SC provides the highest fields

SC provides the highest fields. At a gap of 12 mm a period length of about 50 mm would result in almost 4 T and at a period length of 17 mm still about 0.5 T is possible [4]. EM technology appears appropriate for undulators with long or even very long period lengths, 0 > 200 mm, and for very special applications. This technology is well defined and has many common features with classical EM magnets for accelerator applications.

In recent years, there has been an active search for new accelerator schemes with a high acceleration rate. One of the most promising schemes is a vacuum laser accelerator of the Inverted free-electron laser FEL (IFEL) type, which allows one to obtain both a high acceleration rate and a large percentage of electrons captured in the accelerating process. IFEL is based on the interaction of electron and high-power laser beams in a special undulator [5].

As it is known, there are several methods that allow obtaining a detailed picture of the field inside a magnetic element (devices based on nuclear magnetic resonance, Hall transducers, measuring coils, vibrating wires, etc.) and methods for determining the integral characteristics of magnetic elements (extended integrating coils and longitudinal wires, pulse wire method). It is shown that for tuning magnetic undulators it is advisable to use two methods: for precision measurements required to adjust the amplitudes of magnetic fields, it is best to use Hall sensors; and for the final integral adjustment of the fields, which allows adjusting the trajectory of the electron inside the undulator, it is better to use the pulsed wire method [5].

The Hall transducer method is one of the most accurate and reliable methods. Field measurements are made step by step with stops of the sensor carriage at the measuring points and take a long time. The wire-wound method for measuring magnetic fields is less accurate, but has its advantages. First, it is significantly faster. Secondly, it is more compact and can be used to measure magnetic fields in narrow gaps within the entire undulator. Third, it provides direct information on the first and second integrals of the magnetic field, excluding numerical integration errors. The principle of the method is as follows: a current pulse is passed through a thin wire stretched along the undulator axis (Fig. 4). Under the action of local transverse Lorentz forces, a disturbance region is formed, which propagates in both directions with an acoustic velocity of ~ 300 m/s [6]. An optical sensor located near the beginning of the undulator measures the deflection of the wire, which is proportional to the first or second integral of the magnetic field, depending on the duration of the current pulse