Modeling laser wakefield accelerators in a Lorentz boosted frame

Modeling of laser-plasma wakefield accelerators in an optimal frame of reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of calculations from first principles. Obtaining these speedups requires mitigation of a high-frequency instability that otherwise limits effectiveness in addition to solutions for handling data input and output in a relativistically boosted frame of reference. The observed high-frequency instability is mitigated using methods including an electromagnetic solver with tunable coefficients, its extension to accomodate Perfectly Matched Layers and Friedman's damping algorithms, as well as an efficient large bandwidth digital filter. It is shown that choosing the frame of the wake as the frame of reference allows for higher levels of filtering and damping than is possible in other frames for the same accuracy. Detailed testing also revealed serendipitously the existence of a singular time step at which the instability level is minimized, independently of numerical dispersion, thus indicating that the observed instability may not be due primarily to Numerical Cerenkov as has been conjectured. The techniques developed for Cerenkov mitigation prove nonetheless to be very efficient at controlling the instability. Using these techniques, agreement at the percentage level is demonstrated between simulations using different frames of reference, with speedups reaching two orders of magnitude for a 0.1 GeV class stages. The method then allows direct and efficient full-scale modeling of deeply depleted laser-plasma stages of 10 GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively

Speeding up simulations of relativistic systems using an optimal boosted frame

It can be computationally advantageous to perform computer simulations in a Lorentz boosted frame for a certain class of systems. However, even if the computer model relies on a covariant set of equations, it has been pointed out that algorithmic difficulties related to discretization errors may have to be overcome in order to take full advantage of the potential speedup. We summarize the findings, the difficulties and their solutions, and show that the technique enables simulations important to several areas of accelerator physics that are otherwise problematic, including self-consistent modeling in three-dimensions of laser wakefield accelerator stages at energies of 10 GeV and above.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July 2009, eConf C09072

Would You Choose to be Happy? Tradeoffs Between Happiness and the Other Dimensions of Life in a Large Population Survey

A large literature documents the correlates and causes of subjective well-being, or happiness. But few studies have investigated whether people choose happiness. Is happiness all that people want from life, or are they willing to sacrifice it for other attributes, such as income and health? Tackling this question has largely been the preserve of philosophers. In this article, we find out just how much happiness matters to ordinary citizens. Our sample consists of nearly 13,000 members of the UK and US general populations. We ask them to choose between, and make judgments over, lives that are high (or low) in different types of happiness and low (or high) in income, physical health, family, career success, or education. We find that people by and large choose the life that is highest in happiness but health is by far the most important other concern, with considerable numbers of people choosing to be healthy rather than happy. We discuss some possible reasons for this preference

Evidence of English language proficiency and academic achievement of non-English-speaking background students

The increasing number of international students enrolled in Australian universities over the last decade has met with a corresponding concern that many non-English-speaking background (NESB) students experience considerable difficulty in their courses. Consequently, concerns about admission procedures have been raised regarding how English language proficiency (ELP) is determined for NESB students (both domestic and international). In addition to standardised ELP tests, some universities accept other forms of evidence, such as the completion of English-medium courses. This large-scale quantitative study analysed data on 5675 undergraduate and postgraduate students available from one university's database over a three-year period to ascertain if its ELP requirements were sufficient to ensure the academic progress of adequate numbers of these students. The best evidence for potential academic success was found to be standardised tests while students submitting other forms of ELP evidence tended to have more difficulties

Dancing raisins: levitation and dynamics of bodies in supersaturated fluids

A body immersed in a supersaturated fluid like carbonated water can accumulate a dynamic field of bubbles upon its surface. If the body is mobile, the attached bubbles can lift it upward against gravity, but arrival at a free surface can clean the body of these lifting agents and the body may plummet. The process then begins anew, and continues for as long as the concentration of gas in the fluid supports it. In this work, experiments using fixed and free immersed bodies reveal fundamental features of force development and gas escape. A continuum model which incorporates the dynamics of a surface buoyancy field is used to predict the ranges of body mass and size, and fluid properties, for which the system is most dynamic, and those for which body excursions are suppressed. Simulations are then used to probe systems which are dominated by a small number of large bubbles. Body rotations at the surface are found to be critical for driving periodic vertical motions of large bodies, which in turn can produce body wobbling, rolling, and damped surface 'bouncing' dynamics.Comment: 29 pages, 9 figures. Movies are posted here: https://people.math.wisc.edu/~spagnolie/publications.htm

Addressing the language and literacy needs of Aboriginal high school VET students who speak SAE as an additional language

Vocational Education and Training (VET) in high schools has had positive effects on the retention of Indigenous students, providing important pathways into further education and the workforce. However, low-level literacy (and numeracy) skills can make successful completion difficult, especially for students who speak Standard Australian English as an additional language or dialect. This article describes research undertaken to inform the development of a second language and literacy needs analysis model designed for high school VET teachers to address the needs of Indigenous students. The study draws on second language acquisition research, which demonstrates the value of using tasks as the basis for language teaching syllabus design, with needs analysis as a fundamental aspect of this. The project centred on Aboriginal high school VET students from remote and rural communities in Western Australia, who speak English as an additional language/dialect. Data collected included: individual and focus group interviews, training materials, and observation field notes on the language and literacy practices in classrooms and workplaces. The major findings focus on the development of oral language (for both job-oriented and social interactions in the workplace) and literacy skills, as well as the need to overcome ‘shame’ and develop confidence for speaking to non-Aboriginal people

Quantum Collective QCD String Dynamics

The string breaking model of particle production is extended in order to help explain the transverse momentum distribution in elementary collisions. Inspired by an idea of Bialas', we treat the string using a collective coordinate approach. This leads to a chromo-electric field strength which fluctuates, and in turn implies that quarks are produced according to a thermal distribution.Comment: 6 pages. Presented at SQM 2006. Submitted to J. Phys. G for publication in proceedings. Vers. 2: Minor revisions; final hadron spectrum calculation include

Effects of Hyperbolic Rotation in Minkowski Space on the Modeling of Plasma Accelerators in a Lorentz Boosted Frame

Laser driven plasma accelerators promise much shorter particle accelerators but their development requires detailed simulations that challenge or exceed current capabilities. We report the first direct simulations of stages up to 1 TeV from simulations using a Lorentz boosted calculation frame resulting in a million times speedup, thanks to a frame boost as high as gamma=1300. Effects of the hyperbolic rotation in Minkowski space resulting from the frame boost on the laser propagation in the plasma is shown to be key in the mitigation of a numerical instability that was limiting previous attempts