Wanming Liu

Synchrotron x-ray microscopy studies on electromigration of a two-phase material

Basic issues involving movement of conductive constituents and microstructural evolution from high current density in single phase materials are well documented. Recently, electromigration of conductive constituents in multiphase materials has gained attention due to the necessity of employing such alloys for interconnects in microelectronics. Reported studies on these alloys using complicated industrial geometry suffer from contributions such as current crowding. Hence a basic understanding on operative mechanisms during electromigration in multiphase alloys cannot be gained from these studies. Consequently, several mechanisms proposed from these studies involve fitting parameters and not well-understood complex diffusional processes. A joint configuration designed to avoid current crowding and associated local Joule heating is suitable for evaluating electromigration induced microstructural events. Synchrotron x-ray microscopy has provided information regarding two- and three-dimensional crystallographic orientations and strain fields in such joints, aiding the development of a basic understanding of electromigration in two-phase alloys.Basic issues involving movement of conductive constituents and microstructural evolution from high current density in single phase materials are well documented. Recently, electromigration of conductive constituents in multiphase materials has gained attention due to the necessity of employing such alloys for interconnects in microelectronics. Reported studies on these alloys using complicated industrial geometry suffer from contributions such as current crowding. Hence a basic understanding on operative mechanisms during electromigration in multiphase alloys cannot be gained from these studies. Consequently, several mechanisms proposed from these studies involve fitting parameters and not well-understood complex diffusional processes. A joint configuration designed to avoid current crowding and associated local Joule heating is suitable for evaluating electromigration induced microstructural events. Synchrotron x-ray microscopy has provided information regarding two- and three-dimensional crystallographi...

Planar ultrananocrystalline diamond field emitter in accelerator radio frequency electron injector: Performance metrics

A case performance study of a planar field emission cathode (FEC) based on nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in an RF 1.3u2009GHz electron gun. The FEC was a 100u2009nm (N)UNCD film grown on a 20u2009mm diameter stainless steel disk with a Mo buffer layer. At surface gradients 45–65u2009MV/m, peak currents of 1–80u2009mA (equivalent to 0.3–25u2009mA/cm2) were achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD surface with (1) the beam emittance of 1.5u2009mmu2009×u2009mrad/mm-rms and (2) longitudinal FWHM and rms widths of non-Gaussian energy spread of 0.7% and 11% at an electron energy of 2u2009MeV. Current stability was tested over the course of 36u2009×u2009103 RF pulses (equivalent to 288u2009×u2009106u2009GHz oscillations).

Interaction of an ultrarelativistic electron bunch train with a W-band accelerating structure: High power and high gradient

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91xa0GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to the interference of the wakefields from the two bunches, was measured as a function of bunch separation. Measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.

The Argonne Wakefield Accelerator Facility capabilities and experiments.

A description of the Argonne Wakefield Accelerator is presented, pointing out the unique capabilities of the facility. A photocathode RF gun produces electron bunches with tens of nanocoulombs of charge, which are used to excite wakefields. A second photocathode RF gun generates electron bunches that are used to probe these wakefields. An overview of the experimental program carried out at the facility is also presented.

Observation of Field-Emission Dependence on Stored Energy

Field emission from a solid metal surface has been continuously studied for a century over macroscopic to atomic scales. It is general knowledge that, other than the surface properties, the emitted current is governed solely by the applied electric field. A pin cathode has been used to study the dependence of field emission on stored energy in an L-band rf gun. The stored energy was changed by adjusting the axial position (distance between the cathode base and the gun back surface) of the cathode while the applied electric field on the cathode tip is kept constant. A very strong correlation of the field-emission current with the stored energy has been observed. While eliminating all possible interfering sources, an enhancement of the current by a factor of 5 was obtained as the stored energy was increased by a factor of 3. It implies that under certain circumstances a localized field emission may be significantly altered by the global parameters in a system.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunchs horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Numerical studies of International Linear Collider positron target and optical matching device field effects on beam

For an International Linear Collider (ILC) undulator-based positron source target configuration, a strong optical matching device (OMD) field is needed inside the target to increase the positron yield (by more than 40%) [Y. K. Batygin, Proceedings of the 2005 ALCPG and ILC Workshops, Snowmas, CO, 14–27 August 2005 (unpublished)] It is also required that the positron target be constantly rotated to reduce thermal and radiation damages. Eddy currents, produced by an OMD field in turn, interact with the magnetic field and produce a drag (stopping) force. This force not only produces heat in the disk but also creates a dipole deflecting field, which affects the beam. Therefore it is important to simulate such a system in detail to design the motor and cooling system and also a correction magnet system. In order to guide the ILC target design, an exact simulation of the spinning disk in a magnetic field is required. In this paper we present a simulation method implemented using COMSOL and compare it with the e...

Storage-ring-based, ultrashort positron beam source

We propose a scheme to generate high-flux, short-burst γ-ray radiation and its application for generating ultrashort positron beams. The intense γ-ray bursts are generated from short laser pulses scattering off high-energy electron beams in a storage ring. The γ-ray bursts are then used to irradiate thin metal targets to generate the positron beams via pair production. Using the example of the Advanced Photon Source storage ring, more than 107positrons∕s in 1ps pulses at energies of a few MeV can be generated.

High Power Testing of Dielectric-Loaded Accelerating Structures at 11.424 GHz

Summary form only given. A joint Naval Research Laboratory/Argonne National Laboratory study is under way to investigate the performance of X-band dielectric-loaded accelerating (DLA) structures using high-power 11.424-GHz radiation from the NRL Magnicon facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient RF linear accelerators. The purpose of the high-power tests is to find the RF breakdown limits of these structures and to test their ability to produce high accelerating gradients. We have recently tested DLA structures employing cylindrical ceramic liners fabricated from two different materials, high purity alumina (Al2O3), dielectric constant 9.4, and high-index magnesium calcium titanate (MgxCa1-xTiO3), dielectric constant 20. For alumina, we have seen no evidence of RF breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, strong multipactor effects were found to absorb an increasing fraction of the incident microwave power, as the power level was increased. These effects could be mitigated by use of a TiN coating on the alumina. For magnesium calcium titanate, the multipactor effects were smaller, but a problem of breakdown at dielectric joints between separate ceramic sections limited the achievable gradients. In this case, gradients of ~6 MV/m were achieved at ~1 MW drive power. Detailed experimental results will be presented

Observation of multipactor in an 11.424-GHz dielectric-loaded accelerating structure

Summary form only given. A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. The purpose of the high-power tests is to find the rf breakdown limits of these structures and to test their ability to produce high accelerating gradients. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating a strong multipactor effect in the tube. The multipactor was found to absorb an increasing fraction of the incident microwave power, as the power level was increased, ultimately absorbing half of the incident power. This behavior is unlike that observed in multipactor on rf windows, which typically saturates at /spl sim/1% power absorbed, independent of incident power. We present a simple model that explains these observations, and discuss means to eliminate the effect.