Academic literature on the topic 'SRFT'

This thesis discusses an experiment, which has demonstrated transverse laser cooling of a pulsed supersonic beam of strontium monofluoride (SrF) molecules. Producing ultracold molecules is important because they could advance many fields including many-body physics, quantum chemistry and precision measurements to explore fundamental forces in nature. Direct laser cooling of molecules is a new and promising way to produce molecules with temperatures in the sub-millikelvin range. In the experiment, SrF molecules produced from a pulsed supersonic source were cooled in the transverse direction using light from just two lasers. The molecular beam brightness was increased by about 20%. I discuss the detailed experimental setup, laser system and data analysis. I also present several theoretical models, which give insight into the cooling experiment. Finally, I discuss improvements to this experiment, which should enable higher yields of ultracold molecules to be produced.

The advantages of contemporary particle injectors are high bunch charges and good beam quality in the case of normal conducting RF guns and increased repetition rates in the one of DC injectors. The technological edge of the concept of superconducting radio frequency injectors is to combine the strengths of both these sides. As many future accelerator concepts, such as energy recovery linacs, high power free electron lasers and certain collider designs, demand particle sources with high bunch charges and high repetition rates combined, applying the superconductivity of the accelerator modules to the injector itself is the next logical step. However, emittance compensation — the cornerstone for high beam quality — in case of a superconducting injector is much more challenging than in the normal conducting one. The use of simple electromagnets generating a solenoid field around the gun’s resonator interferes with its superconducting state. Hence, it requires novel and sophisticated techniques to maintain the high energy gain inside the gun cavity, while at the same time alleviating the detrimental fast transverse emittance growth of the bunch. In the case of the ELBE accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, a superconducting electron accelerator provides beam for several independent beamlines in continuous wave mode. The applications include IR to THz free electron lasers, neutron and positron generation, to Thompson backscattering with an inhouse TW laser, and hence, call for a flexible CW injector. Therefore, the development of a 3.5 cell superconducting electron gun was initiated in 1997. The focus of this thesis lies on three approaches of transverse emittance compensation for this photoinjector: RF focusing, the installation of a superconducting solenoid close to the cavity’s exit, and the introduction of a transverse electrical mode of the RF field in the resonator. All three methods are described in theory, examined by numerical simulation, and experimentally reviewed in the particular case of the ELBE SRF Gun II at HZDR and a copy of its niobium resonator at Thomas Jefferson National Laboratory, Newport News, VA, USA.

Serum Response Factor regulates a large array of genes involved in diverse processes including cell proliferation, muscle differentiation and development, and cytoskeletal processes such as cell migration and adhesion. The specificity and versatility of the SRF responses is achieved by combinatorial interactions with accessory factors. SRF binds to the CC(A/T)2A(A/T)3GG CArG box consensus sequence within the promoters of its target genes and acts as a docking platform for diverse signal regulated and cell- type specific cofactors to elicit their distinct responses. In fibroblasts two pathways signal through SRF in a mutually exclusive manner. MAP kinase signalling results in transcriptional activation of a subset of SRF target genes, via the interaction of SRF with members of the TCF family of Ets domain proteins. In contrast Rho-signalling induced changes in actin dynamics result in the association of SRF with members of the Myocardin-related family of SRF cofactors (MAL/MRTF-A/MKL1 and MAL16/MRTF-B/MKL2). The results described in this thesis characterise the molecular mechanism of MAL-SRF complex formation. MAL binds SRF as a dimer via a seven-residue core sequence within the MAL B1 region. Residues in the neighbouring Q-box enhance MAL-SRF complex formation, although these do not contact SRF directly. The MAL-SRF interaction displays the properties of a Rho-regulated cofactor. MAL competes with TCF for SRF binding due to the interaction of both cofactors with the same hydrophobic groove and pocket on SRF. In contrast to TCF, MAL-SRF complex formation depends on the intact N-terminus of the SRF DNA-binding domain. Mutations in the SRF al-helix that reduce DNA bending also impair complex formation with MAL. These mutations however do not affect DNA distortion in the MAL-SRF complex. Efficient MAL-SRF complex formation requires that SRF be bound to its cognate DNA and that MAL directly contacts DNA on either side of the CArG box. My results support a model in which each MAL monomer adds a p-strand consisting of the core B1 sequence, to the p-sheet of the SRF DNA-binding domain in a similar way to TCF, while also making direct DNA contacts in the ternary complex facilitated by SRF- induced DNA distortion. My analysis of complex formation between MAL and SRF demonstrates that members of the MRTF and TCF families of SRF cofactors interact with SRF using related but distinct mechanisms, thus providing a molecular rationale for their mutually exclusive transcriptional responses and the specificity of signalling to SRF.

La réduction du cout de construction et d’exploitation des futurs accélérateurs d particules, a grande et petite échelles, dépend du développement de nouveaux matériaux pour les surfaces actives des structures supraconductrices en radiofréquence (SRF). Les propriétés SRF sont essentiellement un phénomène de surface vu que la profondeur de pénétration (profondeur de pénétration de London, λ) des micro-ondes (RF) est typiquement de l’ordre de 20 à 400 nm en fonction du matériau. Lorsque les procédés de préparation de surface sont optimises, la limite fondamentale du champ RF que les surfaces SRF peuvent supporter est le champ RF maximum, Hc₁, au-delà duquel le flux magnétique commence à pénétrer la surface du supraconducteur. Le matériau le plus utilise pour des applications SRF est le niobium (Nb) massif, avec un champ Hc₁ de l’ordre de 170 mT, qui permet d’atteindre un champ accélérateur de moins de 50 MV/m. Les meilleures perspectives d’amélioration des performances des cavités SRF sont liées à des matériaux et méthodes de production produisant la surface SRF critique de façon contrôlée. Dans cette optique, deux avenues sont explorées pour utiliser des couches minces pour augmenter les performances des structures SRF au-delà du Nb massif, en monocouche ou en structures multicouches Supraconducteur-Isolant-Supraconducteur (SIS) : La première approche est d’utiliser une couche de Nb déposée sur du cuivre (Nb/Cu) à la place du Nb massif. La technologie Nb/Cu a démontré, au cours des années, être une alternative viable pour les cavités SRF. Toutefois, les techniques de dépôt communément utilisées, principalement la pulvérisation magnétron, n’ont jusqu’à présent pas permis de produire des surfaces SRF adaptées aux performances requises. Le récent développement de techniques de dépôt par condensation énergétiques, produisant des flux d’ions énergétiques de façon contrôlée (telles que des sources d’ions ECR sous ultravide) ouvrent la voie au développement de films SRF de grand qualité. La corrélation entre les conditions de croissance, l’énergie des ions incidents, la structure et les performances RF des films produits est étudiée. Des films Nb avec des propriétés proches du Nb massif sont ainsi produits. La deuxième approche est basée sur un concept qui propose qu’une structure multicouche SIS déposée sur une surface de Nb peut atteindre des performances supérieures à celles du Nb massif. Bien que les matériaux supraconducteurs à haute Tc aient un champ Hc₁ inférieur à celui du Nb, des couches minces de tels matériaux d’une épaisseur (d) inférieure à la profondeur de pénétration voient une augmentation de leur champ parallèle Hc₁ résultant au retardement de la pénétration du flux magnétique. Cette surcouche peut ainsi permettre l’écrantage magnétique de la surface de Nb qui est donc maintenue dans l’état de Meissner à des champs RF bien plus importants que pour le Nb massif. La croissance et performance de structures multicouches SIS basées sur des films de NbTiN, pour le supraconducteur, et de l’AlN, pour le diélectrique, sont étudiées. Les résultats de cette étude montrent la faisabilité de cette approche et le potentiel qui en découle pour l’amélioration des performances SRF au-delà du Nb massif<br>The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth λ, typically between 20 and 400 nm depending on the material. When other technological processes are optimized, the fundamental limit to the maximum supportable RF field amplitude is understood to be the field at which the magnetic flux first penetrates into the surface, Hc₁. Niobium, the material most exploited for SRF accelerator applications, has Hc₁~170 mT, which yields a maximum accelerating gradient of less than 50 MV/m. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. In this context, two avenues are pursued for the use of SRF thin films as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high field performance. High quality films can be grown using methods of energetic condensation, such as Electron Cyclotron Resonance (ECR) Nb ion source in UHV which produce higher flux of ions with controllable incident angle and kinetic energy. The relationship between growth conditions, film microstructure and RF performance is studied. Nb films with unprecedented “bulk-like” properties are produced. The second approach is based on the proposition that a Superconductor/Insulator/Superconductor (S-I-S) multilayer film structure deposited on an Nb surface can achieve performance in excess of that of bulk Nb. Although, many higher-Tc superconducting compounds have Hc₁ lower than niobium, thin films of such compounds with a thickness (d) less than the penetration depth can exhibit an increase of the parallel Hc₁ thus delaying vortex entry. This overlayer provides magnetic screening of the underlying Nb which can then remain in the Meissner state at fields much higher than in bulk Nb. A proof of concept is developed based on NbTiN and AlN thin films. The growth of NbTiN and AlN films is studied and NbTiN-based multilayer structures deposited on Nb surfaces are characterized. The results from this work provide insight for the pursuit of major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low footprint compact machines to energy frontier facilities

Modern physics research relies on particle accelerators and available beam time is a very limited resource. The ARIEL eLINAC will strengthen the rare isotope program at TRIUMF by providing an alternative way to create rare isotope beams (RIB). A possible way to add additional use to this machine is to create a return beam line and use the beam to excite a free electron laser (FEL). The remaining beam can be used to drive fields in the SRF cavities to reduce the required RF power. One limitation of these energy recovery LINACs (ERL) is beam-break up. Higher order modes (HOM), especially dipole modes, have a negative influence on the beam which can lead to beam loss. The design of the SRF cavity has to accommodate this to make sure a beam current of up to 10mA can be used for both RIB production and ERL operation. This thesis will go through the design process of the ARIEL 1.3 GHz nine-cell cavity. The design relies on simulations to calculate the fields inside the cavity and with it the shunt impedance of HOMs. The investigations showed that resistive beam line absorbers can be used to reduce the shunt impedance of HOMs sufficiently without interfering with the accelerating mode. The performance of the absorber material has been verified in dedicated low temperature measurements, while the HOM field distribution has been measured via beadpulling on a copper model of the cavity. These measurements showed good agreement with the simulations. The power dissipation in the SRF cavities is of vital importance. The cryogenic system is a signicant part of the capital investment for the accelerator and sets the power budget for each cavity to around 10 W. This corresponds to a Q₀ value of 1 x 10¹⁰ at an operational temperature of 2 K. The gradient goal is 10 MV/m to reach the design energy of 50 MeV with five cavities. Both Q₀ and Eacc specifications have been met in the first two cavities that are installed in cryomodules. Two more cavities have been built and are in their qualification phase.<br>Science, Faculty of<br>Physics and Astronomy, Department of<br>Graduate

Serum Response Factor (SRF) is controlled by actin dynamics at many of its target genes: Rho-induced depletion of G-actin is sensed by MAL, a member of the myocardin family of SRF coactivators. MAL binds G-actin via its N-terminus, the "RPEL domain", containing three RPEL motifs. MAL rapidly circulates between nucleus and cytoplasm in resting NIH3T3 fibroblasts. It accumulates in the nucleus and activates SRF upon serum stimulation, which alters interactions between G-actin and the RPEL domain. In contrast, myocardin (MC) itself is constitutively nuclear and active when expressed in fibroblasts, suggesting that it is not controlled through Rho. This thesis addresses the mode and functions of actin binding by myocardin-family proteins. Actin binding targets MAL for efficient CRM1-mediated nuclear export. Nuclear accumulation of MAL is not sufficient for activation of SRF-mediated transcription unless an inhibitory MAL-actin interaction in the nucleus is released. Actin therefore fulfils a dual role in MAL regulation by controlling MAL localisation as well as activity. The MAL RPEL domain is sufficient to confer actin-regulated nucleocytoplasmic trafficking and binds multiple actin molecules, efficiently sequestering them from polymerisation. Actin-binding toxins directly interfere with the MAL-actin complex. The RPEL motif represents an actin-binding unit: affinities of MAL RPEL motifs 1 and 2 for actin are relatively high while RPEL3 binds actin weakly. RPEL motifs cooperate to regulate MAL. The regulatory contribution of an RPEL3-actin interaction depends on actin binding by the RPEL 1-2 unit, differences in which account for differential regulation of MAL and MC, which binds actin weakly. A model of MAL regulation by differential actin occupancy of multiple RPEL motifs is proposed. Crystal structures of MAL RPEL motifs 1 and 2 bound to G-actin were obtained. RPEL motifs maintain hydrophobic interactions with a hydrophobic cleft at the subdomain 1-3 interface of actin and a "platform" on subdomain 3, both at the "base" of the actin molecule in its conventional view (Kabsch et al., 1990). The RPEL motif also establishes critical polar interactions with actin. Conservation of the RPEL motif reflects actin binding. The structures rationalise RPEL-actin affinities and competition of actin-binding toxins and profilin with MAL. A crystal structure of the MAL RPEL domain bound to three actin molecules revealed an additional actin-binding site within the RPEL 1-2 linker and actin-actin contacts in the RPEL domain-actin complex.

Superconducting radio frequency (SRF) technology is widely adopted in particle accelerators. There remain many open questions, however, in developing a systematic understanding of the fundamental behavior of SRF materials, including niobium treated in different ways and various other bulk/thin film materials that are fabricated with different methods under assorted conditions. A facility that can measure the SRF properties of small samples in a range of 2∼40 K temperature is needed in order to fully answer these questions. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. It consists of a sapphire-loaded cylindrical Nb TE011 cavity at 7.4 GHz with a 50 mm diameter flat sample placed on a non-contacting end plate and uses a calorimetric technique to measure the radio frequency (RF) induced heat on the sample. Driving the resonance to a known field on this surface enables one to derive the surface resistance of a relatively small localized area. TE011 mode identification has been done at room temperature and 4 K, and has been compared with Microwave StudioRTM and SuperFish simulation results. RF loss mechanisms in the SIC system are under investigation. A VCO phase lock loop system has been used in both CW and pulsed mode. Two calorimeters, with stainless steel and Cu as the thermal path material for high precision and high power versions, respectively, have been designed and commissioned for the SIC system to provide low temperature control and measurement. A power compensation method has been developed to measure the RF induced power on the sample. Simulation and experimental results show that with these two calorimeters, the whole thermal range of interest for SRF materials has been covered, The power measurement error in the interested power range is within 1.2% and 2.7% for the high precision and high power versions, respectively. Temperature distributions on the sample surface for both versions have been simulated and the accuracy of sample temperature measurements have been analysed. Both versions have the ability to accept bulk superconductors and thin film superconducting samples with a variety of substrate materials such as Al, A12O3, Cu, MgO, Nb and Si. Tests with polycrystalline and large grain bulk Nb samples have been done at impedance, least-squares fittings have been done using SuperFit2.0, a code developed by G. Ciovati and the author.;Microstructure analyses and SRF measurements of large scale epitaxial MgB2 films have been reported. MgB2 films on 5 cm dia. sapphire disks were fabricated by a Hybrid Physical Chemical Vapor Deposition (HPCVD) technique. The electron-beam backscattering diffraction (EBSD) results suggest that the film is a single crystal complying with a MgB2(0001)//A1 2O3(0001) epitaxial relationship. The SRF properties of different film thicknesses (200 nm and 350 nm) were evaluated using SIC system under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9+/-2 muO has been observed at 2.2 K.;Based on BCS theory with moving Cooper pairs, the electron states distribution at 0K and the probability of electron occupation with finite temperature have been derived and applied to anomalous skin effect theory to obtain the surface impedance of a superconductor with moving Cooper pairs. We present the numerical results for Nb.

Serum response factor (SRF) controls gene activation in response to changes in actin dynamics and mitogen-activated protein kinases. SRF has low intrinsic transcriptional activity and requires the recruitment of one of two families of co-activators: the MRTFs (myocardin-related transcription factors) and the TCFs (ternary complex factors). MRTFs are actin-binding proteins. Disruption of the actin-MRTF interaction is sufficient to induce MRTF nuclear accumulation and transcriptional activation. The TCF family are specifically activated by MAPK signalling. This thesis aims to elucidate how the SRF transcription network is controlled. The work presented encompasses two projects focused on each of the co-activator families. The regulation of MRTF shuttling from the cytoplasm to the nucleus is relatively well understood while its regulation once in the nucleus is still uncharacterized. The work demonstrates that nuclear MRTF activities are influenced by nuclear actin. Nuclear actin interferes with MRTF-DNA binding and target gene activation. In the presence of G-actin, nuclear MRTF can associate with target loci and recruit Pol II that, although traverses the gene body, does not generate stable mRNA. This inhibited state is accompanied by hypo-phosphorylation of the Pol II CTD. Dissociation of MRTF-actin interaction is required to re-establish Pol II phosphorylation and mRNA accumulation. The Erk-TCF signalling pathway was used to investigate how chromatin signatures are established in response to cues. Fibroblasts lacking all three TCFs, or reconstituted with mutant derivatives of the Elk-1 TCF were generated. Following Erk activation, chromatin immunoprecipitation and RNA-sequencing techniques, were employed to study the role of the TCFs in chromatin changes and transcriptional activation. It was possible to show that signal-induced chromatin changes occur in absence of transcription, and the specific chromatin signature requires Elk-1 DNA binding and phosphorylation. In addition analysis of the H3K27me3 mark demonstrated that Elk-1 activation is required to maintain a permissive chromatin landscape.

Dans un contexte mondial où les ressources énergétiques commencent à se faire rares, beaucoup de recherches se font sur l’amélioration de l’efficacité thermique et de la densité de puissance des sources d’énergie existantes. Ainsi, un projet de développement d’une microturbine à gaz avec une architecture de nouveau genre permettant d’augmenter la densité de puissance tout en réduisant les coûts a vu le jour. La recherche proposée dans le présent document se concentre sur la conception et la caractérisation d’une chambre de combustion et d’un banc d’essai pour la turbine SRGT-2. Une chambre de combustion à écoulement inverse est conçue et caractérisée expérimentalement. Un modèle 0D de la chambre est tout d’abord fait. Par la suite, une optimisation numérique est faite jusqu’à l’atteinte des objectifs de conception. Finalement, la chambre de combustion est testée durant 30 secondes avec de l’hydrogène comme carburant. Une température de sortie de la chambre de combustion de 1000 K a été maintenue avec une efficacité de combustion de plus de 85%. Le banc d’essai conçu pour le projet de recherche utilise un démarreur électropneumatique permettant d’accélérer le prototype jusqu’à 102 000 RPM. Le module fluide est la partie du banc d’essai qui contient les différentes parties de la turbine SRGT-2 comme le rotor, les stators et la chambre de combustion. Le module est instrumenté dans le but d’obtenir une caractérisation complète de la turbine. Sa configuration modulaire permet aussi de caractériser chacune des composantes individuellement en changeant certaines sections.