The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30-50 MeV m(-1) gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

/pdf/terahertz-driven-linear-electron-acceleration-2lk771eltu.pdf

Terahertz-driven linear electron acceleration

Higgs Boson Theory and Phenomenology

Naturalness bounds on weak scale supersymmetry in the context of radiative breaking of the electroweak symmetry are analyzed. In the case of minimal supergravity it is found that for low $\mathrm{tan}\ensuremath{\beta}$ and for low values of fine-tuning $\ensuremath{\Phi},$ where $\ensuremath{\Phi}$ is defined essentially by the ratio ${\ensuremath{\mu}}^{2}{/M}_{Z}^{2}$ where $\ensuremath{\mu}$ is the Higgs mixing parameter and ${M}_{Z}$ is the $Z$ boson mass, the allowed values of the universal scalar parameter ${m}_{0},$ and the universal gaugino mass ${m}_{1/2}$ lie on the surface of an ellipsoid with radii fixed by $\ensuremath{\Phi}$ leading to tightly constrained upper bounds $\ensuremath{\sim}\sqrt{\ensuremath{\Phi}}.$ Thus for $\mathrm{tan}\ensuremath{\beta}l~2(l~5)$ it is found that the upper limits for the entire set of sparticle masses lie in the range $l700 \mathrm{GeV} (l1.5 \mathrm{TeV})$ for any reasonable range of fine-tuning $(\ensuremath{\Phi}l~20).$ However, it is found that there exist regions of the parameter space where the fine-tuning does not tightly constrain ${m}_{0}$ and ${m}_{1/2}.$ Effects of nonuniversalities in the Higgs boson sector and in the third generation sector on naturalness bounds are also analyzed and it is found that nonuniversalities can significantly affect the upper bounds. It is also found that achieving the maximum Higgs boson mass allowed in supergravity unified models requires a high degree of fine-tuning. Thus a heavy sparticle spectrum is indicated if the Higgs boson mass exceeds 120 GeV. The prospect for the discovery of supersymmetry at the Fermilab Tevatron and at the CERN LHC in view of these results is discussed.

https://repository.library.northeastern.edu/files/neu:331876/fulltext.pdf

Naturalness, weak scale supersymmetry, and the prospect for the observation of supersymmetry at the Fermilab Tevatron and at the CERN LHC

Characterizing femtosecond X-ray pulses that vary from shot to shot is important for data interpretation. Here, Behrens et al. measure time-resolved lasing effects on the electron beam and extract the temporal profile of X-ray pulses using an X-band radiofrequency transverse deflector.

/pdf/few-femtosecond-time-resolved-measurements-of-x-ray-free-41ai8lsc3h.pdf

Few-femtosecond time-resolved measurements of X-ray free-electron lasers

The design and operation of resonant linear accelerator tanks suitable for use in high energy proton and electron linacs is described. It is shown that construction of these tanks to operate in the π/2 mode yields systems with extraordinary field stability. Theoretical expressions and corresponding experimental observations are obtained for the behavior of cavity chains operating in the π/2 mode. The synthesis of the side coupled cavity chain, a biperiodic chain which combines high acceleration efficiency with π/2 mode field stability, is described. Extensions of π/2 mode operation to nonperiodic chains is also demonstrated.

Standing Wave High Energy Linear Accelerator Structures

Over the past few years, several schemes for *. making significant increases in the energy of the SLAC beam have been proposed. Two of the proposals, namely the use of superconducting accelerating sections1 and recirculation 1 of the beam for a second pass through the existing acceler. &or,’ have been abandoned for technical and economic reasons after extensive investigation. An on-going method of gradually raising the beam energy is the development and installation of 30and 40-MW klystrons by the SLAC Klystron Group. It is clear, however, that the accelerator would have to be completely refitted with klystrons producing about 100 MW in order that the present machine energy be approximate1 doubled. While such an approach is not inconceivable, 3 the realization of such klystrons and the modulators needed to drive them would require further years of development and a high initial capital investment.

/pdf/sled-a-method-of-doubling-slac-s-energy-3nm26601o9.pdf

SLED: A Method of Doubling SLAC's Energy

The authors present a summary of RF breakdown-limited electric fields observed in experimental linac structures at SLAC (Stanford Linear Accelerator Center) and a discussion of how these experiments can be interpreted against the background of existing, yet incomplete, theories. The motivation of these studies, begun in 1984, is to determine the maximum accelerating field gradients that can be used safely in future e/sup +or-/ colliders, to contribute to the basic understanding of the RF breakdown mechanism, and to discover whether a special surface treatment can make it possible to supersede the field limits presently attainable room-temperature copper structures. >

/pdf/rf-breakdown-studies-in-copper-electron-linac-structures-3pas8dlour.pdf

RF breakdown studies in copper electron linac structures

For more than ten years, we have been working on R&D for X-band accelerator structures for the JLC/NLC linear collider. Several types of Detuned (DS) and Damped Detuned Structures (DDS) have been successfully designed and fabricated. They have been experimentally tested at both low power and high power to characterize their mechanical and electrical properties. Recently we started developing a new type of damped detuned structure with optimized round-shaped cavities (RDDS). This paper discusses the special specifications, design methods, fabrication procedures, measurement technologies, and anticipated future improvements for all these structures.

Accelerator structure R&D for linear colliders

The versatility and accuracy of programs such as LALA and specially SUPERFISH to calculate the rf properties of standing-wave cavities for linacs and storage rings is by now well established. Such rf properties include the resonant frequency, the phase shift per periodic length, the E- and H-field configurations, the shunt impedance per unit length and Q. While other programs such as TWAP have existed for some time for traveling-wave structures, the wide availability of SUPERFISH makes it desirable to extend the use of this program to traveling-wave structures as well. That is the purpose of this paper. In the process of showing how the conversion from standing waves to traveling waves can be accomplished and how the group velocity can be calculated, the paper also attempts to clear up some of the common ambiguities between the properties of these two types of waves. Good agreement is found between calculated results and experimental values obtained earlier.

/pdf/computer-calculations-of-traveling-wave-periodic-structure-3mpq2md1hf.pdf

Computer Calculations of Traveling-Wave Periodic Structure Properties

From the following description, it will be seen that SLED raises the accelerator peak energy by 40% if the present 2.7 1s RF pulse length is used. The energy increase ” is 80% if the pulse length is extended to 5 ps. TO do this, however, changes have to be made in the modulators and trigger system, and the maximum repetition rate has to be halved to maintain the present average power level. In addition, more extensive switchyard modifications are required for~handling the higher energy beams. For these reasons, the SLED system will be initially installed and run at the present 2.7 ns pulse length. Performance at both pulse lengths is discussed in this paper.

/pdf/recent-progress-on-sled-the-slac-energy-doubler-38tggpmmp5.pdf

G. A. Loew

Papers

SLED: A Method of Doubling SLAC's Energy

RF breakdown studies in copper electron linac structures

Accelerator structure R&D for linear colliders

Computer Calculations of Traveling-Wave Periodic Structure Properties

Recent Progress on Sled, The SLAC Energy Doubler