RF linear accelerators

Wangler T (1998) Principles of RF linear accelerators. Wiley, New York... 

Generally speaking, electrostatic accelerators operate at lower particle energies (1-5 MeV). RF accelerators (e.g., linear accelerators and RF electron guns) can accelerate particles to much higher energies (GeV). 

As mentioned above, a 3.5-cell RF photocathode gun is in operation as the accelerator for the Brookhaven National Laboratory Laser-Electron Accelerator Facility. Recently, 1.6-cell RF photocathode guns have replaced thermionic cathode systems as injectors for 30 MeV linear accelerators at Osaka University and the Nuclear Engineering Research Laboratory in Tokai-mura, Japan [6]. Another RF photocathode gun accelerator is under construction at the ELYSE facility at the Universite de Paris-Sud at Orsay, France. A magnesium cathode is in use at LEAF, copper is used at NERL, while the Orsay accelerator will use Cs Te. 

Radio-frequency linear accelerators (rf linacs) A linear accelerator that employs radio frequency (rf) cavities for electron acceleration. The particles are accelerated in cylindrical cavities that reqitire a high power soitrce for the rf fields. The rf fields in these cavities may be either traveling or standing waves. In the case of traveling wave conflguratiorts, which are most often employed for electron accelerators, the phase velocity if the rf fields must be synchronized with the desired electron velocity. 

The linear accelerator (also called LINAC) is a series of cylindrical tubes, which are alternately connected to a high-frequency generator, as shown in Rig. 50.17. The charged particles are accelerated in the gaps between the tubes and then they drift inside the tube in a field-free region. While the particles are inside the tube, the direction of the electric field is reversed. If the length of tubes is increased in proportion to the particle speed, the particles will always arrive at the next gap at the correct phase of the RF voltage. 

Schematic structure of the linear accelerator. The arrows indicate the iocai direction of the electric fieid at a particuiar value of the RF phase...

As the speed of particles increases, the drift tubes get longer. The final energy of the particles is limited by the available RF driver frequencies, the lengths of the drift tubes and that of the whole accelerating system. These types of linear accelerators are used to accelerate protons and heavier particles. To produce higher energy ion beams, such structures are coupled in a synchronized accelerating system. 

To produce beams of extreme specifications, for example, very high-energy particles or heavy-ion beams, a chain of accelerators have to be used (see O Tables 50.2-50.4 and the Internet sources referred to there). Generally, the ions coming from the ion source are accelerated in the first step by an electrostatic accelerator and after a bunching system, preparing the DC beam for the acceleration in an RF-based structure, they are accelerated further by cyclic or linear accelerators. 

In the operation of 2D and 3D quadrupole instruments, the modification of ion trajectories by ion/ neutral collisions must be considered. In rf quadrupole fields, an ion/neutral collision reduces both ion kinetic energy and ion excursions such that the ions are cooled and focused to the center of each field. Collisional cooling is an important aspect of the behavior of ions in a quadrupole field for example, in a 2D quadrupole device, collisional cooling is employed to limit the excursions of ions so as to form a tightly focused ion beam of diminished kinetic energy constrained close to the central axis. A focused beam of ions may be transmitted through a relatively small orifice from one section of an instrument to the next such that pumping requirements are reduced, and a focused ion beam can be accelerated with reduced ion loss. When the axial motion of a focused ion beam is arrested within a rod array such that the confined ions can be excited resonantly, a linear ion quadrupole trap is obtained. 

Various types of electric field are used in different mass analyzers. From this perspective the simplest example is provided by the constant field (DC) strengths used to accelerate ions, e.g. out of an ion source into an analyzer, or from an intermediate component into the flight tube of a time of flight analyzer etc. More elaborate fields whose strength is varied in time at frequencies in the range of a few MHz (within the radiofrequency (RF) range) are used in linear quadrupoles and ion traps. However, the same principles of physics are exploited in aU cases, and the following section is an introduction to these principles. 

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