The radio-frequency system, or RF system, supplies power to the ALS in the form of microwaves. Microwaves are radio waves with a wavelength between about one meter and one millimeter, which are the wavelengths used for radio and television broadcasts as well as radar and microwave ovens. Most parts of the RF system supply microwave radiation with a wavelength of about 0.6 meter (see Electromagnetic Radiation for an explanation of wavelengths). Microwave power is used to energize electrons, keeping them whirling around the ALS storage ring at almost the speed of light. Eventually, the electrons release this energy as x rays and ultraviolet light. Scientists use this light, which is called synchrotron radiation, to carry out experiments at the ALS. How important is the RF system? All the energy released as synchrotron radiation originates as RF power.
The basic components of the RF system include:
- RF Cavities
What is a klystron?
A klystron is a very powerful type of microwave amplifier. Radar installations and television broadcast stations use klystrons to generate their broadcast signals. The klystrons at the ALS “broadcast” down special tubes and cables, called waveguides, that lead to the linac, booster synchrotron, and storage ring.
The ALS has four klystrons, two to accelerate electrons in the linac, one to power the RF cavity of the booster synchrotron, and one to maintain the energy of electrons in the storage ring. The klystron for the ALS booster ring (see the following photo) is a commercial model like those used in television broadcasting. As a TV transmitter it would broadcast on channel 18, which corresponds to a frequency of roughly 500 MHz.
Here is a diagram that shows how a klystron works (we’ll explain this in more detail in a minute):
- An electron gun produces an intense flow of electrons into the klystron.
- A low-energy microwave signal intersects this continuous electron beam, breaking it up into a pulsed beam consisting of separate “bunches” of electrons.
- The pulsed electron beam passes through a tuned waveguide, inducing a powerful high-energy microwave signal.
- High-energy microwave power travels along the waveguide to the linac, booster synchrotron, or storage ring, where it passes its energy to electrons, accelerating them to relativistic velocity.
How does the electron beam become pulsed?
In the same way that a radio broadcast signal induces an electrical current in a portable radio antenna, the low-energy microwave signal (lets say it’s 500 MHz) causes the electrons in the electron beam to speed up or slow down at the point where the two intersect. When the microwave is near its crest (peak power) as it intersects the beam, it makes the electrons in that part of the beam at that moment speed up, just as a surfer speeds up when she catches a wave. When the microwave is near its trough as it meets the beam, the electrons in that part of the beam at that moment slow down. The result is an electron beam that is broken into pulses that have the same frequency as the low-energy microwaves: 500 MHz.
How does the pulsed electron beam induce high-energy microwave power?
The pulsing electron beam interacts with the tuned waveguide (a very carefully constructed and adjusted hollow copper tube) causing it to resonate like a bell at 500MHz, transmitting microwave radiation down its entire length, which extends either to the linac, the RF cavity on the booster synchrotron, or the RF cavities on the storage ring.
What is a waveguide?
A waveguide is a conduit for efficiently transmitting electromagnetic radiation. The coaxial cable used for cable television, the optical fiber used in telecommunications, and the linac are other examples of waveguides. The large box-like structures in the following photograph are waveguides attached to the top of the storage ring.
What is an RF cavity?
Radio frequency cavities, or RF cavities, receive RF energy from a klystron and transfer it to electrons as they pass through the cavities on their way around the booster synchrotron and storage ring. Just as in the klystron, RF radiation interacts with electrons, adding energy to increase or maintain their speed. In the booster ring this energy increases the speed of the electrons to 99.999996 percent of the speed of light. In the storage ring two RF cavities resupply the electrons with the energy that they lose in emitting synchrotron light–about 100 MeV for every turn around the storage ring. Here are two photographs of an RF cavity during installation at the ALS booster synchrotron:
The RF cavity is shaped to resonate at 500 MHz, which is the frequency of the RF radiation as well as the frequency at which the electron bunches pass. The beam pipe attaches to the cavity on either side. (In the photos the beam pipe is not attached yet. In the photo on the right you can see a temporary steel plate covering the opening where the beam pipe will attach, pointed toward the yellow block, which is a magnet.) The large duct-like conduit atop the cavity is the waveguide. The cavity is made out of very pure copper so that the current induced by the radiation will be spread evenly over its interior. Notice the slender copper “ribs” banded around the cavity. These are tubes that contain coolant to prevent the copper from overheating and possibly melting. Does this suggest why you shouldn’t put metal in a microwave oven?
The microwaves in the cavity are synchronized to crest just as each bunch of electrons enters the cavity (every 2 billionths of a second), so that the bunch is accelerated by a wave that is approaching peak amplitude. This synchronization takes into account the time it takes the control signal (travelling at the speed of light) to reach the controls for the klystron and the time it takes the microwave radiation to reach the RF cavity.
Temperature in the ALS building is carefully monitored and controlled.
- Can you say why