87 research outputs found
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The influence of the spatial frequency content of discrete roughness distributions on the development of the crossflow instability
An experimental investigation on the influence of the spatial frequency content of roughness distributions on the development of crossflow instabilities has been carried out. From previous research it is known that micro roughness elements can have a large influence on the crossflow development. When the spanwise spacing is chosen such that it is the most unstable wavelength (following linear stability analysis), stationary crossflow waves are amplified. While in earlier studies the focus was on the height or spanwise spacing of roughness elements, in the present study it is chosen to vary the shape of the elements. Through the modification of the shape the forcing at the critical wavelength is increased, while the forcing at the harmonics of the critical wavelength is damped. Experiments were carried in the low turbulence wind tunnel at City University London (Tu=0.006%) on a swept flat plate in combination with displacement bodies to create a sufficiently strong favourable pressure gradient. Hot wire measurements across the plate tracked the development of stationary and travelling crossflow waves. Initially, stronger crossflow waves were found for the elements with stronger forcing, while further downstream the effect of forcing diminished. Spatial frequency spectra showed that the stronger forcing at the critical wavelength (via the roughness shape) dominates the response of the flow while low forcing at the harmonics has no notable effect. Additionally, high resolution streamwise hot wire scans showed that the onset of secondary instability is not significantly influenced by the spatial frequency content of the roughness distribution
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Comparison of experimental data and 3D simulations of ion beam neutralization from the neutralized transport experiment
The Neutralized Transport Experiment (NTX) at Lawrence Berkeley National Laboratory has been designed to study the final focus and neutralization of high perveance ion beams for applications in heavy ion fusion (HIF) and high energy density physics (HEDP) experiments. Pre-formed plasmas in the last meter before the target of the scaled experiment provide a source of electrons which neutralize the ion current and prevent the space-charge induced spreading of the beam spot. NTX physics issues are discussed and experimental data is analyzed and compared with 3D particle-in-cell simulations. Along with detailed target images, 4D phase-space data of the NTX at the entrance of the neutralization region has been acquired. This data is used to provide a more accurate beam distribution with which to initialize the simulation. Previous treatments have used various idealized beam distributions which lack the detailed features of the experimental ion beam images. Simulation results are compared with NTX experimental measurements for 250 keV K{sup +} ion beams with dimensionless perveance of 1-7 x 10{sup -4}. In both simulation and experiment, the deduced beam charge neutralization is close to the predicted maximum value
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End-to-end simulations of an accelerator for heavy ion beam bunching
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Extreme compression of heavy ion beam pulses: Experiments and modeling
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Neutralized transport of high intensity beams
The NTX experiment at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final focus systems for high perveance heavy ion beams. A converging ion beam at the exit of the final focus magnetic system is injected into a neutralized drift section. The neutralization is provided by a metal arc source and an RF plasma source. Effects of a ''plasma plug'', where electrons are extracted from a localized plasma in the upstream end of the drift section, and are then dragged along by the ion potential, as well as the ''volumetric plasma'', where neutralization is provided by the plasma laid down along the ion path, are both studied and their relative effects on the beam spot size are compared. Comparisons with 3-D PIC code predictions will also be presented
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A Space-Charge Neutralizing Plasma Channel for an Intense Beam
Ion bunches have been suggested as means to heat matter to the warm dense matter, or strongly coupled plasma regime (Temperature ~; 0.1 to 10 eV). For a K+ beam at 0.4 MeV, ~;1 J/cm2 is required to reach 1 eV in solid Aluminum. Also the pulse duration must be short (<~; 2 ns) to avoid hydrodynamic cooling. A spot radius ~;0.5 mm, and current ~;10 A are thus need to reach this flux level and pulse duration. Currents will be achieved by compressing the beam axially with an IBM. To further increase the beam intensity on target, an 8T field solenoid, filled with beam neutralizing plasma will be used. A plasma is injected from filtered cathodic arc plasma sources. The Neutralized Drift Compression Experiment (NDCX-1) at LBNL is intended to test these neutralized focusing techniques with the goal of reaching target temperatures ~;0.5 eV. Experimental measurements including the on axis plasma density distribution and, the beam density distribution, will be presented
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