-use zero-leak microfabricated valves. These valves are actuated exclusively by laser, and operate in the same way as the “make-seals” and “break-seals” found in the filling apparatus of traditional cells. Such structures are employed to fill cesium vapor cells at the wafer-level. The make-seal structure consists of a glass membrane that can be locally heated and deflected to seal a microchannel. The break-seal is obtained by breaching a silicon wall between cavities. This new approach allows adapting processes previously restricted to glass-blown cells. It can also be extended to vacuum microelectronics and vacuum-packaging of micro-electro-mechanical systems (MEMS) devices.

Frequency Stabilization of a Cesium Faraday Laser With a Double-Layer Vapor Cell as Frequency Reference

We implement a compact optical frequency standard at the wavelength of 852 nm by the modulation transfer spectroscopy (MTS) technique, using a Faraday laser as the local oscillator and a double-layer cesium vapor cell to provide frequency reference. The vacuum space between the two quartz glass layers of the double-layer atomic vapor cell can effectively suppress the temperature fluctuations inside the internal atomic vapor cell. The influences of probe and pump laser powers, modulation frequency, and vapor cell temperature on the slope of the MTS error signal are measured. Utilizing the self-estimation method, we evaluate the frequency stability of the laser system, which can be up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${5.8\times 10^{-15}/\sqrt{\tau }}$</tex-math></inline-formula> at short term, dropping to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${2.0\times 10^{-15}}$</tex-math></inline-formula> at 50 s. This result is obviously superior to that using single-layer vapor cell as the frequency reference. Such an ultrastable laser source can be widely used in the fields of optical metrology and precision measurement.

A Temperature Programmed Desorption Diagnostic for SPIDER Cs Operations

Cesium evaporation, aimed at enhancing the production of negative ions, has recently started during the last experimental campaigns in Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma (SPIDER). In view of the full-scale ITER heating neutral beam injector (HNBI) future operations, Megavolt ITER Injector and Concept Advancement (MITICA), understanding the behavior of cesium during plasma operation is necessary to optimize the generation of negative hydrogen ions. Determining the contribution of the different mechanisms affecting the cesium distribution in the source requires a characterization of the desorption energies of the cesium compounds deposited over the source walls. The temperature programed desorption (TPD) diagnostic allows to estimate these energies using a pair of tungsten filaments, and can also be used as a Langmuir–Taylor detector. Two identical versions of the diagnostic have been assembled, giving the possibility to perform tests and characterizations on one of the copies, while having the second copy installed and operational in SPIDER. This work presents the design and development of the device, as well as experimental results in a testing vacuum chamber and in SPIDER. Finally, first estimations of the desorption energies of cesium compounds are carried out to support the plasma-Cs models for SPIDER.

Development of a Collisional Radiative Model for Hydrogen-Cesium Plasmas and Its Application to SPIDER

SPIDER is the full scale prototype for the ITER Heating Neutral Beam (HNB) source. The production of negative ions in the source mostly relies on the conversion of H/D atoms and positive ions on a 1.6 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.8$ </tex-math></inline-formula> m converter surface, which is covered with cesium in order to reduce its work function and maximize the conversion efficiency. In SPIDER, optical emission spectroscopy (OES) is used to study the light emitted by the plasma. In particular, the intensities of the cesium emission lines are tracked and used to reconstruct the distribution of its excited states. By coupling these measurements with a collisional radiative (CR) model, estimates of the cesium ground-state density and plasma parameters can be obtained in various parts of the source which are unreachable by other diagnostics. This work describes such a model and its use in conjunction with OES during the first operation with cesium in SPIDER. The electron density at the bottom of the source was found to be much larger than the rest, as a consequence of vertical drifts caused by the horizontal filter field, which would likely indicate a lower negative-ion availability for the beam. The effect of increasing the cesium evaporation rate on the negative-ion and electron densities was also investigated.

Origins of Rydberg-Atom Electrometer Transient Response and Its Impact on Radio-Frequency Pulse Sensing

Rydberg atoms show significant promise as the basis for highly sensitive detectors of continuous radio-frequency (rf) electric fields (E fields). Here, we study their time-dependent response to pulse-modulated rf E fields at 19.4 GHz using a cesium vapor cell at room temperature. We use density-matrix simulations to explain the timescales that shape the transient atomic response under different laser conditions, finding them to be limited by dephasing mechanisms, including transit-time broadening, Rydberg-Rydberg collisions, and ionization. Using a matched filter, we demonstrate the detection of individual pulses with durations from 10 \textmu{}s down to 50 ns and amplitudes from 15 000 \textmu{}V ${\mathrm{cm}}^{\ensuremath{-}1}$ down to about 170 \textmu{}V ${\mathrm{cm}}^{\ensuremath{-}1}$, corresponding to a sensitivity of about 240 nV ${\mathrm{cm}}^{\ensuremath{-}1}$ ${\mathrm{Hz}}^{\ensuremath{-}1/2}$. Finally, we highlight the potential of a Rydberg vapor cell as a receiver by detecting pulse trains from a rotating emitter on a simulated passing aircraft.

Zero-field level-crossing resonances in a small low-temperature cesium vapor cell for atomic magnetometry applications

Zero-field level-crossing resonances (LCRs) underlay one of the most robust and simplest techniques in atomic magnetometry. LCR-based miniaturized magnetic field sensors have already found relevant applications in biology and medicine. Such sensors utilize a single circularly polarized light wave to interact with alkali-metal atoms (usually Rb or Cs) and to observe the LCR in a vapor-cell transmission when a transverse magnetic field is scanned around zero value. A high-temperature regime (T ≈ 120 − 160°C) is required to achieve a desirable sensitivity of measurements. It can be a problem for some applications, especially in a multi-channel mode of operation. Here, we consider two spectroscopy techniques that can provide high-contrast and narrow LCRs under a relatively low temperature of the cell (≤60°C). These techniques imply using two light waves: the pump wave to polarize the atoms and the probe one to register the resonance. A cubic glass cell of 5×5×5 mm3 size is used in the experiments. It is filled with cesium vapors and neon as a buffer gas. The results can be used for developing a miniaturized low-power high-sensitivity magnetic field sensor for biomedical applications.

A digital alkali spin maser

Self-oscillating atomic magnetometers, in which the precession of atomic spins in a magnetic field is driven by resonant modulation, offer high sensitivity and dynamic range. Phase-coherent feedback from the detected signal to the applied modulation creates a resonant spin maser system, highly responsive to changes in the background magnetic field. Here we show a system in which the phase condition for resonant precession is met by digital signal processing integrated into the maser feedback loop. This system uses a modest chip-scale laser and mass-produced dual-pass caesium vapour cell and operates in a 50 mu T field, making it a suitable technology for portable measurements of the geophysical magnetic field. We demonstrate a Cramér-Rao lower bound-limited resolution of 50 fT at 1 s sampling cadence, and a sensor bandwidth of 10 kHz. This device also represents an important class of atomic system in which low-latency digital processing forms an integral part of a coherently-driven quantum system.

High-temperature Knudsen thermionic converter with multicavity emitter

This article describes an experimental study of the electrical and kinetic parameters of a Knudsen-mode thermionic converter (TIC) equipped both with a multicavity and a smooth emitter that converts heat into electrical energy using a combination of cesium and barium vapors. In the study, we recorded and analyzed data on how the maximum electron current density and the specific power are affected by the values of cesium and barium vapors pressure, the width of the interelectrode gap, and such parameters of the emitters as temperature and surface geometry. By analyzing the influence of surface geometry, it was revealed that the values of the electron current density and the electric power are four times higher in a TIC with a multicavity emitter compared to a TIC with a smooth emitter. TIC efficiency was analyzed taking into account the effective emissivity of the multicavity emitter. It is shown that the efficiency of such a TIC exceeds that of a TIC with a smooth emitter despite a slight increase in heat transfer from the emitter to the collector due to radiation.