| 1. |
- Fenstermacher, M.E., et al.
(författare)
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DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy
- 2022
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 62:4
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Tidskriftsartikel (refereegranskat)abstract
- DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-I p steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L-H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at ∼8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-I p beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and regulate β N in the ITER baseline scenario, based on plasma response to probing 3D fields, was demonstrated. Finally, innovative tokamak operation using a negative triangularity shape showed many attractive features for future pilot plant operation.
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| 2. |
- Kehoe, Laura, et al.
(författare)
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Make EU trade with Brazil sustainable
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6438, s. 341-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Conrad, P.G., et al.
(författare)
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Environmental Dynamics and the Habitability Potential at Gale Crater, Mars
- 2013
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Konferensbidrag (refereegranskat)abstract
- The assessment of environmental habitability potential involves measurement of the chemical and physical attributes of the system as well as their dynamic interplay. The environmental dynamics describe the availability of both energy sources and raw materials for meeting the requirements of organisms and for altering the environment. Energetic exchange can also determine the preservation potential for organic materials in the rock record. During its first year at Gale Crater, the Mars Science Laboratory payload has directly measured the chemistry and physical attributes, e.g., temperature, humidity, radiation, pressure, etc. of the martian atmosphere. Curiosity has also acquired chemical and mineralogical data, both from a wind drift deposit of fines and from two examples of a sedimentary rock formation in a region of Gale Crater called Yellowknife Bay, some 445 meters to the east of Bradbury Landing, where Curiosity initially touched down. These data enabled inferences to be made regarding depositional environment and past habitability potential at Gale Crater. The rock chemistry data reveal signs of aqueous interaction i.e., H2O, OH and H2 and sufficient elemental basis (C, H, O, S and possibly N) for plausible nutrient supply, should Mars have ever had autotrophic prokaryotes to exploit it, and a range of redox conditions tolerable to Earth microbes is indicated by the presence of clay minerals. Curiosity’s observations of the chemical, physical and geologic features of Yellowknife Bay point to a formerly habitable environment.
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| 4. |
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| 5. |
- Wannberg, Gudmund, et al.
(författare)
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EISCAT_3D - a next-generation European radar system for upper atmosphere and geospace research
- 2010
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Ingår i: Radio Science Bulletin. - 1024-4530. ; :333, s. 75-88
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Tidskriftsartikel (refereegranskat)abstract
- The EISCAT Scientifi c Association, together with a number of collaborating institutions, has recently completed a feasibility and design study for an enhanced performance research radar facility to replace the existing EISCAT UHF and VHF systems. This study was supported by EU Sixth-Framework funding. The new radar retains the powerful multi-static geometry of the EISCAT UHF, but will employ phased arrays, direct-sampling receivers, and digital beamforming and beam steering. Design goals include, inter alia, a tenfold improvement in temporal and spatial resolution, an extension of the instantaneous measurement of full-vector ionospheric drift velocities from a single point to the entire altitude range of the radar, and an imaging capability to resolve small-scale structures. Prototype receivers and beamformers are currently being tested on a 48-element, 224 MHz array (the "Demonstrator") erected at the Kiruna EISCAT site, using the EISCAT VHF transmitter as an illuminator.
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| 6. |
- Webster, Christopher R., et al.
(författare)
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Background levels of methane in Mars' atmosphere show strong seasonal variations
- 2018
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Ingår i: Science. - : American Association for the Advancement of Science. - 0036-8075 .- 1095-9203. ; 360:6393, s. 1093-1096
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Tidskriftsartikel (refereegranskat)abstract
- Variable levels of methane in the martian atmosphere have eluded explanation partly because the measurements are not repeatable in time or location. We report in situ measurements at Gale crater made over a 5-year period by the Tunable Laser Spectrometer on the Curiosity rover. The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv). This variation is greater than that predicted from either ultraviolet degradation of impact-delivered organics on the surface or from the annual surface pressure cycle. The large seasonal variation in the background and occurrences of higher temporary spikes (~7 ppbv) are consistent with small localized sources of methane released from martian surface or subsurface reservoirs.
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| 7. |
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| 8. |
- Matthaiou, Michail, 1981, et al.
(författare)
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Sum Rate Analysis of ZF Receivers in Distributed MIMO Systems
- 2013
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Ingår i: IEEE Journal on Selected Areas in Communications. - 0733-8716 .- 1558-0008. ; 31:2, s. 180-191
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Tidskriftsartikel (refereegranskat)abstract
- The performance of single-cell distributed multiple-input multiple-output (D-MIMO) systems is not only affected by small-scale Rayleigh fading but also from large-scale fading and path-loss. In this paper, we elaborate on the sum rate of D-MIMO systems employing linear zero-forcing receivers, accounting for both large and small-scale fading effects, as well as spatial correlation at the transmit side. In particular, we consider the classical lognormal model and propose closed-form upper and lower bounds on the achievable sum rate. Using these bounds as a starting point, we pursue a "large-system" analysis and provide asymptotic expressions when the number of antennas at the base station (BS) grow large, and when the number of antennas at both ends grow large with a fixed and finite ratio. A detailed characterization in the asymptotically high and low signal to noise ratio regimes is also provided. An interesting observation from our results is that in order to maximize the sum rate, the RPs should be placed at unequal distances to the BS when they experience the same level of shadowing. The resulting closed-form expressions are compared with the corresponding results on MIMO optimal receivers.
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| 9. |
- Matthaiou, Michail, 1981, et al.
(författare)
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Sum rate analysis of ZF receivers in distributed MIMO systems with Rayleigh/Lognormal fading
- 2012
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Ingår i: IEEE International Conference on Communications. - 1550-3607. - 9781457720529 ; , s. 3857-3861
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Konferensbidrag (refereegranskat)abstract
- This paper presents a detailed sum rate characterization of distributed multiple-input multiple-output systems operating over composite fading channels and employing linear zero-forcing receivers. We consider the Rayleigh/Lognormal fading model and also take into account the effects of path-loss and spatial correlation at the transmit side. New closed-form upper and lower bounds on the achievable sum rate are proposed that apply for arbitrary numbers of antennas. Moreover, we investigate the concept of large-scale multiple-antenna systems when the number of receive antennas grow large. In this asymptotic regime, it is shown that the effects of Rayleigh fading are averaged out and the channel is dominated by the much more slowly varying shadowing. An interesting observation from our results is that in order to maximize capacity, the radio ports should be placed at unequal distances to the base station when they experience the same level of shadowing.
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| 10. |
- Matthaiou, Michail, 1981, et al.
(författare)
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On the condition number distribution of complex Wishart matrices
- 2010
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Ingår i: IEEE Transactions on Communications. - 1558-0857. ; 58:6, s. 1705-1717
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the distribution of the condition number of complex Wishart matrices. Two closely related measures are considered: the standard condition number (SCN) and the Demmel condition number (DCN), both of which have important applications in the context of multiple-input multiple-output (MIMO) communication systems, as well as in various branches of mathematics. We first present a novel generic framework for the SCN distribution which accounts for both central and non-central Wishart matrices of arbitrary dimension. This result is a simple unified expression which involves only a single scalar integral, and therefore allows for fast and efficient computation. For the case of dual Wishart matrices, we derive new exact polynomial expressions for both the SCN and DCN distributions. We also formulate a new closed-form expression for the tail SCN distribution which applies for correlated central Wishart matrices of arbitrary dimension and demonstrates an interesting connection to the maximum eigenvalue moments of Wishart matrices of smaller dimension. Based on our analytical results, we gain valuable insights into the statistical behavior of the channel conditioning for various MIMO fading scenarios, such as uncorrelated/semi-correlated Rayleigh fading and Ricean fading.
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