Helical undulator based on partial redistribution of uniform magnetic field

A new type of helical undulator based on redistribution of magnetic field of a solenoid by ferromagnetic helix has been proposed and studied both in theory and experiment. Such undulators are very simple and efficient for promising sources of coherent spontaneous THz undulator radiation from dense electron bunches formed in laser-driven photo-injectors.

Figure 1

Ferromagnetic helical insertion into solenoid (two periods are shown).

Figure 2

Results of simulations within the CST Studio for the case of steel helix with rectangular cross section having the parameters of $d=2.5\mathrm{cm}$, $a=1.2\mathrm{cm}$, $R_1=0.3\mathrm{cm}$ and $b=0.2\mathrm{cm}$. The helix geometry (a) and the amplitudes of the transverse (b) and axial (c) magnetic fields.

Figure 3

Results of simulations within the CST Studio for the case of iron helix with round cross section, period of 2.5 cm, and inner radius of 5 mm. The helix geometry (a) and the amplitude of the transverse magnetic field as function of axial coordinate [(b), wire diameter is 0.4 cm] and of helix wire diameter [(c), maximum field amplitude]. The predictions of formula (6) for the equivalent square cross section are also shown (c).

Figure 4

Steel helix with square cross section used for experimental measurements of induced transverse magnetic field.

Figure 5

Results of experimental measuring of a transverse magnetic field at the axis of the steel helix of a square cross section. The different curves correspond to different axial positions of the probe. The transverse component of the field is determined as an amplitude of the sinlike dependency of the probe signal on its angular position.