The US Department of Energy funded Argonne Wakefield Accelerator facility provides a platform for developing advanced technologies for high-energy physics machines. The facility aims to develop acceleration methods for more efficient, more compact and less expensive particle accelerators.
A group of scientists at the Argonne National Laboratory in the US is pursuing the novel technique of electron beam-driven wakefield acceleration. This uses the strong electromagnetic fields that follow in the wake of an electron beam to accelerate a second electron beam. A general-purpose wakefield measurement system, invented at Argonne, is being applied to study the technique. It uses a “witness beam” to map out directly the longitudinal and transverse wakefields in any device. This technique has also been used to perform wakefield measurements of the very first prototype detuned accelerating structure for the proposed Next Linear Collider, and to measure critical components for the ill-fated Superconducting Super Collider.
Wakefield acceleration
A charged particle beam leaves wakefields behind when it passes a discontinuity in a beam line. If this discontinuity is a resonant structure, the beam will drive the structure’s accelerating modes, which can then be used to accelerate a second beam. The fundamental concept of electron beam-driven wakefield acceleration is that a high-current, low-energy electron beam can be used to accelerate a separate low-current electron beam to high energies. This idea is also the basis of plasma wakefield acceleration, where the electron beam is used to excite large-amplitude electrostatic oscillations in a plasma, which then accelerate the second beam.
The aim of the Argonne Wakefield Accelerator (AWA) programme is to demonstrate high-gradient acceleration in wakefield devices, and to develop high-current electron sources that are good enough to power them. Commissioned in 1996, the AWA’s present configuration (figure 1) includes a high-current 16 MeV drive line consisting of a drive photoinjector and linac, and a low-current 4 MeV witness beam photoinjector. The drive photoinjector produces a 100 nC beam pulse; this is two orders of magnitude greater than similar photoinjectors had attained at the time it was first switched on.
While the drive beam is always delivered to the drive-line wakefield device, the path of the witness beam depends on the mode of operation. In collinear mode, the witness beam is magnetically steered into the drive-line wakefield device. In parallel mode the witness beam is delivered into the witness-line accelerating structure. Since both beams originate in photoinjectors, the time separation between the beams can easily be varied by adjusting the optical path of one injector’s laser input pulse with respect to the other.
Experimental programme
Among the most interesting experiments carried out at the AWA are those concerning the so-called dielectric step-up transformer. This device has recently been successfully used to accelerate the witness beam. The acceleration method is similar to the two-beam acceleration concept currently being developed in CERN’s compact linear collider (CLIC) project. It uses two dielectric structures, one for the drive and one for the witness beam, coupled by a waveguide. A train of drive bunches (that can easily be generated from a photoinjector by optically splitting the laser pulse) passes through the drive structure.
The radiofrequency (RF) pulse that is generated by the wakefield of the drive-bunch train is transported via the waveguide to the witness structure. If the dielectric material and geometry have been chosen appropriately, the RF pulse is compressed longitudinally as well as transversely. This provides a field step-up so that the ratio of the accelerating field in the witness structure to the decelerating field in the drive structure can be made much greater than two, which is the maximum in most collinear wakefield accelerators.
The AWA team has carried out experiments with a 7.8 GHz prototype step-up transformer. Careful bench measurements and iterative adjustment of the coupling-slot shape and the taper angle of the dielectric tube solved the potential obstacle of obtaining good coupling between the two beam lines. Initial experiments concentrated on directly measuring the RF power envelope extracted from the drive line using a high-frequency diode detector. The forward power levels peaked at 4 MW.
Steps up to success
Witness beam measurements of fields in the witness line showed that the anticipated step-up ratio of four was achieved. Although the gradients in the prototype were a modest 8 MeV/m, these experiments have provided an important proof of principle. Moreover, the high power levels generated in the drive line could be used for powering a conventional RF accelerating cavity. With this in mind, the AWA group has investigated using this device as a CLIC power extraction/transfer structure.
The AWA team has also carried out experiments on beam-driven plasma wakefields, in collaboration with the University of California at Los Angeles. Using the high-charge beams available from the AWA drive line, plasma wakefields with gradients as high as 30 MeV/m have been measured and an electron beam has been observed propagating through a self-focused ion channel in a plasma for the first time in a plasma wakefield accelerator.
Another line of research uses a test stand that allows cavities to be tested under high power without disrupting operation. Several designs from other institutions have been tested including the prototype Tesla Test Facility photoinjector and a photoinjector for Taiwan’s Synchrotron Radiation Research Centre.
Looking ahead
The AWA is currently being upgraded to obtain extended beam acceleration in high-gradient dielectric wakefield structures. The quality of the drive beam will be improved by an upgraded drive photoinjector, higher quantum efficiency photocathodes for the production of long drive beam trains, and an all solid-state titanium-sapphire laser system.
The new photoinjector uses a combination of high photocathode electric field and high exit energy to maintain a short drive bunch. For a 100 MV/m surface field in the cavity, an improvement in beam quality of a factor of 50 is expected over the already record-breaking original AWA drive photoinjector. The upgraded facility is expected to produce 10 kA micropulses at 18 MeV and an emittance that is 10 times lower than the original.
Operating the dielectric wakefield transformer at high gradients over extended distances requires a long, high-current drive pulse train. This is obtained by sending the output of the laser system through a laser-pulse splitter to generate 16 successive bunches at 40 nC each, spaced by 760 ps. Since each laser pulse will have its energy reduced by a factor of 16, a high quantum efficiency photocathode is needed to produce the high-charge electron beams.
The next goal of the AWA programme is to use the step-up transformer to increase the energy of a high-quality 1 nC witness beam by 100 MeV in less than 1 m – a major milestone in the field.
Further reading
The AWA website is at www.hep.anl.gov/awa.
