Argonne Wakefield Accelerator Research Articles

Double-negative metamaterial research for accelerator applications

Material properties are central to the design of particle accelerators. One area of advanced accelerator research is to investigate novel materials and structures and their potential use in extending capabilities of accelerator components. Within the past decade a new type of artificially constructed material having the unique property of simultaneously negative permittivity and permeability has been realized, and is under intense investigation, primarily by the optical physics and microwave engineering communities [C.M. Soukoulis, Science 315 (2007) 47; D.R. Smith, J.B. Pendry, M.C.K. Wiltshire, Science 305 (2004) 788; J.B. Pendry, A.J. Holden, W.J. Stewart, I. Youngs, Phys. Rev. Lett. 76 (1996) 4773]. Although they are typically constructed of arrays of discrete cells, as long as the condition that the wavelength of applied radiation is significantly greater than the cell dimensions is met, the material mimics a continuous medium and can be described with the bulk properties of permittivity, ε , and permeability, μ . When the permittivity and permeability are simultaneously negative in some frequency range, the metamaterial is called double negative (DNM) or left-handed (LHM) and has unusual properties, such as a negative index of refraction. An investigation of these materials in the context of accelerators is being carried out by IIT and the Argonne Wakefield Accelerator Facility [S. Antipov, W. Liu, W. Gai, J. Power, L. Spentzouris, AIP Conf. Proc. 877 (2006); S. Antipov, W. Liu, J. Power, L. Spentzouris, Design, Fabrication, and Testing of Left-Handed Metamaterial, Wakefield Notes at Argonne Wakefield Accelerator, 〈 http://www.hep.anl.gov/awa/wfnotes/wf229.pdf 〉 ]. Waveguides loaded with metamaterials are of interest because the DNM can change the dispersion relation of the waveguide significantly. For example, slow backward waves can be produced in a DNM-loaded waveguide without having corrugations. This article begins with a brief introduction of known design principles for realizing a DNM [J.B. Pendry, A.J. Holden, W.J. Stewart, I. Youngs, Phys. Rev. Lett. 76 (1996) 4773; D.R. Smith, et al., Phys. Rev. Lett. 84 (2000) 4184; J.B. Pendry, A.J. Holden, D.J. Robbins, W.J. Stewart, IEEE Trans. Microwave Theory Tech. 47 (1999) 2075], along with a description of the experimental verification of the basic DNM properties of our designs. We then present our waveguide analysis, starting with the case of a waveguide loaded with a truly continuous medium that is dispersive and anisotropic. We show that the dispersion relation of a waveguide with frequency regions of negative ε ( ω ) and negative μ ( ω ) has several interesting frequency bands. While a DNM approximates a continuous medium, it is still made up of discrete elements. We discuss some implications of the discrete nature of the material for the behavior of a loaded waveguide, particularly at frequencies below the cutoff frequency of the waveguide. We conclude by describing our experimental program at present and in the near future. This includes testing the excitation of TM modes in a DNM loaded waveguide in the interesting frequency bands, both on the bench and from particle beam excitation.

RECENT STRUCTURE BASED HIGH GRADIENT WAKEFIELD EXPERIMENTS AT ARGONNE

Dielectric structures promise to support high field, especially for short wakefield pulses produced by a high charged electron beam traveling in a dielectric tube. To push the gradient higher, we have tested two structures using recent upgraded Argonne wakefield accelerator facility that capable of producing up to 100 nC charge and bunch length of < 13 ps (FWHM). Here we report on the experiment results that more than 80 nC beam passes through a 14 GHz dielectric loaded wakefield structure that produced an accelerating field of ~ 45 MV/m . The two structures consist of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively, and with length of 102 mm and 23 mm long. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step in these experiments, another structure, with an output coupler, has been designed and is presently being fabricated.

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 ps) 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.

Radio-frequency measurements of coherent transition and Cherenkov radiation: Implications for high-energy neutrino detection

We report on measurements of (11-18)-cm wavelength radio emission from interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield Accelerator. The electrons were observed both in a configuration where they produced primarily transition radiation from an aluminum foil, and in a configuration designed for the electrons to produce Cherenkov radiation in a silica sand target. Our aim was to emulate the large electron excess expected to develop during an electromagnetic cascade initiated by an ultrahigh-energy particle. Such charge asymmetries are predicted to produce strong coherent radio pulses, which are the basis for several experiments to detect high-energy neutrinos from the showers they induce in Antarctic ice and in the lunar regolith. We detected coherent emission which we attribute both to transition and possibly Cherenkov radiation at different levels depending on the experimental conditions. We discuss implications for experiments relying on radio emission for detection of electromagnetic cascades produced by ultrahigh-energy neutrinos.

Observation of plasma wakefield acceleration in the underdense regime

Initial experiments which have explored the physics of the underdense (blowout) regime of the plasma wakefield accelerator (PWFA) at the Argonne Wakefield Accelerator facility are reported. In this regime, the relativistic electron beam is denser than the plasma, causing the beam channel to completely rarefy, and leaving a high quality accelerating region which also contains a uniform ion column. This ion column in turn allows the drive and accelerating beams to be well guided over many initial beam beta-function lengths. The results of these experiments, which have taken place over several years, are reviewed. Notable achievements in the course of these studies include the creation and measurement of drive and witness beam generated in an rf photoinjector, as well as previously published studies on drive beam guiding in the underdense regime. In addition, these experiments allowed measurement of both beam energy loss and gain, at a maximum average rate of 25 MeV/m in this regime of the PWFA, which is consistent with a peak acceleration gradient of 62 MeV/m in the excited waves. Difficulties associated with this type of experiment are discussed, as are prospects for mitigating these difficulties and achieving high gradient acceleration in planned future experiments.

Generation and acceleration of high-charge short-electron bunches

We report experimental results on the high peak current electron beam generated at the Argonne Wakefield Accelerator facility for wakefield applications. The facility produces bunch charges in the range 15--100 nC by using a photocathode based RF electron gun. The energy of the beam is 14 MeV. Our measurements show that for bunches of 100 nC, the pulse length can be as short as 30 ps. A detailed systematic study of pulse length versus charge is reported, as well as comparisons with numerical simulations. Dark current beam loading in the high field RF gun is also discussed.

A 3.9 MeV photoinjector and delay system for Wakefield measurements

We report on the characterization of the 3.9 MeV electron beam from a rf photoinjector at the Argonne Wakefield Accelerator. This beam is used to diagnose wakefields produced by the 15 MeV beam from a second rf photoinjector. We measured the emittance with the usual quadrupole scan method but used a fitting algorithm that includes space-charge forces. This photocathode gun has produced a beam charge per pulse ranging from 10 pC to 4 nC, but is normally operated at 100 pC with a measured normalized emittance of 3.4 π mm mrad and bunch length of 8 ps full width at half maximum.

Potential applications of a dual-sweep streak camera system for characterizing particle and photon beams of VUV, XUV, and X-ray FELs

Abstract Initial tests of a dual-sweep streak system whose vacuum interface and Au photocathode would allow time-resolved measurements from the visible to the X-ray photon regime are presented. Although selected to support the diagnostics of the Advanced Photon Source (APS), this type of system could also address the micropulse phenomena of the next generation of FELs whose lasing wavelengths are projected in the VUV, XUV, and even the X-ray regime. First results at 248 nm on the photoelectric injector drive laser at the Argonne Wakefield Accelerator (AWA) are presented, and the scaling on time resolution into the X-ray regime is addressed. The system has capabilities that could support benchmark experiments in the path to UV/X-ray FELs.