Physics of the helicon antenna on the prototype materials exposure experiment

Proto-MPEX has been operating in a high-density “helicon-mode” of operation. The helicon mode of operation is classified by an increase in target on-axis electron density (6e19 m^-3) and a decrease in electron temperature (2 – 3 eV) during a helicon pulse. This transition is observed when Deuterium gas is puffed into the device and is dependent on operating configurations. The Proto-MPEX helicon antenna is a quarter turn right handed helical twist antenna powered by RF at 13.56 MHz and > 110 kW of power. Establishing plasma densities and magnetic field strengths under the antenna that suppress non-resonant mode conversion to the slow-wave are thought to be responsible for operating in the "helicon-mode". Evidence for this phenomena to be responsible for the "helicon-mode" of operation is presented. The experimental results showing evidence of this phenomena are presented here. First, we present time-resolved measurements of an edge-to-core power transition during a "helicon-mode" plasma pulse in the form of infra-red camera imaging of a thin stainless steel target plate. The time-resolved images measure the two-dimensional distribution of power deposition in the helicon discharge. The discharge displays a mode transition characterized by a significant increase in the on-axis electron density and core power coupling, suppression of edge power coupling and the formation of a fast-wave radial normal mode. Although the self-consistent mechanism that drives this transition is not yet understood, the edge-to-core power transition displays characteristics that are consistent with the discharge entering a slow-wave anti-resonant regime. RF magnetic field measurements made across the plasma column, together with the power deposition results, provide direct evidence to support the suppression of the slow-wave in favour of core plasma production by the fast-wave in a light-ion helicon source. A full wave model of the helicon antenna has been made in the finite element analysis software, COMSOL Multiphysics, to investigate the wave fields produced and the power deposition inside the Proto-MPEX device. Core electron density and magnetic field under the helicon is scanned while tracking core power deposition. The peaks of core power deposition in this parameter space are then investigated and the propagating modes are analyzed. These areas of increased core power deposition are then identified as helicon normal modes that are predicted to decrease edge coupling of power and increase core power coupling by suppressing the non-resonant mode conversion of the fast-wave to the slow-wave in the periphery of the plasma

Physics of RF heating systems on proto-MPEX

Realizing controlled fusion as a commercial energy source is faced with many challenges. One of the main challenges being the development of Plasma Facing Components (PFC) that can survive the extreme environment encountered in a fusion reactor. To expedite the testing and development of PFCs Oak Ridge National Laboratory (ORNL) is building the Materials Plasma Exposure eXperiment (MPEX), which is a linear device purposed specifically for studying Plasma Material Interactions (PMI). Current linear devices cannot produce plasmas with fusion divertor relevant electron and ion temperatures and instead rely on electrostatic biasing of the target to simulate the relevant ion energies. This methodology inhibits studying the interaction of the eroded material and recycled neutral gas with a fusion relevant divertor plasma and does not properly simulate the angular energy distribution of the ion fluxes, therefore, PMI studies on these linear devices omit a vast amount of rich physics important to PFC development. MPEX will enable the study of fusion relevant PMI by producing fusion divertor relevant plasma conditions in front of a target station using RF technology. Proto-MPEX is the device that is currently operating at ORNL, where the viability of this RF technology is being demonstrated. The electron density, electron temperature, and ion temperature of the target plasma will be controlled independently with separate RF heating systems. This thesis focuses on the electron density production system and the ion heating systems on Proto-MPEX and their viability for MPEX. The electron density production on Proto-MPEX is accomplished by a helicon plasma source. Helicon plasma sources have been shown to efficiently produce high-density plasmas for a relatively low amount of RF power. Efficient electron density production of helicon plasma sources in light ions is hypothesized to be enabled by strong core power deposition when the plasma conditions allow for the formation of helicon normal modes. Experimental evidence supporting this hypothesis is presented in the form of B-dot probe and IR camera measurements, showing the increase of on-axis RF magnetic field strength and the formation of eigenmode structure concurrently with an increase in core power deposition at the expense of power deposition in the periphery of the plasma column. An RF full-wave model of the helicon region is made, which predicts the formation of cavity-like structures of the RF magnetic field when core power deposition is increased. Next, the problem that Proto-MPEX’s helicon source has been shown to not operate efficiently at higher magnetic field strengths is addressed. The hypothesis that the power balance does not allow enough density production for the helicon antenna to sustain a mode of operation that enhances core power deposition is tested by coupling a 2D axisymmetric full-wave simulation of the helicon antenna to a volume integrated 0D power/particle balance. This model is compared to experimental measurements of electron density and shows that there is a decrease in electron density production due to a decrease in core power coupling in the region where electron density decreases on Proto-MPEX. The model shows that if the Proto-MPEX helicon plasma source is operated at even higher magnetic fields strengths than efficient electron density production is recovered. Finally, the performance of Proto-MPEX is compared with other experimental devices by calculating the ionization cost of the plasma source, which shows that improvements to efficiency can practically be achieved to match the ionization cost of other plasma sources. Experimental improvements to the helicon source region are suggested and quantified with the couple RF and particle/power balance model. The ion heating on Proto-MPEX is accomplished by ion cyclotron heating via the beach heating technique. This technique is expected to increase ion temperatures on Proto-MPEX to values of Ti = 20 eV or more. The beach heating technique has been successfully demonstrated on previous devices, however, these devices were operating at much lower electron densities than Proto-MPEX. A theoretical route to core ion heating is first explored. At magnetic field strengths near the ion cyclotron resonance, the higher electron density in Proto-MPEX brings the existence of the Alfv ́en resonance into the Proto-MPEX plasma. This layer acts to cut-off the cold plasma slow wave, called the inertial Alfv ́en wave for the high-density core plasma. On the high-density side of the Alfv ́en resonance, the kinetic Alfv ́en wave can propagate when the electron temperature allows. In Proto-MPEX this wave is thought to be responsible for the core heat- ing of ions in the device. A simplified kinetic plasma tensor is implemented in COMSOL to simulate the propagation of this wave and to show that at Proto-MPEX relevant conditions this wave is responsible for core heating of the ions. Next, experimental evidence for core ion heating is presented in the form of ion temperature and target heat flux measurements. However, the core heating is shown to transiently cool, which is proposed to be due to the charge exchange with neutral gas born from plasma recombination at the target. When the target material is changed from carbon to stainless steel, the heat flux at the target reaches an increased steady state and higher ion temperatures are measured throughout the plasma column. The axial peak of the ion temperature is also located closer to the target for the case of the stainless steel target. These phenomena are hypothesized to be due to the increased reflection coefficient of the target material, and a model for quantifying this hypothesis shows that the flux of energetic neutral particles born from the reflection of sheath accelerated ions could explain these observations. Finally, experimental optimization of the magnetic field in the ICH region is shown with COMSOL simulations showing good agreement with these experimental results. The numerical simulations are then used to explore the parameter space of driving frequency, antenna length, and distance of the antenna to the ion cyclotron resonance on the predictions of heating in the ion cyclotron region

Measurement of the electric dipole moments for transitions to rubidium Rydberg states via Autler-Townes splitting

We present the direct measurements of electric-dipole moments for $5P_{3/2}\to nD_{5/2}$ transitions with $20<n<48$ for Rubidium atoms. The measurements were performed in an ultracold sample via observation of the Autler-Townes splitting in a three-level ladder scheme, commonly used for 2-photon excitation of Rydberg states. To the best of our knowledge, this is the first systematic measurement of the electric dipole moments for transitions from low excited states of rubidium to Rydberg states. Due to its simplicity and versatility, this method can be easily extended to other transitions and other atomic species with little constraints. Good agreement of the experimental results with theory proves the reliability of the measurement method.Comment: 12 pages, 6 figures; figure 6 replaced with correct versio

Acceptability, Feasibility and Preliminary Evaluation of a Novel, Personalised, Home based Physical Activity Intervention for Chronic Heart Failure (Active-at-Home-HF)::A Pilot Study

Purpose: Less than 10% of heart failure patients in the UK participate in cardiac rehabilitation programmes. The present pilot study evaluated feasibility, acceptability and physiological effects of a novel, personalised, home-based physical activity intervention in chronic heart failure. Methods: Twenty patients (68±7 years old, 20% females) with stable chronic heart failure due to reduced left ventricular ejection fraction (31±8 %) participated in a single group, pilot study assessing the feasibility and acceptability of a 12-week personalised home-based physical activity intervention aiming to increase daily number of steps by 2000 from baseline (Active-at-Home-HF). Patients completed cardiopulmonary exercise testing with non-invasive gas exchange and haemodynamic measurements and quality of life questionnaire pre- and post-intervention. Patients were supported weekly via telephone and average weekly step count data collected using pedometers. Results: 43 patients were screened and 20 recruited into the study. Seventeen patients (85%) completed the intervention, and 15 (75%) achieved the target step count. Average step count per day increased significantly from baseline to 3 weeks by 2546 (5108±3064 to 7654±3849 P=0.03, n=17), and was maintained until week 12 (9022±3942). Following completion of the intervention, no adverse events were recorded, quality of life improved by 4 points (26±18 vs. 22±19). Peak exercise stroke volume increased by 19% (127±34 vs 151±34 m/beat, P=0.05), while cardiac index increased by 12% (6.8±1.5 vs. 7.6±2.0 L/min/m2, P=0.19). Workload and oxygen consumption at anaerobic threshold also increased by 16% (49±16 vs. 59±14 watts, P=0.01) and 10% (11.5±2.9 vs. 12.8±2.2 ml/kg/min, P=0.39). Conclusion: The Active-at-Home-HF intervention is feasible, acceptable and effective for increasing physical activity in CHF. It may lead to improvements in quality of life, exercise tolerance and haemodynamic function

EuGMS 2019 Congress report: evidence-based medicine in geriatrics

This is the final version. Available from Springer Nature via the DOI in this record. The 2019 EuGMS Congress “Evidence-Based Medicine in Geriatrics” was held in Krakow, Poland, and attended by over 1600 participants from 64 different countries. A summary and reflection on the congress was presented in the Closing Ceremony by European Academy for Medicine of Aging graduates, and summarised in this article. Keynote lectures, ‘state of the art’ sessions and symposia presented the evidence relating to different age-related conditions, their prevention, management and treatments. Hot topic areas included frailty and multimorbidity, and evidence-based attempts to address these conditions at different life stages. The field of geriatrics represents unique challenges for evidence-based medicine practice. There is much research going on. Clear leadership is needed to facilitate consensus agreements on standard definitions, methods and relevant outcomes, in collaboration with older people themselves, to maximise the opportunities and benefits of doing this research, and benefiting our patients and society at large

A cold-atoms based processor for deterministic quantum computation with one qubit in intractably large Hilbert spaces

We propose the use of Rydberg interactions and ensembles of cold atoms in mixed state for the implementation of a protocol for deterministic quantum computation with one quantum bit that can be readily operated in high dimensional Hilbert spaces. We propose an experimental test for the scalability of the protocol and to study the physics of discord. Furthermore, we explore the possibility of extending to non-trivial unitaries, such as those associated to many-body physics. Finally develop a scheme to add control to cold atom unitaries in order to facilitate their implementation in our proposal

Ozone observations and a model of marine boundary layer photochemistry during SAGA 3

A major purpose of the third joint Soviet‐American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O3 and photochemical cycles in detail. On leg 1, which took place between Hilo, Hawaii, and Pago‐Pago, American Samoa, in February and March 1990, shipboard measurements were made of O3, CO, CH4, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), H2S, H2O2, organic peroxides, and total column O3. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O3 was observed on SAGA 3, with O3 north of the intertropical convergence zone (ITCZ) at 15–20 parts per billion by volume (ppbv) and less than 12 ppbv south of the ITCZ but never ≤3 ppbv as observed on some previous equatorial Pacific cruises (Piotrowicz et al., 1986; Johnson et al., 1990). Total column O3 (230–250 Dobson units (DU)) measured from the Akademik Korolev was within 8% of the corresponding total ozone mapping spectrometer (TOMS) satellite observations and confirmed the equatorial Pacific as a low O3 region. In terms of number of constituents measured, SAGA 3 may be the most photochemically complete at‐sea experiment to date. A one‐dimensional photochemical model gives a self‐consistent picture of O3‐NO‐CO‐hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O3 is 10 ppbv with a 10–15% diurnal variation and maximum near sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain model‐calculated OH to 9 × 105 cm−3 for 10 ppbv O3 at the equator. For DMS (300–400 parts per trillion by volume (pptv)) this OH abundance requires a sea‐to‐air flux of 6–8 × 109 cm−2 s−1, which is within the uncertainty range of the flux deduced from SAGA 3 measurements of DMS in seawater (Bates et al., this issue). The concentrations of alkyl nitrates on SAGA 3 (5–15 pptv total alkyl nitrates) were up to 6 times higher than expected from currently accepted kinetics, suggesting a largely continental source for these species. However, maxima in isopropyl nitrate and bromoform near the equator (Atlas et al., this issue) as well as for nitric oxide (Torres and Thompson, this issue) may signify photochemical and biological sources of these species