Transverse instability of the antiproton beam in the Recycler Ring

The brightness of the antiproton beam in Fermilab's 8 GeV Recycler ring is limited by a transverse instability. This instability has occurred during the extraction process to the Tevatron for large stacks of antiprotons even with dampers in operation. This paper describes observed features of the instability, introduces the threshold phase density to characterize the beam stability, and finds the results to be in agreement with a resistive wall instability model. Effective exclusion of the longitudinal tails from Landau damping by decreasing the depth of the RF potential well is observed to lower the threshold density by up to a factor of two.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1 Apr 2011. New York, US

Effect of Overdispersion of Lethal Lesions on Cell Survival Curves

We explore the effects of overdispersed DNA lesion distribution on the shapes of cell surviving curves of mammalian cells exposed to hadrons at various doses. To provide a theoretical framework in resolving discrepancies between experimental data and Linear-quadratic (LQ)model predictions, we employ a non-Poisson distribution of lethal lesions together with non-homologous end-joining (NHEJ) pathway of double-strand break (DSB) repair. A negative binomial (NB)distribution is used to study the effect of the overdispersion on the shapes and possible reduction of dose-response curvature at high doses. The error distribution is customized to include an adjustable parameter so that the overdispersion parameter of NB is not constant but depends on the mean of the distribution. The trends in predicted cell survival responses are compared with the experimental data in low and high dose regions at various LET values for proton, helium, and carbon ions. The cell survival responses calculated by the present method reveal straightening of survival curves at high doses. This suggests that the overdispersion causes the cell survival dose-response to approximate log-linear behaviour at high doses. Comparison of the cell survival predictions with the Particle Irradiation Data Ensemble (PIDE) shows that the NB model provides better fits to the experimental data following low and intermediate doses. Whereas the model predictions are not validated at tiny and very high doses, nonetheless, the presented approach provides insight into underlying microscopic mechanisms which may help to improve the radiobiological responses along the dose-response curves and resolve discrepancies between experimental data and current cell survival models.Comment: 11 pages, 9 figure

Non-resonant microwave absorption studies of superconducting MgB_2

Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported. The NRMA results indicate near absence of intergranular weak links. A linear temperature dependence of the lower critical field H_c1 is observed indicating a non s-wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d-wave symmetry is also not observed.Comment: 8 pages, 2 figure

A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses

Multipath propagation in the interstellar medium distorts radio pulses, an effect predominant for distant pulsars observed at low frequencies. Typically, broadened pulses are analyzed to determine the amount of propagation-induced pulse broadening, but with little interest in determining the undistorted pulse shapes. In this paper we develop and apply a method that recovers both the intrinsic pulse shape and the pulse broadening function that describes the scattering of an impulse. The method resembles the CLEAN algorithm used in synthesis imaging applications, although we search for the best pulse broadening function, and perform a true deconvolution to recover intrinsic pulse structre. As figures of merit to optimize the deconvolution, we use the positivity and symmetry of the deconvolved result along with the mean square residual and the number of points below a given threshold. Our method makes no prior assumptions about the intrinsic pulse shape and can be used for a range of scattering functions for the interstellar medium. It can therefore be applied to a wider variety of measured pulse shapes and degrees of scattering than the previous approaches. We apply the technique to both simulated data and data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical Journa

Dynamics of Team Teaching and Research in a Management School: Learning and Imperatives

This paper discusses the experience of team teaching to address the issues of integration of academic inputs by bringing multi-disciplinary perspective together and thereby enhance learning experience of participants. The paper delves on the process, events and outcome of team teaching by four faculty members at IIM Ahmedabad who collaborated in teaching, writing cases, and doing research for a period of more than three years. The experience has been summarized using the following dimensions: need for team-teaching, existing mechanisms and barriers, opportunities and potential, imperatives, fall-outs and challenges experienced in the process. The cohesion, trust and mutual respect are key imperatives. The other factors contributing to the success of team-teaching are strong felt need by the members for integration in programmes, complementary skills and experiences of team members, frequent programmes with integrated components that provided continuous opportunities for learning, co-location of the instructors, and off-site programmes that provided opportunities for close get-togethers. The autonomy granted by the institution to instructors and co-coordinators to design and execute learning opportunities was also instrumental in the success. In an environment where the rule of the game is individualism, forming teams creates fears of loss of importance and recognition. The experience shows that the competency of the members and the overall effectiveness of the tasks are strengthened if the team believes in “reciprocating interdependence”. This can be possible only if members allow themselves opportunities to experiment, improvise and review

1.57 μm InGaAsP/InP surface emitting lasers by angled focus ion beam etching

The characteristics of 1.57 μm InGaAsP/InP surface emitting lasers based on an in-plan ridged structure and 45° beam deflectors defined by angled focused ion beam (FIB) etching are reported. With an externally integrated beam deflector, threshold currents and emission spectra identical to conventional edge emitting lasers are achieved. These results show that FIB etching is a very promising technique for the definition of high quality mirrors and beam deflectors on semiconductor heterostructures for a variety of integrated optoelectronic devices