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Adielectric wall accelerator (DWA) is a compact linearparticle accelerator concept designed and patented[1] in the late 1990s, that works byinducing a travellingelectromagnetic wave in a tube which is constructed mostly from adielectric material. The main conceptual difference to a conventional disk-loadedlinac system is given by the additional dielectric wall and the coupler construction.[clarification needed]
Possible uses of this concept include its application inexternal beam radiotherapy (EBRT) usingprotons orions.
An external alternating-current power supply provides an electromagnetic wave that is transmitted to the accelerator tube using awaveguide. The power supply is switched on only a very short time (pulsed operation).[2]
Electromagnetic induction creates a travelingelectric field, which accelerates charged particles. The traveling wave overlaps with the position of the charged particles, leading to their acceleration inside as they pass through the tube's vacuum channel.[2] The field inside the tube is negative just ahead of the proton and positive just behind the proton. Because protons are positively charged, they accelerate toward the negative and away from the positive. The power supply switches the polarity of the sections, so they stay synchronized with the passing proton.
The accelerator tube is made from sheets offused silica, only 250 μm thick. After polishing, the sheets are coated with 0.5 μm of chromium and 2.5 μm of gold. About 80 layers of the sheets are stacked together, and then heated in a brazing furnace, where they fuse together. The stacked assembly is then machined into a hollow cylinder. Fused silica is pure transparent quartz glass, a dielectric, which is why the machine is called a "dielectric wall accelerator."
A sketch of one of the assembled modules of the accelerator is shown in the patent sketch. The module is about 3 cm long, and the beam travels upward. The dielectric wall is seen as item number 81. It is surrounded by a pulse-forming device called aBlumlein. In figure 8A, the power supply charges the Blumlein. In figure 8B, silicon carbide switches surrounding the Blumlein close, shorting out the edge of the Blumlein. The energy stored in the Blumlein rushes toward the dielectric wall as a high voltage pulse.
Dielectric wall accelerators have the potential to replace the currently used proton accelerators in radiation therapy, due to their smaller size, cost advantages, and reduced shielding requirements.
The DWA[3] addresses the main issues with the currentproton therapy systems—cost and size. Depending on the desired finalbeam energy, the conventional medical accelerator solutions (cyclotrons and smallsynchrotrons) can have large cost factors and space requirements, which could be circumvented by DWAs. The cost estimate for a DWA is about 20 millionUS dollars.
DWAs are expected to reach acceleration gradients around 100 MV/m.[2]
The system is a spinoff of a DOE device to inspect nuclear weapons. This system requires several new advances because of the high energies, e.g., high gradient insulators.[4]A wide band-gapphotoconductive switch, about 4,000, is needed.A symmetricBlumlein, typical width 1 mm.