Contents I. Introduction
Insersion devices are the key components of the synchrotron radiation source (SRS) and free electron laser (FEL) source. There are interests and technology upgrades of undulator technology for SRS and FEL applications [1], [2], [3], [4], [5], [6]. In the SRS, the relativistic electron beam propagates in the magnetic undulator field and emits spontaneous emission of radiation. In the FEL scheme, the relativistic electron beam exchanges the kinetic energy to a co-propagating radiation field at the resonant frequency. Most insertion devices are undulators designed either as pure permanent magnet or hybrid permanent magnet according to Halbach field configuration. The modern synchrotron and FEL facilities have undertaken key technology upgrades for the undulator to provide superior spectral and gain qualities enabling experiments in a variety of disciplines and applications. The resonant frequency of the SRS or the FEL is tuned via electron beam energy or the undulator field strength and period. The energy tuning is associated with the accelerator technology and the complicated difficult task of focusing and trajectory issue in the undulator magnets. An alternative way of wavelength tuning is the variation of the field amplitude via gap variations [7]. The wavelength tuning via variation of the field amplitude is an issue in long undulators used X-ray FEL. The field amplitude in the long undulator of the X-ray FEL requires precision better than 0.1% in different undulator sections of the beam line. Adjustment of the gaps of all undulator sections with such high precision is technological issue. The SRS and FEL performances have been upgraded by the use of step tapered undulator. The scheme of the step tapered undulator alters the resonance condition halfway through the undulator [8], [9], [10]. A step tapered undulator provides two independently tunable resonances for two color operation of the FEL. Alternately, two stream FEL and two stream electron cyclotron resonance masers [11], [12], [13], [14], [15] are reported with efficiency enhancements. A third possibility exists in the changing mechanism of the undulator periods. The period length of the installed operational undulator provides easy technological tool to the tuning of radiation wavelength without interrupting the accelerator and the gap precision [16], [17], [18], [19]. As a further technology upgrade, composite biperiod undulator and triple period undulator have been implemented [20], [21]. These devices increase the wavelength tunability of the FELs. The staggered array undulator is a popular short period undulator scheme. In the scheme, the peak field strength is derived from a solenoid. The periodic field is constructed from the staggered array arrangement of magnet poles along the undulator length [22], [23], [24], [25], [26], [27]. The staggered array undulator performance has been improved by implementing superconducting technology for the solenoids [28], [29], [30]. The staggered array undulator works with variable periods. The variation of the period often requires to achieve broad photon energies [31], [32], [33], [34]. The concept of inverse FEL accelerator with a staggered undulator has been reported recently [35].
