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Soft–X-ray (SXR) brightness measurements contain information on a number of physics parameters in fusion plasmas; however, it is nearly impossible to extract the information without modeling. A validated forward model is therefore necessary for the accurate interpretation of SXR measurements and will be critical in the burning plasma era, where medium- and high-Z impurities are ever present. The Atomic Data and Analysis Structure (ADAS) database is a powerful interpretive tool that is extensively used to model and predict atomic spectra, level populations, and ionization balance for fusion plasmas. These predictions are in good agreement with experimental measurements. However, continuum radiation in the X-ray range, while also modeled in ADAS, has not been rigorously verified or tested against experimental data. We therefore performed a systematic comparison of ADAS to a simplified model called PFM. PFM only calculates continuum radiation but shows good agreement with experimental data when only ...

Local helicity injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate tokamak discharges. Viable non-solenoidal startup techniques require high central Te to combat resistive losses and enhance coupling to auxiliary methods of current drive/heating. Thomson scattering measurements of LHI discharges in Pegasus showed peaked Te profiles at Ip∼0.15 MA and Bt∼0.15 T with Te,0∼100– 150 eV. These results are similar to Te profiles observed with Ohmic induction. At lower levels of Bt, LHI Te profiles were hollow with Te,0∼40 eV and Te,max≤120 eV depending upon the helicity input. Regardless of the Bt level and helicity input, the electron pressure profiles were flat/peaked with hollow JR profiles. Equilibrium reconstructions and measurements of core absolute extreme ultraviolet radiation suggest the hollow Te profiles are a result of very low resistive heating power in the core due to the edge-localized nature of LHI and low-Z line radiation losses. Estimates of Zeff from the plasma conductivity indicate averaged values of ∼1 or ∼3 assuming neoclassical or Spitzer conductivity, respectively. When auxiliary heating power from magnetic reconnection is considered, this observed LHI performance is comparable to expectations from a linear Ohmic confinement scaling estimate and a collisional stochastic confinement scaling estimate of the core plasma region.

Results from one of the world’s largest cement plants are presented in this paper. The plant incorporates many innovative technologies including being the largest single kiln and a preheater tower with dual-sting ‘hybrid’ low-NOx and CO inline calciner. Commissioned with coal firing, the plant later switched to firing a blend of coal and petcoke in the kiln. The calciner has a seven-second residence time and is capable to burn several types of alternative fuels including bigger and denser fuels such as tire chips and/or solid recovered fuels.Prior to calciner installation, a detailed mathematical modeling exercise using mineral interactive computational fluid dynamics (MI-CFD) was carried out. The simulated results showed higher level of coal burnout in seven seconds’ gas residence time with about 95% meal calcination levels. The CO and NOx emissions were found to be extremely low. The predicted values were later found to be reasonably close to plant data.Plant analysis for the last ten years of operation are presented. The lower NOx emissions enabled the plant to use no or very little SNCR allowing for savings of between 1-2 $M per year, as compared with other plants. Some data and simulated results are presented to highlight calciner performance and, where possible, results are compared with other state of the art calciners. The achieved success from the plant can be replicated to other plants through creating similar combustion and process conditions.

A multi-energy soft x-ray pinhole camera has been designed and built for the Madison Symmetric Torus reversed field pinch to aid the study of particle and thermal-transport, as well as MHD stability physics. This novel imaging diagnostic technique combines the best features from both pulse-height-analysis and multi-foil methods employing a PILATUS3 x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. Further improvements implemented on the new cooled systems allow a maximum count rate of 10 MHz per pixel and sensitivity to the strong Al and Ar emission between 1.5 and 4 keV. The local x-ray emissivity will be measured in multiple energy ranges simultaneously, from which it is possible to infer 1D and 2D simultaneous profile measurements of core electron temperature and impurity density profiles with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations, or need of shot-to-shot reproducibility. The expected time and space resolutions will be 2 ms and

The analysis approach called integrated data analysis (IDA) provides a means to exploit all information present in multiple streams of raw data to produce the best inference of a plasma parameter. This contrasts with the typical approach in which information (data) from a single diagnostic is used to measure a given parameter, e.g., visible bremsstrahlung → Zeff. Data from a given diagnostic usually contain information on many parameters. For example, a Thomson scattering diagnostic is sensitive to bremsstrahlung and line emission in addition to electron temperature. This background light is typically subtracted off and discarded but could be used to improve knowledge of Zeff. IDA encourages explicit awareness of such information and provides the quantitative framework to exploit it. This gives IDA the ability to increase spatial and temporal resolution, increase precision and accuracy of inferences, and measure plasma parameters that are difficult or impossible to measure using single diagnostic techniques. One example is the measurement of Zeff on Madison symmetric torus using IDA since no single diagnostic can provide a robust measurement. As we enter the burning plasma era, application of IDA will be critical to the measurement of certain parameters, as diagnostic access in the harsh fusion environment will be extremely limited.The analysis approach called integrated data analysis (IDA) provides a means to exploit all information present in multiple streams of raw data to produce the best inference of a plasma parameter. This contrasts with the typical approach in which information (data) from a single diagnostic is used to measure a given parameter, e.g., visible bremsstrahlung → Zeff. Data from a given diagnostic usually contain information on many parameters. For example, a Thomson scattering diagnostic is sensitive to bremsstrahlung and line emission in addition to electron temperature. This background light is typically subtracted off and discarded but could be used to improve knowledge of Zeff. IDA encourages explicit awareness of such information and provides the quantitative framework to exploit it. This gives IDA the ability to increase spatial and temporal resolution, increase precision and accuracy of inferences, and measure plasma parameters that are difficult or impossible to measure using single diagnostic techniqu...

The first observation of rf heating in a reversed field pinch (RFP) using the electron Bernstein wave (EBW) is demonstrated on the Madison Symmetric Torus. Propagation across and heating in a stochastic magnetic field is observed. Novel techniques are required to measure the suprathermal electron tail generated by EBW heating in the presence of intense Ohmic heating. rf-heated electrons directly probe the edge transport properties in the RFP; measured loss rates imply a large noncollisional radial diffusivity.

We present the method of characteristics with mass, momentum, and energy conservation to solve the nonlinear wave equation with shock formation in a two layer one-dimensional rod made of cellular material. We show that the rigid-perfectly-plastic-locking model cannot predict shock formation at a material interface, so we propose an elastic–plastic-densifying model to describe the stress–strain behavior of the cellular materials. The conditions for shock formation at a material interface are provided. We conduct a two-layer analysis to gain insights into the behavior of two layer cellular systems and to determine which material properties are most important for design. Finally, we optimize the significant parameters to reduce the length of one and two layered cellular systems with impulse and mass constraints subject to pulse loading. The results reinforce the concept of sandwich structures and show that two layer systems can achieve a 30% reduction in length over single layer ones.

The first successful gaseous boronization during a pulsed discharge is reported. Sublimation of o-carborane (C2B10H12) combined with pulsed discharge plasmas with a repetition rate of 1 Hz is used to produce a hard boron-containing coating for reversed field pinch (RFP) plasmas in the Madison Symmetric Torus. X-ray photoelectron spectroscopy with Ar ion beam etching for silicon coupons installed at the plasma boundary shows about 60% boron concentration in the deposited layer. Both profilometer and scanning electron microscope analyses of the silicon coupons imply a strong toroidally non-uniform deposition depending on the location of the o-carborane injection. The layer thickness ranges from 50 to 300 nm. Ellipsometry calibrated with the profilometer results yields a refractive index of 2.2–2.3 for the films. The high refractive index implies that the coating is hard and has a well-ordered morphology. A reduction in wall recycling has consistently been observed after all boronization sessions. Comparison of the X-ray spectra in standard RFP plasmas before and after boronization indicates a slight decrease in the effective ionic charge.

With the increased use of improvised explosive devices in regions at war, the threat to military and civilian life has risen. Cabin penetration and gross acceleration are the primary threats in an explosive event. Cabin penetration crushes occupants, damaging the lower body. Acceleration causes death at high magnitudes. This investigation develops a process of designing armour that simultaneously mitigates cabin penetration and acceleration. The hybrid cellular automaton (HCA) method of topology optimization has proven efficient and robust in problems involving large, plastic deformations such as crash impact. Here HCA is extended to the design of armour under blast loading. The ability to distribute two metallic phases, as opposed to one material and void, is also added. The blast wave energy transforms on impact into internal energy (IE) inside the solid medium. Maximum attenuation occurs with maximized IE. The resulting structures show HCA's potential for designing blast mitigating armour structures.

Confinement in the reversed field pinch (RFP) has been shown to increase strongly with current profile control. The MST RFP can operate in two regimes: the standard regime with a naturally occurring current density profile, robust reconnection and dynamo activity; and the improved confinement regime with strong reduction in reconnection, dynamo and transport. New results in standard plasmas include the observation of a strong two-fluid Hall effect in reconnection and dynamo, the determination that the m = 0 edge resonant mode is nonlinearly driven, and the determination that tearing modes can lock to the wall via eddy currents in the shell. New results in improved confinement plasmas include observations that such plasmas are essentially dynamo-free, contain several isolated magnetic islands (as opposed to a stochastic field) and contain reduced high frequency turbulence. Auxiliary current drive and heating is now critical to RFP research. In MST, a programme to apply auxiliary systems to the RFP is underway and progress has accrued in several techniques, including lower hybrid and electron Bernstein wave injection, ac helicity injection current drive, pellet injection and neutral beam injection.

High power density, phased antenna operation can often be limited by antenna voltage handling and/or impurity and density production. Using a pair of two-strap antennas for comparison, the performance of a four-strap, fast wave antenna is assessed for a variety of configurations and antenna phases in Alcator C-Mod. To obtain robust voltage handling, the antenna was reconfigured to eliminate regions where the RF E-field is parallel to B or to reduce the RF E-field to

Research on the Alcator C-Mod tokamak [1] is focused on high particle- and power-density plasma regimes to understand particle and energy transport in the core, the dynamics of the H-mode pedestal, and scrape-off layer and divertor physics. The auxiliary heating is provided exclusively by RF waves, and both the physics and technology of RF heating and current drive are studied. The momentum which is manifested in strong toroidal rotation, in the absence of direct momentum input, has been shown to be transported in from the edge of the plasma following the L–H transition, with timescale comparable to that for energy transport. In discharges which develop internal transport barriers, the rotation slows first inside the barrier region, and then subsequently outside of the barrier foot. Heat pulse propagation studies using sawteeth indicate a very narrow region of strongly reduced energy transport, located near r/a = 0.5. Addition of on-axis ICRF heating arrests the buildup of density and impurities, leading to quasi-steady conditions. The quasi-coherent mode associated with enhanced D-Alpha (EDA) H-mode appears to be due to a resistive ballooning instability. As the pedestal pressure gradient and temperature are increased in EDA H-mode, small ELMs appear; detailed modelling indicates that these are due to intermediate n peeling–ballooning modes. Phase contrast imaging has been used to directly detect density fluctuations driven by ICRF waves in the core of the plasma, and mode conversion to an intermediate wavelength ion cyclotron wave has been observed for the first time. The bursty turbulent density fluctuations, observed to drive rapid cross-field particle transport in the edge plasma, appear to play a key role in the dynamics of the density limit. Preparations for quasi-steady-state advanced tokamak studies with lower hybrid current drive are well underway, and time dependent modelling indicates that regimes with high bootstrap fraction can be produced.

Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases ten-fold (to ~10 ms), which is comparable with L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker–Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

For several reasons, improved-confinement achieved in the reversed field pinch (RFP) during the last few years can be characterized as 'tokamak-like'. Historically, RFP plasmas have had relatively poor confinement due to tearing instability which causes magnetic stochasticity and enhanced transport. Tearing reduction is achieved through modification of the inductive current drive, which dramatically improves confinement. The electron temperature increases to >1?keV and the electron heat diffusivity decreases to ~5?m2?s?1, comparable with the transport level expected in a tokamak plasma of the same size and current. This corresponds to a 10-fold increase in global energy confinement. Runaway electrons are confined, and Fokker?Planck modelling of the electron distribution reveals that the diffusion at high energy is independent of the parallel velocity, uncharacteristic of stochastic transport. Improved-confinement occurs simultaneously with increased beta ~15%, while maintaining a magnetic field strength ten times weaker than a comparable tokamak. Measurements of the current, magnetic, and electric field profiles show that a simple Ohm's Law applies to this RFP sustained without dynamo relaxation.

Silicon photodiodes used for soft x-ray detection typically have a thin metal electrode partially covering the active area of the photodiode, which subtly alters the spectral sensitivity of the photodiode. As a specific example, AXUV4BST photodiodes from International Radiation Detectors have a 1.0 μm thick aluminum frame covering 19% of the active area of the photodiode, which attenuates the measured x-ray signal below ∼6 keV. This effect has a small systematic impact on the electron temperature calculated from measurements of soft x-ray bremsstrahlung emission from a high-temperature plasma. Although the systematic error introduced by the aluminum frame is only a few percent in typical experimental conditions on the Madison Symmetric Torus, it may be more significant for other instruments that use similar detectors.

The famous discovery of the Omega − in 1964 put the quark model on firm ground and since then a lot of effort has been spent on mapping out the baryonic and mesonic states. Over the following decades, many excited baryons with light quarks (up, down and strange) have been measured, but by most predictions, only a small percentage of those expected have been found. In this talk, I will discuss a newly developing technique using an (unflavored) photon beam to excite protons to doubly-strange "Cascade" (Xi) states. Advantages of such an experiment and associated difficulties will be presented, along with recent results from the CLAS detector at Jefferson Lab in Virginia.

The effect of divertor baffling on impurity neutral and impurity ion compression in ohmic discharges in the Alcator C-Mod tokamak is explored. Experiments are performed using a novel divertor bypass which allows in situ variations to the divertor baffling. The results indicate that divertor baffling in Alcator C-Mod plays an important role in impurity neutral compression, but little role in impurity ion compression. The apparent separation between neutral compression and ion compression is surprising, but is consistent with a model in which the neutral compression (both in deuterium and argon) is determined by a leakage flux through the baffle structure from the divertor region to the main chamber which is independent of the leakage conductance. Instead, the leakage fluxes appear to be determined by the divertor plasma conditions (including impurity and fuel ions), which are not influenced by the baffle structure.

The formation of internal transport barriers (ITBs) has been observed in the core region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement to low confinement mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes). Experiments with ion cyclotron range of frequencies power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for ten energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Simi...

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ≤ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.

Detailed measurements of the magnitude and location of volumetric recombination occurring in the detached divertor of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The drop in divertor plate ion current during detachment is due to two mechanisms: (1) volumetric recombination in the divertor plasma; and (2) reductions in the divertor ion source. Depending on plasma conditions, each of these can be the primary mechanism for the observed ion current reduction in detachment. The ion source during detachment is inferred and its magnitude is consistent with the measured divertor power flow. A scaling of the density in the divertor recombining region for L- (low confinement) mode plasmas is found, ne,r∝ne0.8⋅PSOL2/7. A model based on pressure variation along a flux surface during detachment is consistent with the main features of this scaling.

Recent breakthroughs in silicon detector technology now permit measurement of radiated power over a wide range in photon energies. These detectors (also known as AXUV photodiodes) have a flat spectral power response from ultraviolet to x-ray energies, and with a slightly reduced efficiency all the way down to visible wavelengths. Since they can be made small, multichannel detectors allow high spatial resolution to be combined with an intrinsic high temporal resolution, which can reach the microsecond range, depending on the application. Additional features include ease of use and installation, and relatively low cost compared to other techniques. A combination of two multichannel toroidally viewing systems has been recently installed on the Alcator C-Mod tokamak. The first array, which is composed of 16 channels, sees tangentially the outer-half of the plasma at the midplane, and is used to measure the total power radiated. The second array, also located at the midplane, consists of 19 channels and views ...

The Plasma Spectroscopy Group of The Johns Hopkins University is developing diagnostics for spectroscopic characterization of the tokamak plasma scrape-off layer and divertor regions. A far ultraviolet polychromator has been designed for radiative divertor studies at the Alcator C-Mod and D-IIID tokamaks. Local measurements of resonant transitions of lithium- to boron-like ions of intrinsic or seeded low Z impurity elements will be performed along multiple chords around the X point. Planar diffraction gratings and stacked grids will be used for dispersion and angular collimation of radiation. Phosphor wavelength converters coupled to a photomultiplier tube by an optical fiber will be used as detectors. The design provides a wavelength resolution of ≃10 A, a spatial resolution of ⩽2 cm, and an adequate photometric sensitivity. The in-vessel instrument, proposed for the Alcator C-Mod tokamak, will measure intensities of the lines at 1240 (N V), 765, 923 (N IV), and 990 A (N III). The port-mounted polychromator at DIII-D will be able to monitor intensities of 1550 (C IV), 977, 1176 (C III), and 1335 A (C II) lines. This, together with visible and bolometric diagnostics, should enable estimates of power losses, charge state distribution and local transport of the impurity ions in the divertor. A one-channel prototype of the C-Mod and D-IIID instruments is being built for the CDX-U spherical tokamak. Line-integrated brightnesses of the 2s–2p transition at 1550 A will be measured and inverted to obtain C IV emissivity distribution.

The intrinsic impurity content of molybdenum and carbon was measured in the Alcator C-Mod tokamak using low resolution, multilayer mirror (MLM) spectroscopy ( Delta lambda ~1-10 AA). Molybdenum was the dominant high-Z impurity and originated from the molybdenum armour tiles covering all of the plasma facing surfaces (including the inner column, the poloidal divertor plates and the ion cyclotron resonant frequency (ICRF) limiter) at Alcator C-Mod. Despite the all metal first wall, a carbon concentration of 1 to 2% existed in the plasma and was the major low-Z impurity in Alcator C-Mod. Thus, the behaviour of intrinsic molybdenum and carbon penetrating into the main plasma and the effect on the plasma must be measured and characterized during various modes of Alcator C-Mod operation. To this end, soft X-ray extreme ultraviolet (XUV) emission lines of charge states, ranging from hydrogen-like to helium-like lines of carbon (radius/minor radius, r/a~1) at the plasma edge to potassium to chlorine-like (0.4

A series of experiments, examining the confinement properties of ion cyclotron range of frequencies (ICRF) heated H mode plasmas, has been carried out on the Alcator C-Mod tokamak. Alcator C-Mod is a compact tokamak that operates at high particle, power and current densities at toroidal fields up to 8 T. Under these conditions the plasma is essentially thermal with very little contribution to the stored energy from energetic ions (typically no more than 5%) and with Ti~Te. Most of the data were taken with the machine in a single null `closed' divertor configuration with the plasma facing components clad in molybdenum tiles. The data include those taken both before and after the first wall surfaces were coated with boron, with emphasis on the latter. H modes obtained from plasmas run on boronized walls typically had a lower impurity content and radiated power and attained a higher stored energy than those run on bare molybdenum. Confinement enhancement, the energy confinement time normalized to L mode scaling, for discharges with boronized walls, ranged from 1.6 to 2.4. The unique operating regime of the Alcator C-Mod device provided a means for extending the tests of global scaling laws to parameter ranges not previously accessible. For example, the Alcator C-Mod edge localized mode (ELM)-free data were found to be 1.1 to 1.6 times the ITERH93 scaling and the ELMy data almost 2.0 to 2.8 times the ITERH92 ELMy scaling law, suggesting that the size scaling in both scalings may be too strong. While both ELM-free and ELMy discharges were produced, the ELM characteristics were not easily compared with observations on other devices. No large, low frequency ELMs were seen despite the very high edge pressure and temperature gradients that were attained. For all of our H mode discharges, a clear linear relationship between the edge temperature pedestal and the temperature gradient in the core plasma was observed; the discharges with the `best' transport barriers also showing the greatest improvement in core c

Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjunction with an atomic physics package for individual line emission. During Low-Confinement-Mode (L-mode) plasmas, the transport can be characterized by pure diffusion, with coefficients ∼5000 cm2/s, reflecting the ∼20 ms decay in the x-ray and VUV line brightnesses. During High-Confinement-Modes (H-modes), the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (o...

A series of experiments has been carried out to determine the relative screening of the low recycling gaseous impurities, nitrogen and methane, in Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] discharges. The impurity density in the core plasma is directly proportional to the rate of impurity injection. Screening was found to vary weakly with plasma density but to be markedly dependent on the poloidal position of injection. Detachment of the plasma results in significantly less screening both in Ohmic and in high confinement (H-mode) discharges, particularly when the gas is injected into the divertor or at the inner vessel wall. Evidence of impurity transport from the scrape-off layer into the divertor using bolometry and visible spectroscopy is presented and modeling of the transport using a two-dimensional Monte Carlo code is described.

Highly localized direct electron heating [full width at half-maximum (FWHM)≅0.2a] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center (r/a⩾0.3) has been observed in H(3He) discharges at B0=(6.0–6.5) T and ne(0)≅1.8×1020 m−3. [Here, the minority ion species is indicated parenthetically.] Off-axis heating (r/a⩾0.5) has also been observed in D(3He) plasmas at B0=7.9 T. The concentration of 3He in these experiments was in the range of n3He/ne≅(0.2–0.3) and the locations of the mode conversion layer and electron heating peak could be controlled by changing the 3He concentration or toroidal magnetic field (B0). The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to ...

Enhanced confinement modes up to a toroidal field of BT=8 T have been studied with up to 3.5 MW of radiofrequency (rf) heating power in the ion cyclotron range of frequencies (ICRF) at 80 MHz. H-mode is observed when the edge temperature exceeds a threshold value. The high confinement mode (H-mode) with higher confinement enhancement factors (H) and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement (H=2.0), high normalized beta (βN=1.5), low radiated power fraction (Pradmain/Ploss=0.3), and low effective charge (Zeff=1.5) has been obtained in Enhanced Dα H-mode. This type of H-mode has enhanced levels of continuous Dα emission and very little or no edge localized mode (ELM) activity, and reduced core particle confinement time relative to ELM-free H-mode. The pellet enhanced performance (PEP) mode is obtained by combining core fueling with pellet injection and core heating. A highly peaked pressure profile with a central value of 8 atmospheres was observed. The steep pressure gradient drives off-axis bootstrap current, resulting in a shear reversed safety factor (q) profile. Suppression of sawteeth appears to be important in maintaining the highly peaked pressure profile. Lithium pellets were found to be more effective than deuterium pellets in raising q0.

Local measurements of line emission of either intrinsic or gas puffed impurities will be performed around and below the X point. Such spectroscopic measurements, together with divertor Thomson scattering and bolometric imaging will enable estimates of divertor impurity particle distributions and power losses. A proposed multilayer mirror (MLM) diagnostic mounted inside the tokamak vessel will measure XUV (100–200 A) emission from impurities at multiple locations in the divertor region of the C-Mod tokamak. A prototype of this instrument measuring N V at 162.6 A is scheduled to be installed on C-Mod with a single chordal view of the X point. A multichordal/multispectral device using MLMs and gratings has been designed for the DIII-D tokamak. The instrument will measure the resonance emission of C II to C VI from 40–2000 A, thus enabling a more direct and reliable estimate of the impurity content in the divertor. At DIII-D, the device will be mounted on a port below the mid plane avoiding spectroscopic cont...

Results of ICRF heating experiments in Alcator C-Mod during the November 1994 to June 1995 campaign are summarized. Efficient heating of high-density plasmas was demonstrated with high power densities (up to volume averaged, surface averaged). These experiments were carried out with RF powers up to 3.5 MW at 80 MHz, at magnetic fields up to 8 T and plasma currents up to 1.2 MA. For on-axis hydrogen minority heating at 5.3 T, near complete absorption is achieved. Energy confinement in L-mode plasmas was found to be consistent with the ITER89-P scaling. H-mode is routinely observed when the ion drift is directed toward the X-point. The H-mode power threshold was found to scale as low as , which is a factor of two lower than the scaling observed on other tokamaks. PEP modes with highly peaked density and ion temperature profiles and highly enhanced fusion reactivity were obtained with Li pellet injection followed by on-axis ICRF heating at both (H minority heating) and 8 T ( minority heating). In H - plasmas at T highly localized direct electron heating by the mode converted ion Bernstein wave was observed. Nearly complete absorption by electrons in a small volume resulted in an extremely high electron heating power density of with .

The sign of the toroidal magnetic field has a major effect on the divertor in Alcator C-Mod, determining whether the higher recycling is on the inboard or outboard side and leading to inboard/outboard temperature and density differences of up to a factor of ten. A recently published paper (Hutchinson et al 1995 Plasma Phys. Control. Fusion 37 1389) reported in detail on these observations, which are an indication of the importance of plasma particle drifts for understanding and modelling the divertor. The conference presentation covered these published results and also some additional results, documented here, in which the plasma flows associated with the field-direction-dependent asymmetries are measured using a Mach probe and a localized impurity puffing plume analysis technique.

The divertor of the Alcator C‐Mod tokamak [Phys. Plasmas 1, 1511 (1994)] routinely radiates a large fraction of the power entering the scrape‐off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MW m−3 in ion cyclotron range of frequency (ICRF) heated discharges, have been measured using bolometer arrays. An analysis of both Ohmic and ICRF‐heated discharges has demonstrated some of the relative merits of detached divertor operation versus high‐recycling divertor operation. An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases.

The regime of high particle and energy confinement known as the H mode [Phys. Rev. Lett. 49, 1408 (1982)] has been extended to a unique range of operation for divertor tokamaks up to toroidal fields of nearly 8 T, line‐averaged electron densities of 3×1020 m−3, and surface power densities of nearly 0.6 MW/m2 in the compact high‐field tokamak Alcator C Mod [Phys. Plasmas 1, 1511 (1994)]. H modes are achieved in Alcator C Mod with Ion Cyclotron Resonant Frequency (ICRF) heating and with Ohmic heating alone without boronization of the all molybdenum tiled first wall. Large increases in charge exchange flux are observed during the H mode over the entire range of energies from 2 to 10 keV. There appears to be an upper limit to the midplane neutral pressure, of about 0.08 Pa above which no H modes have been observed. The plasmas with the best energy confinement have the lowest midplane neutral pressures, below 0.01 Pa. There is an edge electron temperature threshold such that Te≥280 eV ±40 eV for sustaining the...

Imbalances between the inboard and outboard legs of the single null divertor in tokamak Alcator C-Mod are observed to reverse when the direction of the toroidal field is reversed. These imbalances are measured by embedded probes in the target plates, tomographic reconstructions of bolometry and line radiation, and visible imaging. Density imbalances of about a factor of ten at the targets are observed at moderate density, decreasing as the density is raised until they are almost balanced. The data indicate that the electron pressure is not imbalanced, thus arguing against momentum imbalance as the cause of these drift-induced effects. Instead, power flux imbalance caused by Er V-product B convection, and enhanced by radiation, is suggested as the underlying cause.

The two-color soft x-ray (SXR) tomography diagnostic on the Madison Symmetric Torus is capable of making electron temperature measurements via the double-filter technique; however, there has been a 15% systematic discrepancy between the SXR double-filter (SXRDF) temperature and Thomson scattering (TS) temperature. Here we discuss calibration of the Be filters used in the SXRDF measurement using empirical measurements of the transmission function versus energy at the BESSY II electron storage ring, electron microprobe analysis of filter contaminants, and measurement of the effective density. The calibration does not account for the TS and SXRDF discrepancy, and evidence from experiments indicates that this discrepancy is due to physics missing from the SXRDF analysis rather than instrumentation effects.

A series of transport experiments has been carried out in Alcator‐C‐Mod. [Phys Plasmas 1, 1511 (1994)]. Data from both Ohmic and ICRF (ion cyclotron range of frequencies) heated plasmas can be fitted with an L‐mode (low mode) scaling law. The Ohmic τE’s show no scaling with density in any regime and can reach values of 2–3 times neo‐Alcator. Impurity confinement has been studied with the laser blow‐off technique with τI showing nearly linear scaling with plasma current. Ohmic and ICRF H modes are obtained over a wide range of discharge parameters, extending the range in the international database for nB, by almost a factor of 10. The power threshold for ELM‐free (edge localized mode) discharges is in rough agreement with the scaling P/S=0.044nB. Energy diffusivities of Ohmic and ICRF heated plasmas have been measured from local analysis of plasma profiles and power fluxes. The same analysis produces a value for plasma resistivity which lies between the Spitzer and neoclassical calculations. Analysis of pl...

Detailed measurements and transport analysis of divertor conditions in Alcator C‐Mod [Phys. Plasmas 1, 1511 (1994)] are presented for a range of line‐averaged densities, 0.7

Early results from the Alcator C-Mod tokamak indicate that ohmic energy confinement does not obey the Neo-Alcator scaling. Instead 'Ohmic L-mode' confinement, close to ITER89-P, is obtained, which substantially exceeds Neo-Alcator under some circumstances. ICRF-heated plasmas follow the same scaling. H-modes are observed both in ohmic and ICRF heated plasmas, with confinement enhancements of about 2 for ELM-free conditions. Divertor operation into the closed chamber shows different regimes, depending on the density. Of particular interest is the progressive detachment at moderate to high densities. Power to the divertor plates is generally low, reflecting the influence of strong radiation in the divertor. The scaling of parameters from Alcator's growing edge database is briefly summarized.

Ohmic H modes have been achieved in ALCATOR C-MOD with toroidal fields up to BT ≈ 5.25 T and line averaged densities up to ne ≈ 1.4 × 1020 m-3 with less than 1 MW of input power. In ELM-free cases at lower toroidal fields, the energy confinement increases by at least 60% and the particle confinement increases by more than a factor of two. The ELM-free H modes appear to fit the threshold power scaling observed on other tokamaks, P/S (MW/m2)=4.4 × 10-3 neBT (1019 m-3.T), up to power densities of about 0.15 MW/m2. At higher toroidal fields, ELMy H modes were observed after the injection of lithium pellets at about half of the scaled threshold power density. All of these results were obtained with molybdenum plasma facing components and the ELM-free H modes were obtained without boronization or other wall coatings

A soft x-ray (SXR) diagnostic to measure electron temperature on the Madison Symmetric Torus using two complementary methods is presented. Both methods are based on the double-foil technique, which calculates electron temperature via the ratio of SXR bremsstrahlung emission from the plasma in two different energy ranges. The tomographic emissivity method applies the double-foil technique to a tomographic reconstruction of SXR emissivity, creating a two-dimensional map of temperature throughout the plasma. In contrast, the direct brightness method applies the double-foil technique directly to the measured brightness and generates vertical and horizontal radial profiles. Extensive modeling demonstrates advantages and limitations in both techniques. For example, although the emissivity technique provides a two-dimensional mapping of temperature, its reliance on multiple tomographic inversions introduces some artifacts into the results. On the other hand, the more direct brightness technique avoids these artifacts but is only able to provide a radial profile of electron temperature.

Design for structural topology optimization is a method of distributing material within a design domain of prescribed dimensions. This domain is discretized into a large number of elements in which the optimization algorithm removes, adds, or maintains the amount of material. The resulting structure maximizes a prescribed mechanical performance while satisfying functional and geometric constraints. Among di erent topology optimization algorithms, the hybrid cellular automaton (HCA) method has proven to be efcient and robust in problems involving large, plastic deformations. The HCA method has been used to design energy absorbing structures subject to crash impact. The goal of this investigation is to extend the use of the HCA algorithm to the design of advanced armor systems subject to a blast load. The proposed algorithm drives the optimal distribution of a metallic phase and void, or two metallic phases, within the design domain. When the blast pressure wave hits the targeted structure, the uids kinetic energy is transformed into internal energy (IE) inside the solid medium. Maximum attenuation is reached when IE is maximized. Along with an optimum use of material, this condition is satised when IE is uniformly distributed in the design domain. This work makes use of the CONWEP model developed by the Army Research Laboratory and the DRDC plate model developed at Valcartier. The resulting structures show the potential of the HCA method when designing blast mitigating armor structures.

This work presents an application of the Hybrid Cellular Automaton (HCA) approach to the shape optimization of aluminum thin walled structures for blast mitigation. The design objective is to minimize the kinetic energy transferred from the blast event to the target. Fluid structure interaction between blast products and the target structure is considered as the driving factor in the blast mitigation problem. The method proposed couples an Arbitrary Lagrange Eulerian Fluid Structure Interaction FEA model with a reformulated HCA methodology to generate shape function concepts with improved pressure response attributes. This novel methodology is applied to the design of a target plate subject to a simulated surface detonation and shallow buried detonation. The proposed approach is shown to generate intricate shape designs dicult to obtain by function decomposition methods and is computationally ecient in terms of the total function evaluations required to achieve algorithm convergence. Solutions obtained by this method are shown to produce a considerable reduction in peak pressure and impulse imparted to the target over the original design. The optimized shape is dependant upon loading condition: whether the target is loaded by a surface or buried detonation.

A first order structural optimization problem is examined to evaluate the effects of structural geometry on blast energy transfer in a fully coupled fluid structure interaction problem. The fidelity of the fluid structure interaction simulation is shown to yield significant insights into the blast mitigation problem not captured in similar empirically based blast models. An emphasis is placed on the accuracy of simulating such fluid structure interactions and its implications on designing continuum level structures. Higher order design methodologies and algorithms are discussed for the application of such fully coupled simulations on vehicle level optimization problems.Copyright © 2010 by ASME

In order to characterize the Madison Symmetric Torus (MST) reversed-field pinch (RFP) plasmas that bifurcate to a helical equilibrium, the V3FIT equilibrium reconstruction code was modified to include a conducting boundary. RFP plasmas become helical at a high plasma current, which induces large eddy currents in MST's thick aluminum shell. The V3FIT conducting boundary accounts for the contribution from these eddy currents to external magnetic diagnostic coil signals. This implementation of V3FIT was benchmarked against MSTFit, a 2D Grad-Shafranov solver, for axisymmetric plasmas. The two codes both fit Bθ measurement loops around the plasma minor diameter with qualitative agreement between each other and the measured field. Fits in the 3D case converge well, with q-profile and plasma shape agreement between two distinct toroidal locking phases. Greater than 60% of the measured n = 5 component of Bθ at r = a is due to eddy currents in the shell, as calculated by the conducting boundary model.

In the general area of confinement improvement and concept advancement, recent results in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) include good confinement of both thermal and large-orbit ions and near doubling of total beta to 26% with deuterium pellet injection. Current profile control enables MST to reduce stochastic transport and achieve tokamak-like confinement. In standard RFP operation, substantial MHD tearing mode activity results in stochastic transport and an energy confinement time of about 1 ms in MST. Application of inductive current profile control reduces MHD activity and accompanying stochasticity, improving confinement by about a factor of ten. Previous work concentrated on electron confinement in improved-confinement RFP operation. Recent work confirms that ions are also well confined, and that high beta and improved confinement can be achieved simultaneously. PACS numbers: 52.55.Hc, 52.55.Dy (Some figures in this article are in colour only in the electronic version)