Single-pass Absorption Research Articles

Computational studies on fast wave current drive in high beta SST-1 and SST-2 plasmas

The non-inductive current drive in low toroidal beta (β) steady-state superconducting tokamak-1 (SST-1) is obtained using slow waves or Lower Hybrid waves (LHW). If β of the plasma is increased either by increasing the plasma parameters (density/temperature) or by decreasing the toroidal magnetic field, the non-inductive current drive may be supported by Fast Waves (FWs), as wave accessibility is limited for slow waves at these high β plasmas. Non-inductive current drive using FWs is explored for these high β plasmas to identify the optimized parameters (operating frequency and parallel refractive index (n||)) of the fast wave. The studies indicate that FWs may be launched with an operating frequency of 0.5 GHz RF source and n|| around 4 in SST-1 plasma. The GENRAY code is employed to carry out detailed studies on Fast Wave Current Drive (FWCD) analysis using ray tracing techniques. Computational studies on FWCD are carried out where central electron density (ne0) and central electron temperature (Te0) of 6×1019m−3 and 3 keV respectively is assumed. The FWCD displays central depositions of RF power and is attributed to fast radial propagation compared to the parallel propagation along the toroidal magnetic field, resulting in ray transits with large number of reflections. The off-axis current drive in SST-1 plasma requires reduced radial penetration of the wave and demands unrealistic plasma parameter in terms of β. However, for SST-2 plasmas, having enhanced plasma parameters (βN∼1.8), it is found that if FWs are launched with n||∼3 and f∼1.5 GHz, the radial penetration of the wave is impeded and single pass absorption in toroidal direction is observed, thus providing conducive condition for off axis current drive. It indicates that FWs would drive plasma current of 11 kA/MW at ρ∼0.7 for ne0∼2.2×1020m−3 and Te0∼7.7 keV values.

Physics basis for the ICRF system of the SPARC tokamak

Ion cyclotron range of frequencies (ICRF) heating will be the sole auxiliary heating method on SPARC for both full-field (Bt0 ~ 12 T) D–T operation and reduced field (Bt0 ~ 8 T) D–D operation. Using the fast wave at ~120 MHz, good wave penetration and strong single-pass absorption is expected for D–T(3He), D(3He), D(H) and4He(H) heating scenarios. The dependences of wave absorption on${k_\parallel }$,3He concentration, resonance location, antenna poloidal location and losses on alpha particles and ash have been studied. The antenna loading has been assessed by comparison with the Alcator C-Mod antennae. An antenna spectrum of${k_\parallel }\sim 15\text{--}18\,{\textrm{m}^{ - 1}}$is shown to be good for both core absorption and edge coupling. For the control of impurity sources, the antenna straps are rotated ~10° to be perpendicular to the B field and the straps can run with different power levels in order to optimize the antenna spectrum and to minimize the image current on the antenna frame. Combining the physics constraints with the SPARC port design, maintenance requirement and contingency against antenna failure during D–T operation, we plan to mount on the inner wall of the vacuum vessel a total of 12 4-strap antennae in 6 ports while keeping 3-strap antennae that are insertable and removable on port plugs as the backup option.

Laser-based CO concentration and temperature measurements in high-pressure shock-tube studies of n-heptane partial oxidation

This paper presents a laser-based absorption technique for measuring temperature and CO concentration in high-pressure shock tubes. Two fundamental vibrations of CO (v" = 0, P8, 4.73 µm and v" = 1, R21, 4.56 µm) were selected for high-temperature sensitivity with a reduced influence from pressure broadening compared to previous work. Single-pass absorption (80 mm path length) was measured with two quantum-cascade lasers. The technique was demonstrated by measuring time-resolved temperature for non-reactive mixtures at 1100–1960 K and 1.2–9.7 bar. During partial oxidation of n-heptane, temperature and CO concentrations were measured with 4 µs time resolution at 1360–1670 K and 5.8–8.2 bar. Interference from broadband CO2 absorption was quantified and subtracted. Measured data in the burnout state are in excellent agreement with predictions from kinetics mechanisms (Mehl et al. Proc Combust Inst 33:193, 2011; Zhang et al. Combust Flame 172:116, 2016) over the entire range of operating conditions, which validates the performance of the current laser-absorption technique in reactive-mixture measurements. Additionally, time-resolved temperature and CO-concentration measurements agree well with predictions based on the Mehl et al. mechanism.

Optically-thick 300 nm GaAs solar cells using adjacent photonic crystals.

Ultra-thin photovoltaics offer the potential for increasing efficiency while minimizing costs. However, a suitable light trapping strategy is needed to reach the optically thick regime for otherwise thin-film structures. III-V materials can benefit from simple adjacent light trapping structures, if correctly designed. Here we present three strategies for a 300 nm thick GaAs cell using front photonic crystals, back photonic crystals, and both front and back combined, predicting a maximum photocurrent, Jsc=29.9 mA/cm2 under the radiative limit, including an enhanced absorption in the Urbach-tail. We analyze the increased absorption isolating the Fabry-Perot resonances, the single pass absorption and the scattered contribution from the incident light.

Ion cyclotron resonance heating scenarios for DEMO

The present paper offers an overview of the potential of ion cyclotron resonance heating (ICRH) or radio frequency heating for the DEMO machine. It is found that various suitable heating schemes are available. Similar to ITER and in view of the limited bandwidth of about 10 MHz that can be achieved to ensure optimal functioning of the launcher, it is proposed to make core second harmonic tritium heating the key ion heating scheme, assisted by fundamental cyclotron heating 3He in the early phase of the discharge; for the present design of DEMO—with a static magnetic field strength of Bo = 5.855 T—that places the T and 3He layers in the core for f = 60 MHz and suggests centering the bandwidth around that main operating frequency. In line with earlier studies for hot, dense plasmas in large-size magnetic confinement machines, it is shown that good single pass absorption is achieved but that the size as well as the operating density and temperature of the machine cause the electrons to absorb a non-negligible fraction of the power away from the core when core ion heating is aimed at. Current drive and alternative heating options are briefly discussed and a dedicated computation is done for the traveling wave antenna, proposed for DEMO in view of its compatibility with substantial antenna–plasma distances. The various tasks that ICRH can fulfill are briefly listed. Finally, the impact of transport and the sensitivity of the obtained results to changes in the machine parameters is commented on.

A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target.

Methodology development for the control of NOx emissions in Aerospace Industry

A large number of studies conducted in Russia and abroad have been devoted to the development of low NOx emission gas turbine engines for aircraft and power stations. However, the continual improvement of the environmental requirements of ICAO (International Civil Aviation Organization) forces new research to be carried out to meet the future goals of reduced emissions produced by gas turbine combustors (GTE), a better understanding of the process of formation of various pollutants is required. Both empirical and theoretical approaches will be studied in this present research work to provide the exhaust concentrations of NOx emissions. In recent years different methods are already developed for this problem, one of them is associated with a theoretical approach. It is well known that correct theoretical models are economical (help’s to avoid experimental work) and can be useful for deep examination of NOx emissions generated in the combustion chamber process. The main objective of this study is to predict NOx emissions in GTE based on the mathematical simulations and chemical kinetics approach.To validate the simulation results, dispersion type optical system was developed in the laboratory. This system includes UV LED source (λ=245-280nm), single-pass absorption cell with length (L=500 mm) and spectrometer DFS-452 (spectral dispersion up to Δλ = 0.2nm/mm) equipped photo detector MORS-1 with a detection range Δλ=50nm. It allowed us to measure concentrations of some matters which absorbed light in the UVC range.

Measurement of stray millimeter-wave radiation from a 70-GHz ECH/ECCD system in Heliotron J

Stray millimeter-wave radiation from a 70-GHz electron cyclotron heating and current drive (ECH/ECCD) system has been measured in the Heliotron J helical device. Two rotatable diode detectors located at the outboard side ports are used to pick up the stray radiation: one is installed at the ECH launcher port and the other is installed at a toroidal angle of 135 deg far from the ECH launcher port. Both detectors are rotated to measure the polarization of the stray radiation. The results show that at the toroidal position far from the ECH launcher, the polarization is not fully randomized before plasma breakdown, whereas the polarization is uniform after a quasi-stationary plasma is generated. The polarization near the ECH launcher is not uniform even in a quasi-stationary plasma. Plasma experiments scanning the electron density indicate that the EC power absorption estimated from the stray radiation at the toroidal position far from the ECH launcher is correlated with the single-pass absorption rate calculated by the TRAVIS ray-tracing code. These results indicate that the diagnostic using a simple diode detector can be used as a real-time monitor of EC power absorption only when the plasma with finite density is produced and the detector is placed far from the ECH launcher port.

Simulation of Polarising and Reflector Gratings for High Power mm Waves

High power mm waves for fusion plasma heating need to be elliptically polarised to ensure good absorption in the plasma. In some scenarios, electron cyclotron resonance heating (ECRH) at higher harmonics (X3 and O2) is used, but this has significant shine-through because of low single pass absorption. Grating reflectors at the inboard strike point form a holographic mirror that reflects the beam back into the plasma. This paper investigates the optical properties and ohmic losses of both the polariser and the reflectors with the 3D fullwave code IPF-FD3D. The reflection properties of a reflector for ASDEX Upgrade and the improved ohmic losses of a waveguide polariser were confirmed.

Wave coupling simulation of ICRF antenna in EAST relying on the perfectly matched layer technique

A coupling model for the waves in ion cyclotron resonance frequency (ICRF) in tokamak plasmas is described. The simulation is performed based on the finite element method. A cold plasma dielectric tensor is used. A perfectly matched layer (PML) is applied to describe the ICRF wave single-pass absorption in the core plasma. The selection of PML characteristic parameters is optimized in a wide wave frequency range and validated for plane waves and ICRF waves. The dependencies of the coupling resistance on the plasma-antenna gap, average plasma density, and plasma profiles are given. The simulation results are in good agreement with the experimental results.

Review of recent experimental and modeling advances in the understanding of lower hybrid current drive in ITER-relevant regimes

Progress in understanding lower hybrid current drive (LHCD) at high density has been made through experiments and modeling, which is encouraging given the need for an efficient off-axis current profile control technique in burning plasma. By reducing the wall recycling of neutrals, the edge temperature is increased and the effect of parametric instability (PI) and collisional absorption (CA) is reduced, which is beneficial for increasing the current drive efficiency. Strong single pass absorption is preferred to prevent CA and high LH operating frequency is essential for wave propagation to the core region at high density, presumably to mitigate the effect of PI. The dimensionless parameter that characterizes LH wave accessibility and wave refraction for the experiments in this joint study is shown to bracket the region in parameter space where ITER LHCD experiments will operate in the steady state scenario phase. Further joint experiments and cross modeling are necessary to understand the LHCD physics in weak damping regimes which would increase confidence in predictions for ITER where the absorption is expected to be strong.

Real-time control of electron cyclotron wave polarization in the LHD

Peripheral plasma with finite electron density gradients and finite magnetic shear is known to affect polarization of electron cyclotron (EC) waves. Calculation of the ratio between the ordinary (O) mode and the extraordinary (X) mode, integrated in the ray-tracing code developed for EC heated plasmas in the Large Helical Device (LHD), enables the search for the optimum EC wave polarization in order to excite the pure O/X mode at the EC resonance layer. The real-time control system of the incident polarization was developed for maximum single-pass absorption of EC waves, based on the dependence of optimum EC wave polarization on peripheral density profiles. The polarization control system is equipped with a fast field programmable gate array, which processes in real time the calculation of the peripheral electron density profile and the optimum EC wave polarization for motion control of the polarization rotator and the elliptical polarizer on the transmission line. The real-time control in LHD experiments functioned properly in maintaining the absorbed power of the EC wave higher than that without the control, demonstrating that the purer heating mode was successfully excited.

AORSA full wave calculations of helicon waves in DIII-D and ITER

Helicon waves have been recently proposed as an off-axis current drive actuator for DIII-D, FNSF, and DEMO tokamaks. Previous ray tracing modeling using GENRAY predicts strong single pass absorption and current drive in the mid-radius region on DIII-D in high beta tokamak discharges. The full wave code AORSA, which is valid to all order of Larmor radius and can resolve arbitrary ion cyclotron harmonics, has been used to validate the ray tracing technique. If the scrape-off-layer (SOL) is ignored in the modeling, AORSA agrees with GENRAY in both the amplitude and location of driven current for DIII-D and ITER cases. These models also show that helicon current drive can possibly be an efficient current drive actuator for ITER. Previous GENRAY analysis did not include the SOL. AORSA has also been used to extend the simulations to include the SOL and to estimate possible power losses of helicon waves in the SOL. AORSA calculations show that another mode can propagate in the SOL and lead to significant (~10%–20%) SOL losses at high SOL densities. Optimizing the SOL density profile can reduce these SOL losses to a few percent.

Temperature-dependence laws of absorption line shape parameters of the CO2 ν3 band

To improve the understanding of temperature-dependence laws of spectral line shape parameters, spectra of the ν3 rovibrational band of CO2 perturbed by 10, 30, 100, 300 and 1000 mbar of N2 were recorded at nine temperatures between 190 K and 330 K using a 22 cm long single-pass absorption cell in a Bruker IFS125 HR Fourier Transform spectrometer. The spectra were fitted employing a quadratic speed-dependent hard collision model in the Hartmann–Tran implementation extended to account for line mixing in the Rosenkranz approximation by means of a multispectrum fitting approach developed at DLR. This enables high accuracy parameter retrievals to reproduce the spectra down to noise level and we present the behavior of line widths, shifts, speed-dependence-, collisional narrowing- and line mixing-parameters over this 140 K temperature range.

Simulations of High Harmonic Fast Wave Heating on the C-2U Advanced Beam-Driven Field-Reversed Configuration Device

Numerous efforts have been made at Tri-Alpha Energy (TAE) to theoretically explore the physics of microwave electron heating in field-reversed configuration (FRC) plasmas. For the fixed 2D profiles of plasma density and temperature for both electrons and thermal ions and equilibrium field of the C-2U machine, simulations with GENRAY-C ray-tracing code have been conducted for the ratios of ω/ω ci[D] in the range of 6 - 20. Launch angles and antenna radial and axial positions have been optimized in order to simultaneously achieve good wave penetration into the core of FRC plasmas and efficient power damping on electrons. It is found that in an optimal regime, single pass absorption efficiency is 100% and most of the power is deposited inside the separatrix of FRC plasmas, with power damping efficiency of about 72% on electrons and less than 19% on ions. Calculations have clearly demonstrated that substantial power absorption on electrons is mainly attributed to high beta enhancement of magnetic pumping; complete power damping occurs before Landau damping has a significant effect on power absorption.

High Field Side Lower Hybrid Current Drive Simulations for Off- axis Current Drive in DIII-D

Efficient off-axis current drive scalable to reactors is a key enabling technology for developing economical, steady state tokamak. Previous studies have focussed on high field side (HFS) launch of lower hybrid current drive (LHCD) in double null configurations in reactor grade plasmas and found improved wave penetration and high current drive efficiency with driven current profile peaked near a normalized radius, ρ, of 0.6-0.8, consistent with advanced tokamak scenarios. Further, HFS launch potentially mitigates plasma material interaction and coupling issues. For this work, we sought credible HFS LHCD scenario for DIII-D advanced tokamak discharges through utilizing advanced ray tracing and Fokker Planck simulation tools (GENRAY+CQL3D) constrained by experimental considerations. For a model and existing discharge, HFS LHCD scenarios with excellent wave penetration and current drive were identified. The LHCD is peaked off axis, ρ∼0.6-0.8, with FWHM Δρ=0.2 and driven current up to 0.37 MA/MW coupled. For HFS near mid plane launch, wave penetration is excellent and have access to single pass absorption scenarios for variety of plasmas for n|| =2.6-3.4. These DIII-D discharge simulations indicate that HFS LHCD has potential to demonstrate efficient off axis current drive and current profile control in DIII-D existing and model discharge.

Broadband absorption spectroscopy for rapid pH measurement in small volumes using an integrated porous waveguide.

Broadband absorption spectroscopy integrated with microfluidics is widely used to measure the pH of samples available in small volumes. Single pass absorption spectroscopy, however, suffers from poor minimum detectable absorption coefficient because the optical path length is determined by the physical dimensions of microchannels. As a result, either large amounts of indicator dyes are added or instrumentally complex techniques are used to determine sample pH. This work addresses these limitations by the integration of metal-clad leaky waveguide (MCLW) with microfluidics to perform broadband absorption spectroscopy. The MCLW consisted of a porous agarose waveguide with an estimated thickness and swelling ratio values of 1.405 ± 0.009 μm and 20.96 ± 0.65% respectively coated onto a titanium layer 32.8 ± 0.43 nm thick on a glass slide. MCLW permits obtaining absorption spectra over most of the wavelength range of the visible region (i.e. 450 nm to 700 nm) with a speed of 12.5 spectra per second using 3.2 nl of sample. The minimum detectable decadic absorption coefficient measured using MCLW was 0.43 cm-1, which is intermediate between single pass absorption spectroscopy and instrumentally complex techniques such as broadband cavity enhanced absorption spectroscopy and whispering gallery mode resonators.

Extension of electron cyclotron heating at ASDEX Upgrade with respect to high density operation

The ASDEX Upgrade electron cyclotron resonance heating operates at 105 GHz and 140 GHz with flexible launching geometry and polarization. In 2016 four Gyrotrons with 10 sec pulse length and output power close to 1 MW per unit were available. The system is presently being extended to eight similar units in total. High heating power and high plasma density operation will be a part of the future ASDEX Upgrade experiment program. For the electron cyclotron resonance heating, an O-2 mode scheme is proposed, which is compatible with the expected high plasma densities. It may, however, suffer from incomplete single-pass absorption. The situation can be improved significantly by installing holographic mirrors on the inner column, which allow for a second pass of the unabsorbed fraction of the millimetre wave beam. Since the beam path in the plasma is subject to refraction, the beam position on the holographic mirror has to be controlled. Thermocouples built into the mirror surface are used for this purpose. As a protective measure, the tiles of the heat shield on the inner column were modified in order to increase the shielding against unabsorbed millimetre wave power.

Time-resolved sub-ppm CH3 detection in a shock tube using cavity-enhanced absorption spectroscopy with a ps-pulsed UV laser

In this study we report a novel UV laser absorption diagnostic for sensitive and time-resolved measurement of methyl radicals (CH3) in shock tube kinetics studies, utilizing a frequency-quadrupled, high-repetition-rate (78 MHz) ps-pulsed Ti-sapphire laser targeted at the strong rovibronic transition of CH3 at 216.62 nm. This diagnostic employs a cavity-enhanced absorption spectroscopy (CEAS) strategy that amplifies the absorbance signal by a gain factor of about 50, which enables sub-ppm detection limit of CH3 to be achieved under typical shock conditions of 1200 K < T < 2500 K and P ∼ 1 atm, in a shock tube of ∼15 cm inner diameter. The use of a pulsed laser also enables an on-axis CEAS configuration, which yields a time resolution of 2–4 µs. For example, CH3 measurements were performed in shock tube decomposition experiments of dilute ethane and methane mixtures, demonstrating the substantial improvement in detection sensitivity of the current diagnostic compared to conventional single-pass absorption measurements. This new diagnostic promises to be a powerful tool for future shock tube chemical kinetics studies, and can be potentially extended to the detection of other UV-absorbing species.

Perfect single-sided radiation and absorption without mirrors

Highly directional radiation from photonic structures is important for many applications, including high power photonic crystal surface emitting lasers, grating couplers, and light detection and ranging devices. However, previous dielectric, few-layer designs only achieved moderate asymmetry ratios, and a fundamental understanding of bounds on asymmetric radiation from arbitrary structures is still lacking. Here, we show that breaking the 180$^\circ$ rotational symmetry of the structure is crucial for achieving highly asymmetric radiation. We develop a general temporal coupled-mode theory formalism to derive bounds on the asymmetric decay rates to the top and bottom of a photonic crystal slab for a resonance with arbitrary in-plane wavevector. Guided by this formalism, we show that infinite asymmetry is still achievable even without the need of back-reflection mirrors, and we provide numerical examples of designs that achieve asymmetry ratios exceeding $10^4$. The emission direction can also be rapidly switched from top to bottom by tuning the wavevector or frequency. Furthermore, we show that with the addition of weak material absorption loss, such structures can be used to achieve perfect absorption with single-sided illumination, even for single-pass absorption rates less than $0.5\%$ and without back-reflection mirrors. Our work provides new design principles for achieving highly directional radiation and perfect absorption in photonics.