arXiv subjects

W. Gai

Publications and source records attributed to W. Gai.

9 recordsLinked to original sources

arXiv2017

Spatial Control of Photoemitted Electron Beams using a Micro-Lens-Array Transverse-Shaping Technique

A common issue encountered in photoemission electron sources used in electron accelerators is the transverse inhomogeneity of the laser distribution resulting from the laser-amplification process and often use of frequency up conversion in nonlinear crystals. A inhomogeneous laser distribution on the photocathode produces charged beams with lower beam quality. In this paper, we explore the possible use of microlens arrays (fly-eye light condensers) to dramatically improve the transverse uniformity of the drive laser pulse on UV photocathodes. We also demonstrate the use of such microlens arrays to generate transversely-modulated electron beams and present a possible application to diagnose the properties of a magnetized beam.

physics.acc-ph↗

arXiv2016

Microlens Array Laser Transverse Shaping Technique for Photoemission Electron Source

A common issue encountered in photoemission electron sources used in electron accelerators is distortion of the laser spot due to non ideal conditions at all stages of the amplification. Such a laser spot at the cathode may produce asymmetric charged beams that will result in degradation of the beam quality due to space charge at early stages of acceleration and fail to optimally utilize the cathode surface. In this note we study the possibility of using microlens arrays to dramatically improve the transverse uniformity of the drive laser pulse on UV photocathodes at both Fermilab Accelerator Science \& Technology (FAST) facility and Argonne Wakefield Accelerator (AWA). In particular, we discuss the experimental characterization of the homogeneity and periodic patterned formation at the photocathode. Finally, we compare the experimental results with the paraxial analysis, ray tracing and wavefront propagation software.

physics.acc-ph↗

arXiv2014

Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 6: Accelerator Capabilities

These reports present the results of the 2013 Community Summer Study of the APS Division of Particles and Fields ("Snowmass 2013") on the future program of particle physics in the U.S. Chapter 6, on Accelerator Capabilities, discusses the future progress of accelerator technology, including issues for high-energy hadron and lepton colliders, high-intensity beams, electron-ion colliders, and necessary R&D for future accelerator technologies.

hep-ex↗

arXiv2013

Experimental Demonstration of Energy Chirp Compensation by a Tunable Dielectric Based Structure

A 60 MeV beam at the BNL Accelerator Test Facility (ATF) was manipulated by a planar tunable de-chirper made out of two 10 cm long dielectric slabs with copper plated backs. While the gap was changed from 5.8 mm to 1 mm, the correlated energy chirp of the low charge electron bunch was reduced from approximately 330 keV/mm to zero. This result is in agreement with simulations. Calculations show that similar devices, properly scaled to account for the expected electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers (FEL). Potentially, this technique could significantly simplify linac design and improve FEL performance.

physics.acc-ph↗

arXiv2012

Experimental demonstration of wakefield effects in a THz planar diamond accelerating structure

We have directly measured THz wakefields induced by a subpicosecond, intense relativistic electron bunch in a diamond loaded accelerating structure via the wakefield acceleration method. We present here the beam test results from the first diamond based structure. Diamond has been chosen for its high breakdown threshold and unique thermoconductive properties. Fields produced by a leading (drive) beam were used to accelerate a trailing (witness) electron bunch which followed the drive bunch at a variable distance. The energy gain of a witness bunch as a function of its separation from the drive bunch describes the time structure of the generated wakefield.

physics.acc-ph↗

arXiv2012

Experimental Observation of Energy Modulation in Electron Beams Passing Through Terahertz Dielectric Wakefield Structures

We report observation of a strong wakefield induced energy modulation in an energy-chirped electron bunch passing through a dielectric-lined waveguide. This modulation can be effectively converted into a spatial modulation forming micro-bunches with a periodicity of 0.5 - 1 picosecond, hence capable of driving coherent THz radiation. The experimental results agree well with theoretical predictions.

physics.acc-ph↗

arXiv2007

Observations of Microwave Continuum Emission from Air Shower Plasmas

We investigate a possible new technique for microwave measurements of ultra-high energy cosmic ray (UHECR) extensive air showers which relies on detection of expected continuum radiation in the microwave range, caused by free-electron collisions with neutrals in the tenuous plasma left after the passage of the shower. We performed an initial experiment at the AWA (Argonne Wakefield Accelerator) laboratory in 2003 and measured broadband microwave emission from air ionized via high energy electrons and photons. A follow-up experiment at SLAC (Stanford Linear Accelerator Center) in summer of 2004 confirmed the major features of the previous AWA observations with better precision and made additional measurements relevant to the calorimetric capabilities of the method. Prompted by these results we built a prototype detector using satellite television technology, and have made measurements indicating possible detection of cosmic ray extensive air showers. The method, if confirmed by experiments now in progress, could provide a high-duty cycle complement to current nitrogen fluorescence observations of UHECR, which are limited to dark, clear nights. By contrast, decimeter microwave observations can be made both night and day, in clear or cloudy weather, or even in the presence of moderate precipitation.

astro-ph↗

arXiv2007

Observation of Enhanced Transformer Ratio in Collinear Wakefield Acceleration

One approach to future high energy particle accelerators is based on the wakefield principle: a leading high-charge drive bunch is used to excite fields in an accelerating structure or plasma that in turn accelerates a trailing low-charge witness bunch. The transformer ratio R is defined as the ratio of the maximum energy gain of the witness bunch to the maximum energy loss of the drive bunch. In general, for configurations in which the two beams traverse the accelerator along the same trajectory (collinear wakefield acceleration). A number of techniques have been proposed to overcome the transformer ratio limitation. We report here the first experimental study of the ramped bunch train (RBT) technique in which a dielectric loaded waveguide was used as the accelerating structure. A single drive bunch was replaced by two bunches with charge ratio of 1:2.5 and a separation of 10.5 wavelengths of the fundamental mode. An average measured transformer ratio enhancement by a factor of 1.31 over the single drive bunch case was obtained in this experiment.

physics.acc-ph↗

arXiv2004

Transverse Beam Envelope Measurements and the Limitations of the 3-Screen Emittance Method for Space-Charge Dominated Beams

In its normal mode of operation the Argonne Wakefield Accelerator Facility uses a high charge (10-100 nC), short pulse (3-5 psec) drive bunch to excite high-gradient accelerating fields in various slow-wave structures. To generate this bunch, we designed a 1.5 cell, L-band, rf photocathode gun with an emittance compensating solenoid to give optimal performance at high-charge; it has recently completed commissioning. More recently, we have begun to investigate the possibility of using this gun in a high-brightness, low-charge operating mode, with charge equal to approximately 1 nC, for high-precision measurements of wakefields. Two related measurements are reported on in this paper: (1) measurements of the transverse beam envelope are compared to predictions from the beam dynamics code PARMELA; and (2) investigations into the use of a modified 3-screen emittance measurement method that uses a beam envelope model that includes both space-charge and emittance effects. Both measurements were made for the 1 nC, 8 MeV beam in the drift region directly following the rf photocathode gun.

physics.acc-ph↗