Measuring radio emission from air showers offers a novel way to determine properties of the primary cosmic rays such as their mass and energy. Theory predicts that relativistic time compression ...effects lead to a ring of amplified emission which starts to dominate the emission pattern for frequencies above MHz. In this article we present the first detailed measurements of this structure. Ring structures in the radio emission of air showers are measured with the LOFAR radio telescope in the frequency range of 110-190 MHz. These data are well described by CoREAS simulations. They clearly confirm the importance of including the index of refraction of air as a function of height. Furthermore, the presence of the Cherenkov ring offers the possibility for a geometrical measurement of the depth of shower maximum, which in turn depends on the mass of the primary particle.
We have developed thin film amorphous silicon alloy (a-Si:H) and nanocrystalline silicon (nc-Si:H) based multijunction solar cells on lightweight polymer substrate ~25 μm thick for space and ...near-space applications. The baseline cells use an a-Si:H/a-SiGe:H/a-SiGe:H structure deposited by conventional Radio Frequency (RF) plasma enhanced CVD using roll-to-roll deposition. The best initial performance for the baseline cells is aperture-area efficiency 9.84% and specific power ~1200 W/kg. The baseline cells are available to potential customers in large quantities. In order to increase the solar cell efficiency, we have pursued two new approaches. In the first, we use a Modified Very High Frequency (MVHF) technique to deposit the multijunction a-SiGe:H based cells. In the second, we have investigated nc-Si:H based multijunction cells. In this paper, we present the solar cell efficiency results on the three different device structures.
LOFAR sparse image reconstruction Garsden, H.; Girard, J. N.; Starck, J. L. ...
Astronomy and astrophysics (Berlin),
03/2015, Letnik:
575, Številka:
A90
Journal Article
Recenzirano
Odprti dostop
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components ...of the sky brightness. Recovering the original brightness distribution with aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods. Aims. Recent papers have established a clear link between the discrete nature of radio interferometry measurement and the “compressed sensing” (CS) theory, which supports sparse reconstruction methods to form an image from the measured visibilities. Empowered by proximal theory, CS offers a sound framework for efficient global minimization and sparse data representation using fast algorithms. Combined with instrumental direction-dependent effects (DDE) in the scope of a real instrument, we developed and validated a new method based on this framework. Methods. We implemented a sparse reconstruction method in the standard LOFAR imaging tool and compared the photometric and resolution performance of this new imager with that of CLEAN-based methods (CLEAN and MS-CLEAN) with simulated and real LOFAR data. Results. We show that i) sparse reconstruction performs as well as CLEAN in recovering the flux of point sources; ii) performs much better on extended objects (the root mean square error is reduced by a factor of up to 10); and iii) provides a solution with an effective angular resolution 2−3 times better than the CLEAN images. Conclusions. Sparse recovery gives a correct photometry on high dynamic and wide-field images and improved realistic structures of extended sources (of simulated and real LOFAR datasets). This sparse reconstruction method is compatible with modern interferometric imagers that handle DDE corrections (A- and W-projections) required for current and future instruments such as LOFAR and SKA.
Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications or extracting of the cosmological 21 cm signal from the ...LOw-Frequency ARray - Epoch of Reionization (LOFAR-EoR) data Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. Results. The brightness temperature of the detected Galactic emission is on average similar to 4 K in polarized intensity and covers the range from -10 to +13 rad m(-2) in Faraday depth, The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to approximate to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data. in line with results at higher frequencies Conclusions. The wide frequency range. high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of similar to 1-2 rad m(-2) in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.
The existence of double-double radio galaxies (DDRGs) is evidence for recurrent jet activity in active galactic nuclei (AGN), as expected from standard accretion models. A detailed study of these ...rare sources provides new perspectives for investigating the AGN duty cycle, AGN-galaxy feedback, and accretion mechanisms. Using wide-field imaging with the LOFAR telescope, we study both a well-known DDRG as well as a large number of radio sources in the field of view. We present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using LOFAR commissioning data. The spectral fit of the four components is consistent with a scenario in which the outer lobes are still active or the jets recently switched off, while emission of the inner lobes is the result of a mix-up of new and old jet activity. Finally, we show that the significant challenges of wide-field imaging can be solved using LOFAR commissioning data, thus demonstrating the potential of the full LOFAR telescope to discover millions of powerful AGN at redshift z ~ 1.
Context. The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), ...the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Aims. Here, the combined high spatial, spectral, and temporal resolution of the LOw Frequency ARray (LOFAR) was used to study solar Type III radio bursts at 30–90 MHz and their association with CMEs. Methods. The Sun was imaged with 126 simultaneous tied-array beams within ≤5 R⊙ of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (~83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR’s standard interferometric output is currently limited to one image per second. Results. Over a period of 30 min, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (~4 R⊙ from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. Conclusions. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.
Summary form only given. Hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin ...film silicon solar cells due to its superior long-wavelength response and stability against light-induced degradation. Due to the indirect band gap in crystalline silicon, the absorbing nc-Si:H layer needs to be much thicker than the corresponding a-SiGe:H layer. For nc-Si:H based solar cells to be commercially viable, the greatest challenge is to deposit the absorbing layers at a high rate with good spatial uniformity, while maintaining the same superior quality achieved at lower deposition rate. In this paper, we report on the development of our proprietary High Frequency (HF) glow discharge deposition technology to fabricate high efficiency, large area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate ≥1 nm/s. We have improved our nc-Si:H and a-Si:H processes to fabricate high performance component cells used in the triple-junction solar cells. We have fabricated small area cells (0.25 cm 2 ) and mini module (1.2 cm 2 ) cut out from the large deposited area. We have attained initial, active-area efficiency as high as ~14.0% and light-stabilized, active-area efficiency ~12.8% on these cells. SIMS analysis on the device show low impurity levels in the nc-Si:H absorbing layers. We have also fabricated large area encapsulated modules. We have attained initial aperture-area (~212 cm 2 ) efficiency of ~11.8% on an encapsulated module. These are the highest values measured at United Solar for such high rate samples. Detailed results will be presented at the conference.
Hydrogenated nanocrystalline silicon (nc-Si:H) is a promising candidate to replace the hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells due to ...its superior long-wavelength response and stability against light-induced degradation. Due to its indirect bandgap, the absorbing nc-Si:H layer needs to be much thicker than its amorphous counterpart. For nc-Si:H-based solar cells to be commercially viable, the challenge is to deposit the nc-Si:H layer at a high rate with good quality. In this paper, we report on the development of our proprietary high-frequency glow discharge deposition technology to fabricate high-efficiency, large-area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate >;1 nm/s. The National Renewable Energy Laboratory (NREL) has confirmed stable efficiency of 12.41% on a 1.05-cm 2 solar cell. We have attained initial efficiency of 12.33% on an encapsulated cell of aperture area ~400 cm 2 ; the corresponding stable efficiency is projected to be 11.7-11.9%.
Because of its superior long-wavelength response and stability against light-induced degradation, hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace ...hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells. In this paper, we report on the development of our proprietary high-frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high-quality nc-Si:H materials with good spatial uniformity. We studied the HF-deposited nc-Si:H material using various analytical techniques. We fabricated a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells that are deposited on textured Ag/ZnO back reflectors. Large-area cells were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. National Renewable Energy Laboratory (NREL) has confirmed (1) initial aperture-area efficiency of 11.8% on an 807.8-cm 2 encapsulated cell and (2) stable aperture-area efficiency of 11.2% on a 400-cm 2 encapsulated cell.
We present our progress in attaining high efficiency nc-Si:H solar cells at high deposition rates with superior light soaking stability. We have focused our effort on three areas: (i) improving the ...spatial uniformity and homogeneous properties for nc-Si:H, such as crystallite grain size and volume fraction, (ii) optimizing nucleation and seed layer during the initial growth of the nc-Si:H film, and (iii) optimizing nc-Si:H bulk growth and grain evolution. We have conducted an extensive study of the effect of process parameters including hydrogen dilution profiling, VHF power, and substrate temperature on the nc-Si:H film properties and component cell characteristics. We also conducted light soaking tests both indoors and outdoors. The a-Si:H/nc-Si:H/nc-Si:H triple-junction cells incorporating the optimized nc-Si:H component cells show significantly higher performance, achieving an 11.2% AM1.5 stabilized efficiency for both encapsulated large-area (464 cm 2 ) cells and inter-connected modules (2320 cm 2 ). To the best of our knowledge, this is the highest stabilized efficiency for a large-area thin-film silicon module.
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