Stochastic cooling

Encyclopedia : S : ST : STO : Stochastic cooling

Stochastic cooling is a technique to reduce the momentum spread in a storage ring for charged particles by detecting fluctuations in the momentum of the bunches and applying a correction to the momentum (a "steering pulse" or "kick") with the opposite sign. This is an application of negative feedback.

In a storage ring the charged particles travel in bunches in potential wells. The oscillation of the center of mass of the bunches is easily damped, but the internal temperature of the bunch is not affected by this longitudinal cooling. The key to cooling is to address individual particles within the bunches using electromagnetic radiation.

The bunches pass an optical scanner, which detects the position of the individual particles in a wide bandwidth. In reality the transverse motion of particles is easily damped by synchrotron radiation, which has a short wavelength and wide bandwidth, but the longitudinal motion can only be increased by simple devices (see for example Free electron laser). To achieve cooling the information about the position is fed back into the particle bunches, in a produce an overall negative feedback loop.

The particles travel near but not exactly at light-speed, so some devices are needed to slow the light, or to phase match light and particles.

Klystron cavity
traveling wave tube
undulator
Cherenkov radiation ( a thin walled ceramic tube passing through gas. The gas has a refractive index a bit above 1 )

The last three devices are directional couplers, that is they allow to integrate measurement and steering (in this context often called kicking) in one device. For maximum bandwidth multiple devices tuned to different frequencies are used, so that about 20 GHz can be covered. For most devices the bunches are focused through a small hole, so that the devices have access to the near-field of the radiation.

The word “stochastic” in the title stems from the fact, that usually not all particles can unambiguously be addressed at once. Then they cannot be cooled down in one but in multiple steps. The better the addressing the faster the cooling.

As the particles in storage ringe travel at nearly the speed of light, the feedback loop, in general, has to wait until the bunch returns to make the correction. The detector and the kicker can in general be placed on different positions on the ring with appropriately chosen delays to match the eigenfrequencies of the ring.

The cooling is more efficient for long bunches. Optimally bunches are as short as possible in the accelerators of the ring and as long as possible in the coolers. The devices for this are intuitively called stretcher, compressor, or buncher, debuncher. The links point to the equivalent devices for light pulses, so please exchange the prisms by dipole magnets.

For low energy rings the bunches can be overlapped with freshly created and thus cool (1000 K) electron bunches from a linac. This is a direct coupling to a temperature bath, but afterwards the electrons can also be analized and stochasitic cooling applied.

For transversal cooling the same devices are used like in an oscilloscope or in a Streak camera.

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.