Miaomiao Zhou

Energy doubling of 42 GeV electrons in a metre-scale plasma wakefield accelerator

The energy frontier of particle physics is several trillion electron volts, but colliders capable of reaching this regime (such as the Large Hadron Collider and the International Linear Collider) are costly and time-consuming to build; it is therefore important to explore new methods of accelerating particles to high energies. Plasma-based accelerators are particularly attractive because they are capable of producing accelerating fields that are orders of magnitude larger than those used in conventional colliders. In these accelerators, a drive beam (either laser or particle) produces a plasma wave (wakefield) that accelerates charged particles. The ultimate utility of plasma accelerators will depend on sustaining ultrahigh accelerating fields over a substantial length to achieve a significant energy gain. Here we show that an energy gain of more than 42u2009GeV is achieved in a plasma wakefield accelerator of 85u2009cm length, driven by a 42u2009GeV electron beam at the Stanford Linear Accelerator Center (SLAC). The results are in excellent agreement with the predictions of three-dimensional particle-in-cell simulations. Most of the beam electrons lose energy to the plasma wave, but some electrons in the back of the same beam pulse are accelerated with a field of ∼52u2009GVu2009m-1. This effectively doubles their energy, producing the energy gain of the 3-km-long SLAC accelerator in less than a metre for a small fraction of the electrons in the injected bunch. This is an important step towards demonstrating the viability of plasma accelerators for high-energy physics applications.

A nonlinear theory for multidimensional relativistic plasma wave wakefields

A nonlinear kinetic theory for multidimensional plasma wave wakes with phase velocities near the speed of light is presented. This theory is appropriate for describing plasma wakes excited in the so-called blowout regime by either electron beams or laser pulses where the plasma electrons move predominantly in the transverse direction. The theory assumes that all electrons within a blowout radius are completely expelled. These radially expelled electrons form a narrow sheath just beyond the blowout radius which is surrounded by a region which responds weakly (linearly). This assumption is reasonable when the spot size of the electron beam and laser are substantially less than the blowout radius. By using this theory one can predict the wakefield amplitudes and blowout radius in terms of the electron beam or laser beam parameters, as well as predict the nonlinear modifications to the wake’s wavelength and wave form. For the laser case, the laser spot size must also be properly matched in order for a narrow sheath to form. The requirements for forming a spherical wave form, i.e., “bubble,” are also discussed. The theory is also used to show when linear fluid theory breaks down and how this leads to a saturation of the logarithmic divergence in the linear Green’s function.

Limits of linear plasma wakefield theory for electron or positron beams

The validity and usefulness of linear wakefield theory for electron and positron bunches is investigated. Starting from the well-known Green’s function for a cold-fluid plasma, engineering formulas for the maximum accelerating field for azimuthally symmetric bi-Gaussian beams of the form nb=nbe−r2∕2σr2e−z2∕2σz2 are derived. It is also found that for fixed beam parameters the optimum wake is obtained for kpσz=21∕2, for kpσr⩽1. The validity and usefulness of linear-fluid theory is studied using fully nonlinear particle-in-cell simulations. It is found that linear theory can be useful beyond the nominal range of validity for narrow bunches. The limits of usefulness differ significantly between electron and positron bunches. For electron bunches, scaling laws are found for three limits for optimal plasma density (kpσz=21∕2), characterized by the normalized spot size kpσr and the normalized charge per unit length of the beam, Λ≡(nb∕np)kp2σr2. These are e≡eE∕mcωp=1.3(nb∕np) for kpσr>1 and nb∕np<1, e=1.3Λln(1∕kp...

ErbB2-intronic microRNA-4728: a novel tumor suppressor and antagonist of oncogenic MAPK signaling.

Although the role of the ErbB2/HER2 oncogene in cancers has been extensively studied, how ErbB2 is regulated remains poorly understood. A novel microRNA, mir-4728, was recently found within an intron of the ErbB2 gene. However, the function and clinical relevance of this intronic miRNA are completely unknown. Here, we demonstrate that mir-4728 is a negative regulator of MAPK signaling through directly targeting the ERK upstream kinase MST4 and exerts numerous tumor-suppressive properties in vitro and in animal models. Importantly, our patient sample study shows that mir-4728 was under-expressed in breast tumors compared with normal tissue, and loss of mir-4728 correlated with worse overall patient survival. These results strongly suggest that mir-4728 is a tumor-suppressive miRNA that controls MAPK signaling through targeting MST4, revealing mir-4728’s significance as a potential prognostic factor and target for therapeutic intervention in cancer. Moreover, this study represents a conceptual advance by providing strong evidence that a tumor-suppressive miRNA can antagonize the canonical signaling of its host oncogene.

Energy Measurements of Trapped Electrons from a Plasma Wakefield Accelerator

Recent electron beam driven plasma wakefield accelerator experiments carried out at SLAC indicate trapping of plasma electrons. More charge came out of than went into the plasma. Most of this extra charge had energies at or below the 10 MeV level. In addition, there were trapped electron streaks that extended from a few GeV to tens of GeV, and there were mono-energetic trapped electron bunches with tens of GeV in energy.

Electron Bunch Length Measurements in the E‐167 Plasma Wakefield Experiment

Bunch length is of prime importance to beam driven plasma wakefield acceleration experiments due to its inverse relationship to the amplitude of the accelerating wake. We present here a summary of work done by the E167 collaboration measuring the SLAC ultra-short bunches via autocorrelation of coherent transition radiation. We have studied material transmission properties and improved our autocorrelation traces using materials with better spectral characteristics.

A Meter‐Scale Plasma Wakefield Accelerator

Plasma wakefield accelerators (PWFA) have recently shown substantial progress, attaining accelerating fields of more than 30 GV/m. The goal of the present experiment is to show that such accelerating fields can be sustained over the scale of a meter, resulting in a total energy gain comparable to the entire SLAC linear accelerator. We also seek to determine which factors limit the length of the interaction and determine the maximum achievable energy.

Measurement of the Decelerating Wake in a Plasma Wakefield Accelerator

Recent experiments at SLAC have shown that high gradient acceleration of electrons is achievable in meter scale plasmas. Results from these experiments show that the wakefield is sensitive to parameters in the electron beam which drives it. In the experiment the bunch lengths were varied systematically at constant charge. The effort to extract a measurement of the decelerating wake from the maximum energy loss of the electron beam is discussed.

Advanced accelerator simulation research: miniaturizing accelerators from kilometers to meters

Advanced accelerator research is aimed at finding new technologies that can dramatically reduce the size and cost of future high-energy accelerators. Supercomputing is already playing a dramatic and critical role in this quest. One of the goals of the SciDAC Accelerator Modeling Project is to develop code and software that can ultimately be used to discover the underlying science of new accelerator technology and then be used to design future high-energy accelerators with a minimum amount of capital expenditure on large-scale experiments. We describe the existing hierarchy of software tools for modelling advanced accelerators, how these models have been validated against experiment, how the models are benchmarked against each other, and how these tools are being successfully used to elucidate the underlying science.