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Particle Accelerators and Detectors

Wakefield Acceleration in Plasma

Weakfield Simulation Instead of accelerating the charged particles along circular path, the SLAC has developed linear colliders since 1966. Lately in 2014, a more efficient method has been discovered via "Wakefield Acceleration". Essentially, the technique involves sending a drive bunch of electrons through a tube of plasma, the negative charges in the bunch propel the electrons in the plasma to the rear creating a wake. A very strong electric field is generated in this wake, which accelerates the trailing bunch of electrons efficiently with energy gain of 1.6 Gev in 30 cm. Figure 40 shows the simulation of such technique as explained briefly below :

Diagram (a) portrays the drive bunch (in red) with its wake (white) but without the trailing bunch through the plasma (blue). The vertical axis on the left is the physical dimension (in m), that on the right is the electric field Ez (the red curve) in the direction along the tube. The blue dotted curve is the current of the input beam. The color scales on top is for the density of the beam and plasma (the minus sign denotes negative charge).

Diagram (b) shows that same thing with the trailing bunch loaded into the plasma wake, the energy gain for which is at the expense of the electric energy (see modification of the red curve in the region of the wake).

Figure 40 Weakfield Simulation [view large image]

The key to achieve the high gain is in placing an appropriately shaped trailing bunch at precisely the correct distance behind the drive bunch to flatten Ez for a uniform gain throughout the bunch.

Weakfield Experiment Figure 41 shows examples of electron spectra for three different beam shots in an experiment :

(a) No interaction between the 20 Gev drive and trailing bunches since there is no plasma source (density color scale on the right side, size scale is on the left, the length of the tube is 30 cm with the scale shown on top).

(b) The spectrometer is set to image 20.35 Gev showing only the drive bunch.

(c) The spectrometer is set to image 22.35 Gev showing the trailing bunch with energy gain of about 2 Gev.

(d) This is a plot of the charge density (in unit of picoCoulomb/mm) as function of energy (corresponding to the configuration in diagram c) for simulation, data (experiment), and the core of the trailing bunch. About 10% of the electrons in the that bunch survive the ride in the wake to attain the energy gain.

Figure 41 Wakefield Experiment [view large image]

BTW, one electron carries the smallest charge of 1.6x10-19 Coul, thus 1 pC contains about 6x106 electrons.

SLAC's Main Accelerator The SLAC's main accelerator delivers the two bunches of 20 Gev electrons to the Wakefield experiment over a linear distance of 2 km. This amounts to boost 10-4 Gev/cm to each electron. In comparison, the Wakefield delivers 0.05 Gev/cm to each electron in the trailing bunch. (see Figure 42 for an aerial view of the linear accelerator and the small apparatus in the laboratory used to run the Wakefield experiment)

Figure 42 SLAC's Linear Accelerator and Wakefield Apparatus [view large image]

"This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology." - quoted from "High-efficiency acceleration of an electron beam in a plasma wakefield accelerator".

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