Physics of shock waves and high-temperature hydrodynamic phenomena - Ya.B. Zeldovich and Yu.P. Raizer (translated from the Russian and then edited by Wallace D. Hayes and Ronald F. Probstein); Dover Publications, New York, 2002, 944 pp., $34.

An overview is given of the physics issues relevant to the plasma wakefield accelerator, the plasma beat-wave accelerator, the laser wakefield accelerator, including the self-modulated regime, and wakefield accelerators driven by multiple electron or laser pulses. Basic properties of linear and nonlinear plasma waves are discussed, as well as the trapping and acceleration of electrons in the plasma wave. Formulas are presented for the accelerating field and the energy gain in the various accelerator configurations. The propagation of the drive electron or laser beams is discussed, including limitations imposed by key instabilities and methods for optically guiding laser pulses. Recent experimental results are summarized.

Overview of plasma-based accelerator concepts

19세기말 애국적 담론과 新小說의 정체성

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

High‐energy accelerators have been physicists’ main tools for exploring the building blocks of matter for more than 60 years. During this time the particle energy has increased exponentially as a result of a combination of improvements in existing machines and the invention of new acceleration techniques. Historically, whenever a given type of accelerator has reached the limit of its performance, an innovative idea for particle manipulation, storage, cooling or acceleration has made possible experiments at ever higher energies. The tremendous increase in the energy of accelerators has not, however, been without an increase in capital costs. The cancellation of the Superconducting Super Collider makes timely an examination of possible alternative concepts for investigating some of the same physics.

Advanced accelerator concepts

We have measured the effects of high (0--4.5 T) magnetic fields on the operating conditions of 805 MHz accelerating cavities, and discovered that the maximum accelerating gradient drops as a function of the axial magnetic field. While the maximum gradient of any cavity is governed by a number of factors including conditioning, surface topology and materials, we argue that $\mathbf{J}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ forces within the emitters are the mechanism for enhanced breakdown in magnetic fields. The pattern of emitters changes over time and we show an example of a bright emitter which disappears during a breakdown event. We also present unique measurements of the distribution of enhancement factors, $\ensuremath{\beta}$, of secondary emitters produced in breakdown events during conditioning. We believe these secondary emitters can also be breakdown triggers, and the secondary emitter spectrum helps to determine the maximum operating field.

/pdf/effects-of-high-solenoidal-magnetic-fields-on-rf-42j0sxhj2z.pdf

Effects of high solenoidal magnetic fields on rf accelerating cavities

The accelerating gradient in a RF cavity is limited by many factors such as the surface material properties, RF frequency, the external magnetic field and the gas pressure inside the cavity. In the MuCool Program, RF cavities are studied to understand these basic mechanisms with the aim of improving the maximum stable accelerating gradient of RF cavities in magnetic field. These cavities are being developed for muon ionization cooling channels for a Neutrino Factory or Muon Collider. We report studies using 805 MHz and 201 MHz RF pillbox cavities in the MuCool Test Area (MTA) at Fermilab. New results include data from buttons of different materials mounted in the 805 MHz cavity, a study of the accelerating gradient in the 201 MHz cavity and X-ray background radiation rates from the cavities due to Bremsstrahlung. The 201 MHz cavity has been shown to be stable at 21 MV/m at zero magnetic field, well in excess of its 16 MV/m design gradient. We also discuss results from the 201 MHz cavity operation in magnetic field and introduce the test of E × B effects with the 805 MHz box cavity.

RF Studies at Fermilab MuCool Test Area

The MuCool Experiment has been continuing to take data with 805 and 201 MHz cavities in the MuCool Test Area. The system uses rf power sources from the Fermilab Linac. Although the experimental program isprimarily aimed at the Muon Ionization Cooling Experiment (MICE), we have been studying the dependence of rf limits on frequency, cavity material, high magnetic fields, gas pressure, coatings, etc. with the general aim of understanding the basic mechanisms involved. The 201 MHz cavity, essentially a prototype for the MICE experiment, was made using cleaning techniques similar to those employed for superconducting cavities and operates at its design field with very little conditioning.

/pdf/recent-rf-results-from-the-mucool-test-area-escholarship-58m6u3xnum.pdf

Recent RF Results from the MuCool Test Area - eScholarship

The MuCool Experiment has been continuing to take data with 805 and 201 MHz cavities in the MuCool Test Area. The system uses RF power sources from the Fermilab Linac. Although the experimental program is primarily aimed at the Muon Ionization Cooling Experiment (MICE), we have been studying the dependence of RF limits on frequency, cavity material, high magnetic fields, gas pressure, coatings, etc. with the general aim of understanding the basic mechanisms involved. The 201 MHz cavity, essentially a prototype for the MICE, was made using cleaning techniques similar to those employed for superconducting cavities and operates at its design field with very little conditioning.

/pdf/recent-rf-results-from-the-mucool-test-area-y4e2h313xl.pdf

Recent RF results from the MuCool Test Area

This paper describes the surface environment of the dense plasma arcs that damage rf accelerators, tokamaks, and other high gradient structures. We simulate the dense, nonideal plasma sheath near a metallic surface using molecular dynamics (MD) to evaluate sheaths in the non-Debye region for high density, low temperature plasmas. We use direct two-component MD simulations where the interactions between all electrons and ions are computed explicitly. We find that the non-Debye sheath can be extrapolated from the Debye sheath parameters with small corrections. We find that these parameters are roughly consistent with previous particle-in-cell code estimates, pointing to densities in the range 10 24 ‐10 25 m � 3 . The high surface fields implied by these results could produce field emission that would short the sheath and cause an instability in the time evolution of the arc, and this mechanism could limit the maximum density and surface field in the arc. These results also provide a way of understanding how the properties of the arc depend on the properties (sublimation energy, for example) of the metal. Using these results, and equating surface tension and plasma pressure, it is possible to infer a range of plasma densities and sheath potentials from scanning electron microscope images of arc damage. We find that the high density plasma these results imply and the level of plasma pressure they would produce is consistent with arc damage on a scale 100 nm or less, in examples where the liquid metal would cool before this structure would be lost. We find that the submicron component of arc damage, the burn voltage, and fluctuations in thevisible light production of arcs may be the most direct indicators of the parameters of the dense plasma arc, and the most useful diagnostics of the mechanisms limiting gradients in accelerators.

Sheath parameters for non-Debye plasmas: Simulations and arc damage

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