## 1 Introduction  
Generation of pulsed nanosecond THz radiation in the frequency range of 0.3-30 THz at a difference frequency with an average power of \(\approx\)1-10 mW is an intensively developing area of laser technology and nonlinear optics [1, 2]. Effective parametric generation at the difference frequency requires the use of nonlinear crystals with high optical transparency at all wavelengths--pumping, signal, and idler waves; a large second-order nonlinear susceptibility; high laser-induced damage threshold (LIDT); and good spatial homogeneity. The negative uniaxial GaSe crystal is one of the most famous and demanded nonlinear crystals in terms of its characteristics among the extensive variety of nonlinear optical crystals for oscillating THz radiation [1]. The transparency is in the range of 0.65-18 um. The absorption coefficient of this single crystal amounts to \(\approx\)0.1 cm\({}^{-1}\) at a wavelength of \(\approx\)2 um [3], and, alongside ZnGeP\({}_{2}\), it is characterized by the lowest absorption among all nonlinear crystals in the THz spectral range (90-1000 um) [4]. The material has high birefringence, nonlinear susceptibility of GaSe is \(\approx\)54 pm/V [3], and high thermal conductivity reaching \(\approx\)16 W m\({}^{-1}\) K\({}^{-1}\) (which allows for phase matching to be carried out over a wide range). GaSe has a layered structure along the \(C\) axis. Weak Van der Waals forces occur between the layers, while strong covalent bonds and an ionic component are inside the layers. Such structure makes the material soft with the Mohs hardness [3] of \(\approx\)1. This is the main disadvantage of GaSe. GaSe samples are difficult to cut at various angles to the \(C\) axis and polish. Therefore, in almost all known devices, GaSe crystals are made by splitting, and the working surfaces are parallel to the c-face. The use of a dual-wavelength degenerate optical parametric oscillator (OPO) based on a nonlinear KTP crystal with intracavity pumping by Nd\({}^{3+}\):YAG laser as a pumping source is one of the most effective ways to generate powerful tunable THz radiation at a difference frequency in nonlinear crystals including GaSe [5]. These sources are capable of generating powerful tunable dual-wavelength radiation with an orthogonal polarization near the degeneration point of 2.12 um with an average power of \(\approx\)10 W and a pulse power of \(\approx\)100 KW at a pulse repetition rate of \(\approx\)1-10 kHz and a pulse duration of \(\approx\)10-20 ns [6; 7; 8]. The most efficient nonlinear crystals for generating THz radiation, such as GaSe and ZnGeP\({}_{2}\), have minimum absorption in this wavelength range, which compares favorably this pump source with dual-wavelength tunable (or with a discrete set of wavelengths) systems operating in the spectral region of \(\approx\)1 um or \(\approx\)10 microns. The problem of GaSe damage by laser radiation at the wavelengths of 1-10 um was covered in several previously published works [9; 10; 11; 12]. In particular, these works described that in terms of the intensity of the acting beam at a wavelength of 9.55 um, the GaSe LIDT was 121 MW/cm\({}^{2}\) with pulse duration of \(\approx\)30 ns and the repetition rate of 1 Hz [9]. In Reference [12], GaSe LIDT was studied when exposed to radiation from the following lasers: Ti:Sapphire laser (\(\lambda\)-800 nm, pulse duration--100 fs); frequency-tunable optical parametric amplifier (\(\lambda\)-1.1-2.9 um, pulse duration--60-90 fs); TEA (Transversely Excited Atmospheric) CO\({}_{2}\) laser (\(\lambda\)-10.6 um, pulse duration--50 ns); Er\({}^{3+}\):YSGG laser (\(\lambda\)-2.79 um, pulse duration--250 ns). It was shown that when GaSe is exposed to femtosecond radiation, the local microdefects and the induced field effects--for example, dissociation of GaSe and multiphoton absorption--have strong impact on LIDT. It is also noted that under the influence of pulse nanosecond laser radiation, the thermal effects have a considerable influence on GaSe LIDT.