Executive Summary of the T8 Advanced Acceleration Technique Working Group

Phillip Sprangle and Chan Joshi, Convenors

The advanced accelerator (AA) R&D program supported by the DOE is important for the long term vitality of the high energy physics (HEP) effort. In addition, the program contributes essential technology and accelerator science to the benefit of all fields using accelerators in their research. Although the program is not directed at any particular project, such as the NLC, its long term focus, i.e., 10 years or more, is to advance the state-of-the-art for HEP. The program addresses fundamental issues which could lead to new or improved, high-gradient accelerators, rf sources, computational techniques, beam control devices and new diagnostic tools. The program is not only a source of new ideas, it is a source of new talent for HEP. The program provides an exciting and stimulating field of physics, which continues to attract young and talented researchers. The program is also responsible for a large number of high quality scientific publications and is invaluable as a training ground for new Ph.D. students.

Over 75 invited talks were presented within the T8: Advanced Acceleration Techniques Working Group. These talks highlighted the recent progress, developments and results in the field. The AA program is progressing along many fronts, one of which is the next generation of advanced acceleration. Experiments are being designed to produce an electron beam of well-defined energy in the multi-GeV range. The first generation laser wakefield accelerator (LWFA) has generated ³100 MeV electrons with an accelerating gradient of ~100 GeV/m and energy spread of ³100%. The second generation LWFA will require optical guided beams, properly controlled phased beam injection and stable wakefield generation. The very encouraging research results on these issues were reported and discussed in the working group.

To increase the acceleration length, the high intensity laser pulse must be optically guided in a plasma channel. This has been demonstrated over distances of many ten’s of Rayleigh lengths (several centimeters) at several institutions, e.g., NRL, U.MD, U. Texas and LBL. A tapered plasma channel with a drive laser of ten’s of TW and optical laser injection may lead to a final energy of several GeV in a distance of several ten’s of cm. To have a well-defined accelerated beam energy an injected beam occupying a small phase angle is necessary. Several all-optical injection concepts are being investigated that may be capable of producing such pulses (U. Mich, NRL, LBL)

The plasma wakefield accelerator (PWFA) mechanism utilizes a relativistic electron beam propagating in a plasma to excite a large amplitude wakefield which accelerates the tail end of the beam. A number of laboratories (UCLA, FLAB, SLAC, ANL) are presently performing experiments on this concept. One of these experiments is the E-157 project at the FFTB of SLAC. This joint effort (SLAC/UCLA/USC/LBL) involved the propagation of a 30 GeV electron beam through a 1.4m plasma column in the blowout regime of the PWFA. Simulations indicate an accelerating gradient of ~1 GeV/m can be achieved. The experiment observed, a) the betatron oscillation of the electron beam and related synchrotron radiation, b) induced transverse effects such as hosing, and c) electron beam refraction as the beam crosses the beam plasma boundary. An ongoing related experiment is the E-162 joint project (SLAC/UCLA/USC), in which a 30 GeV positron beam propagates through the plasma column. Experimental results demonstrate that the plasma column acted as a focusing lens for the positron beam. Preliminary results on a related experiment, E-150, were presented in which focusing by a factor of 2 was observed in both transverse directions using a thin plasma lenses.

The Neptune Laboratory at UCLA is being used for 2^nd generation experiments on the plasma beat wave acceleration (PBWA) of electrons, plasma wake field generation and acceleration, plasma lenses, IFELs and Cherenkov wakes in magnetized plasmas.

Recently proton acceleration experiments (GA, U.Mich) have observed high energy ~10-50 MeV protons from surface contaminants when a high intensity laser pulse is focused on to a thin solid target. The resulting proton beam can have a small energy spread, emittance (~1mm-mrad) and bunch charge (~1nC). The accelerated proton pulse (~1ps, > 10¹³protons) may find applications in basic nuclear physics studies, fast ignitor fusion, production of radionucleides, and injectors for ion accelerators.

The computational community is developing a hierarchy of new codes for AA research. Full-scale 3D modeling is presently at hand, and it is expected that the computational run time can be reduced from a month to minutes with a combination of reduced description particle models and parallelized algorithms.

New rf sources are being developed either as candidate tubes for future colliders operating from 11.4 to 91 GHz, or simply to carry out high-power tests of structures and components. High frequency gyroklystrons are being developed at the U. MD(80 MW design at 17 GHz) and Calabazas Creek Research (10 MW design at 91 GHz). Magnicons are being developed at 11.4 GHz (NRL/ Omega-P, Inc.) and 34 GHz (Omega-P, Inc.)

The three largest areas of work in the non plasma area are the IFEL, dielectric wake field acceleration (DWFA), and small vacuum structures. The STELLA IFEL experiment at the ATF facility at BNL for the first time demonstrated bunching into ~3fs microbunches, and energy gain in an IFEL using a CO₂ laser. A new method of chopping ps bunches into fs pieces by the LACARA (Yale/Omega-P/Columbia) has been devised and will be tested.

The upgraded ANL facility for wakefield studies was presented. A successful test has been made of their two-beam accelerator concept, and higher energy tests are planned that may soon demonstrate gradients in excess of 100MeV/m. A test at NRL of an ANL dielectric-loaded TM₀₁ slow-wave structure using high power X-Band microwaves generated by a magnicon is planned soon. Whereas most wakefield work involves exciting a spectrum of microwave TM modes in a cylindrical dielectric wakefield device, it was pointed out (Columbia) that one might well imagine tall rectangular dielectric structures having optical-scale dimensions, that would be excited by fs bunches containing pC of charge.

The field of wakefield accelerator research is demonstrating great progress, and results permitting an assessment of its ability to develop an advanced accelerator technology should be obtained in the next 3-5years. Tests of optical structures (Stanford U.) for vacuum acceleration, are planned for the near future. The IFEL, while it cannot achieve TeV energies, can contribute to parts of a staged accelerator system, or as an injector for plasma-based accelerators. If the issues of stability and breakdown can be resolved, the dielectric wakefield accelerator (DWFA), which may have gradients of 100MeV/m to 1GeV/m, may play an important role in accelerator physics of the future. All areas of this topic have contributed “spin-offs” to the rest of accelerator physics, and to the science community at large.

In summary, plasma wakefield schemes demonstrated jets of electrons and ions with broad energy spectra and impressive acceleration gradients, exceeding 100 MeV in a mm. Presently research is directed towards a 2^nd generation of wakefield device employing various injection and channel guiding schemes to produce relatively mon-energetic beams in the GeV range. Several facilities around the country are engaged in this research, including, NRL, ATF (BNL), Neptune (UCLA), L’OASIS (LBNL), AWL (ANL) and the planned ORION facility at SLAC. From the E-154 experiments, an idea for an energy doubler has emerged, called the Afterburner. The advanced accelerator community may, within 3-5 years, propose application of these ideas to the HEP community.

The schedule for the T8: Working Group on Advanced Acceleration Techniques was

Subgroups # of Talks Subgroup Convenors

Plasma Based Acceleration 38 P. Sprangle, A. Ting, E. Esarey

Plasma Based Injectors 8 W. Leemans, D. Umstadter

Computational Techniques (Joint with T7) 8 T. Antonsen, W. Mori

Non-Plasma Based Acceleration 13 T. Marshall

Plasma Based Processes 5 P. Chen

Advanced RF Sources (Joint with T3) 8 J. Hirshfield