A dielectric wall accelerator (DWA) is a compact linear

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...

concept designed and patented in the late 1990s, that works by inducing a travelling

electromagnetic wave In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength, ...

in a tube which is constructed mostly from a

dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...

material. The main conceptual difference to a conventional disk-loaded

linac A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear bea ...

system is given by the additional dielectric wall and the coupler construction. Possible uses of this concept include its application in

external beam radiotherapy External beam radiation therapy (EBRT) is a form of radiotherapy that utilizes a high-energy collimated beam of ionizing radiation, from a source outside the body, to target and kill cancer cells. The radiotherapy beam is composed of particl ...

(EBRT) using

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

s or

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

Operation

An external alternating-current power supply provides an electromagnetic wave that is transmitted to the accelerator tube using a

waveguide A waveguide is a structure that guides waves by restricting the transmission of energy to one direction. Common types of waveguides include acoustic waveguides which direct sound, optical waveguides which direct light, and radio-frequency w ...

. The power supply is switched on only a very short time (pulsed operation).

Electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force, electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1 ...

creates a traveling

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

, 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. 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.

Construction

The accelerator tube is made from sheets of

fused silica Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses, such as soda-lime glass, lead glass, or borosil ...

, 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 a Blumlein. 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.

Usage in proton therapy

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.

Advantages and limitations

The DWA addresses the main issues with the current

proton therapy In medicine, proton therapy, or proton radiotherapy, is a type of particle therapy that uses a beam of protons to irradiate diseased tissue, most often to treat cancer. The chief advantage of proton therapy over other types of external beam ...

systems—cost and size. Depending on the desired final beam energy, the conventional medical accelerator solutions ( cyclotrons and small

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The strength of the magnetic field which bends the particle beam i ...

s) can have large cost factors and space requirements, which could be circumvented by DWAs. The cost estimate for a DWA is about 20 million

US dollar The United States dollar (symbol: $; currency code: USD) is the official currency of the United States and several other countries. The Coinage Act of 1792 introduced the U.S. dollar at par with the Spanish silver dollar, divided it int ...

s. DWAs are expected to reach acceleration gradients around 100 MV/m. 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. A wide band-gap photoconductive switch, about 4,000, is needed. A symmetric Blumlein, typical width 1 mm.

References

{{reflist, refs= {{US patent reference , inventor-last = Sampayan , inventor-first =Stephen , inventorlink = , y = 2001 , m = 12 , d = 18 , title
Process for manufacturing hollow fused-silica insulator cylinder
, number = 6331194

External links

Dielectric Wall Accelerator
G. J. Caporaso, Y.-J. Chen, S. E. Sampayan September 3, 2009, Reviews of Accelerator Science and Technology
High Gradient Dielectric Wall Accelerators
Muon Collider Design Workshop, December 8–12, 2008, Thomas Jefferson National Laboratory
Ultra‐High‐Current Electron Induction Accelerators
Physics today 031-9228Kapetanakos, C yr:1985 vol:38 iss:2 pg:58
US Patent 7924121
Dispersion-Free Radial Transmission Lines, April 12, 2011 Accelerator physics