Engineering topics
Ng, K. Y.
Publications and source records attributed to Ng, K. Y..
Probabilistic Cosmological Mass Mapping from Weak Lensing Shear
We infer gravitational lensing shear and convergence fields from galaxy ellipticity catalogs under a spatial process prior for the lensing potential. We demonstrate the performance of our algorithm with simulated Gaussian-distributed cosmological lensing shear maps and a reconstruction of the mass distribution of the merging galaxy cluster Abell 781 using galaxy ellipticities measured with the Deep Lens Survey. Given interim posterior samples of lensing shear or convergence fields on the sky, we describe an algorithm to infer cosmological parameters via lens field marginalization. In the most general formulation of our algorithm we make no assumptions about weak shear or Gaussian-distributed shape noise or shears. Because we require solutions and matrix determinants of a linear system of dimension that scales with the number of galaxies, we expect our algorithm to require parallel high-performance computing resources for application to ongoing wide field lensing surveys.
Method of phase space beam dilution utilizing bounded chaos generated by rf phase modulation
Not Available
Selected Topics in Microwave Instabilities and Linacs
The Shanghai Institute of Applied Physics (SINAP) is embarking on its first X-ray free-electron laser (FEL) project. It is a cascading high-gain harmonic generation FEL. Microwave instabilities driven by various effects, especially the space-charge force, will degrade the quality of the electron beam before entering into the undulator. However, inside the undulator, the occurrence of microbunching becomes an utmost important ingredient for the generation of coherent radiation. In short, controlled and uncontrolled microwave instabilities must be fully understood in such a project. These are the slides of a series of eight-hour lectures given at the SINAP in June of 2013, with the intention of a fully understanding of the microbunching phenomenon. The sections of wake field and impedance theory are added as an introduction for those who are not familiar with the subject.
Theory and simulations on g-2 proton beam preparation
Booster batch consisting of 84 53-MHz bunches needs to be debunched and recaptured into four 2.5-MHz super bunches, to be extracted to hit a target to generate muons for the g-2 experiment. These super bunches must be each of full width ≤ 120 ns. In addition, the whole beam preparation procedure as well as eventual bunch-by-bunch extraction must be performed within two Booster cycles or 133 ms. Machine studies are required in the Main Injector to demonstrate the feasibility of the preparation. The theory, including adiabatic debunching, adiabatic re-capture, and some other relevant issues necessary for the machine studies, is discussed.
Space-Charge Effects on Beam Rotation at the Compressor Ring
Bunch-width compression can be accomplished by rf rotating an elongated bunch with minimal energy spread. The longitudinal space-charge force counteracts the rf force. When it is larger than the rf force, the focusing effect of the rf will be lost completely. This possibility is studied with analytic formulas as well as simulations. The transverse space-charge force can drive quadrupole breathing modes in the beam. If the frequencies of these modes fall into the stopbands of parametric resonances, especially the half-integer resonances, emittances will increase. Application is made to the bunch-width compression in the Fermilab Compressor Ring proposed by Alexahin and Neuffer [1], destined for pion and subsequently muon production.
Local Longitudinal Microwave Instability Limits During Bunch Rotation
Bunch width compression can be accomplished by rf rotating an elongated bunch with minimal energy spread. The formation of tails at the two bunch ends produces disconnected regions in many time-advance slices. The stability limits of longitudinal microwave growth for each time-advance slice of the beam will be affected. This effect is studied and the Keil-Schnell stability limits for such time-advance slices are derived. Application is made to the bunch-width compression in the Fermilab Compressor Ring, destined for pion and subsequently muon production
Adiabatic Capture and Debunching
In the study of beam preparation for the g-2 experiment, adiabatic debunching and adiabatic capture are revisited. The voltage programs for these adiabbatic processes are derived and their properties discussed. Comparison is made with some other form of adiabatic capture program. The muon g-2 experiment at Fermilab calls for intense proton bunches for the creation of muons. A booster batch of 84 bunches is injected into the Recycler Ring, where it is debunched and captured into 4 intense bunches with the 2.5-MHz rf. The experiment requires short bunches with total width less than 100 ns. The transport line from the Recycler to the muon-production target has a low momentum aperture of ~ ±22 MeV. Thus each of the 4 intense proton bunches required to have an emittance less than ~ 3.46 eVs. The incoming booster bunches have total emittance ~ 8.4 eVs, or each one with an emittance ~ 0.1 eVs. However, there is always emittance increase when the 84 booster bunches are debunched. There will be even larger emittance increase during adiabatic capture into the buckets of the 2.5-MHz rf. In addition, the incoming booster bunches may have emittances larger than 0.1 eVs. In this article, we will concentrate on the analysis of the adiabatic capture process with the intention of preserving the beam emittance as much as possible. At this moment, beam preparation experiment is being performed at the Main Injector. Since the Main Injector and the Recycler Ring have roughly the same lattice properties, we are referring to adiabatic capture in the Main Injector instead in our discussions.
Mysterious dipole synchrotron oscillations during and after adiabatic capture
Strong synchrotron oscillations were observed during and after the 2.5-MHz rf adiabatic capture of a debunched booster batch in the Main Injector. Analysis shows two possible sources for the synchrotron oscillations. One is the frequency drift of the 2.5-MHz rf after the turning off of the 53-MHz rf voltage, thus resulting in an energy mismatch with the debunched beam. The second source is the energy mismatch of the injected booster beam with the frequency of the 53-MHz rf. We have been able to rule out the first possibility.
Some issues on g-2 proton beam preparation
In the preparation of proton beam for the g — 2 experiment, one debunched Booster batch will be captured into four 2.5-MHz bunches in the Recycler Ring. The initial capture voltage is determined in order to limit leakage from the 2.5-MHz buckets. The 2.5-MHz rf voltage required to produce four bunches each of total width 10 ns is computed. In order to accomplish the required beam preparation in Recycler Ring, the maximum emittance growth and bunch length for the present experiments at the Main Injector are estimated.
Theoretical Study of Proton Beam Preparation for the g-2 Experiment
Beam preparation for the g-2 experiment is studied. Because of limited 2.5-MHz rf voltage and the 66-ms limited time duration allowance for the whole process, the Booster batch must be first captured to a lower 2.5-MHz rf voltage followed by a rotation in the 2.5-MHz bucket in order to achieve the required narrow half bunch width of 50 ns. The appearance of bunch tails can be reduced by first rotation by a linear barrier voltage before the adiabatic capture.
Landau damping
Section 2.5.8 of the Handbook of Accelerator Physics and Engineering on Landau damping is rewritten. An solvable example is first given to demonstrate the interplay between Landau damping and decoherence. This example is an actual one when the beam oscillatory motion is driven by a wake force. The dispersion relation is derived and its implication on Landau damping is illustrated. The rest of the article touches on the Landau damping of transverse and longitudinal beam oscillations. The stability criteria are given for a bunched beam and the changes of the criteria when the beam is lengthened and becomes unbunched.