Direct thrust measurement of a 30-cm ion thruster

A direct thrust measurement of a 30-cm diameter ion thruster was accomplished by means of a laser interferometer thrust stand. The thruster was supported in a pendulum manner by three 3.65-m long wires. Electrical power was provided by means of 18 mercury filled pots. A movable 23-button planar probe rake was used to determine thrust loss due to ion beam divergence. Values of thrust, thrust loss due to ion beam divergence, and thrust loss due to multiple ionization were measured for ion beam currents ranging from 0.5 A to 2.5 A. Measured thrust values indicate an accuracy of approximately 1% and are in good agreement with thrust values calculated by indirect measurements

Protective telescoping shield for solar concentrator

An apparatus is described for use with a solar concentrator such as a parabolic dish which concentrates sunlight onto a small opening of a solar receiver, for protecting the receiver in the event of a system failure that could cause concentrated sunlight to damage the receiver. The protective apparatus includes a structure which can be moved to a stowed position where it does not block sunlight, to a deployed position. In this position, the structure forms a tube which substantially completely surrounds an axis connecting the receiver opening to the center of the concentrator at locations between the receiver and the concentrator

Inference of epidemiological parameters from household stratified data

We consider a continuous-time Markov chain model of SIR disease dynamics with two levels of mixing. For this so-called stochastic households model, we provide two methods for inferring the model parameters---governing within-household transmission, recovery, and between-household transmission---from data of the day upon which each individual became infectious and the household in which each infection occurred, as would be available from first few hundred studies. Each method is a form of Bayesian Markov Chain Monte Carlo that allows us to calculate a joint posterior distribution for all parameters and hence the household reproduction number and the early growth rate of the epidemic. The first method performs exact Bayesian inference using a standard data-augmentation approach; the second performs approximate Bayesian inference based on a likelihood approximation derived from branching processes. These methods are compared for computational efficiency and posteriors from each are compared. The branching process is shown to be an excellent approximation and remains computationally efficient as the amount of data is increased

Evaluation of the InDUCKtion project at UCL

Executive summary: There is evidence that a good induction to university life can help with student retention; however, there is also a danger of overwhelming students during the intense period of fresher’s week. Under the auspices of a small grant from the Higher Education Academy’s ‘Changing the Learning Landscape’ funding stream, staff at two universities (University College London and Southampton Solent University) collaborated to produce an innovative and engaging induction project entitled ‘InDUCKtion’, based on the idea of an induction duck being a fun character for students to interact with. At UCL, the InDUCKtion duck existed in the form of a physical plastic duck included in international postgraduate student induction packs, and they were encouraged to take photos of themselves in and around UCL and London as part of a photo challenge using social media. It was anticipated that this would enable students to familiarise themselves with the locale, make friends and have fun at the same time. The InDUCKtion duck was also evident on flyers and posters with QR codes advertising an online tour to enable students to gain an accelerated familiarisation with the campus and its facilities. Within UCL, the project was a collaborative, cross-departmental venture instigated by members of UCL’s E-Learning Environments (ELE) working in partnership with the Centre for the Advancement of Learning and Teaching (CALT) and Student Support and Wellbeing (SSW). The logistics of the project meant that the team members also had to liaise with a number of other individuals and departments around UCL, to help promote and implement the project. Despite a rapid following on Twitter in a relatively short period, a reasonable hit rate on the QR code for the main page of the online tour resource, and some engagement with the photo challenges using social media, participation in the project was lower than anticipated. Lessons learned from an evaluation perspective revealed that adding another activity to an already overwhelming fresher’s week was problematic, despite its innovative and interactive nature. The use of QR codes was problematic for a number of reasons, and the project needed more buy-in from student representatives and academics to provide institutional endorsement. Recommendations for future instances of the project include securing student representation and academic endorsement, integrating the activity with parallel induction activities – particularly with academic departments, replacing QR codes with an alternative technology-enhanced learning approach and optimising the learning design to better motivate students and promote groupwork

Towards Universal Topological Quantum Computation in the ν=5/2\nu=5/2 Fractional Quantum Hall State

The Pfaffian state, which may describe the quantized Hall plateau observed at Landau level filling fraction $\nu = 5/2$, can support topologically-protected qubits with extremely low error rates. Braiding operations also allow perfect implementation of certain unitary transformations of these qubits. However, in the case of the Pfaffian state, this set of unitary operations is not quite sufficient for universal quantum computation (i.e. is not dense in the unitary group). If some topologically unprotected operations are also used, then the Pfaffian state supports universal quantum computation, albeit with some operations which require error correction. On the other hand, if certain topology-changing operations can be implemented, then fully topologically-protected universal quantum computation is possible. In order to accomplish this, it is necessary to measure the interference between quasiparticle trajectories which encircle other moving trajectories in a time-dependent Hall droplet geometry.Comment: A related paper, cond-mat/0512072, explains the topological issues in greater detail. It may help the reader to look at this alternate presentation if confused about any poin

A Supersymmetric Flipped SU(5) Intersecting Brane World

We construct an N=1 supersymmetric three-family flipped SU(5) model from type IIA orientifolds on $T^6/(\Z_2\times \Z_2)$ with D6-branes intersecting at general angles. The spectrum contains a complete grand unified and electroweak Higgs sector. In addition, it contains extra exotic matter both in bi-fundamental and vector-like representations as well as two copies of matter in the symmetric representation of SU(5).Comment: 17 pages, 3 tables, v2 published in Phys.Lett.

A Lattice Test of 1/N_c Baryon Mass Relations

1/N_c baryon mass relations are compared with lattice simulations of baryon masses using different values of the light-quark masses, and hence different values of SU(3) flavor-symmetry breaking. The lattice data clearly display both the 1/N_c and SU(3) flavor-symmetry breaking hierarchies. The validity of 1/N_c baryon mass relations derived without assuming approximate SU(3) flavor-symmetry also can be tested by lattice data at very large values of the strange quark mass. The 1/N_c expansion constrains the form of discretization effects; these are suppressed by powers of 1/N_c by taking suitable combinations of masses. This 1/N_c scaling is explicitly demonstrated in the present work.Comment: 13 pages, 20 figures; v2 version to be published in PR

The JCMT dense gas survey of the Perseus Molecular Cloud

We present the results of a large-scale survey of the very dense gas in the Perseus molecular cloud using HCO+ and HCN (J = 4 - 3) transitions. We have used this emission to trace the structure and kinematics of gas found in pre- and protostellar cores, as well as in outflows. We compare the HCO+/HCN data, highlighting regions where there is a marked discrepancy in the spectra of the two emission lines. We use the HCO+ to identify positively protostellar outflows and their driving sources, and present a statistical analysis of the outflow properties that we derive from this tracer. We find that the relations we calculate between the HCO+ outflow driving force and the Menv and Lbol of the driving source are comparable to those obtained from similar outflow analyses using 12CO, indicating that the two molecules give reliable estimates of outflow properties. We also compare the HCO+ and the HCN in the outflows, and find that the HCN traces only the most energetic outflows, the majority of which are driven by young Class 0 sources. We analyse the abundances of HCN and HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is much more enhanced than the HCO+ in the outflow lobes. We suggest that this is indicative of shock-enhancement of HCN along the length of the outflow; this process is not so evident for HCO+, which is largely confined to the outflow base.Comment: 25 pages, 14 figures, 9 table

Feedback Control of Quantum Transport

The current through nanostructures like quantum dots can be stabilized by a feedback loop that continuously adjusts system parameters as a function of the number of tunnelled particles $n$. At large times, the feedback loop freezes the fluctuations of $n$ which leads to highly accurate, continuous single particle transfers. For the simplest case of feedback acting simultaneously on all system parameters, we show how to reconstruct the original full counting statistics from the frozen distribution.Comment: 4 pages, 2 figure