Influence of the gain profile on the laser dynamics

Abstract

Several important topics relating to MHD behaviour in JT-60U are presented. An extremely hollow current profile was found for the first time in JT-60U with a very small plasma current in the central region, which is called the current hole. Analyses of equilibrium and stability of the current hole discharge show that, as β p increases, the Pfirsh-Schlüter current increases, which reduces the poloidal field on the inboard side. It may cause the null point of the magnetic field. β limits due to ideal MHD instability in the current hole do not change very much compared to that of the weakly reversed shear plasma. In high β long pulse plasmas, it is important to suppress the resistive MHD instability, neo-classical tearing mode (NTM), induced by the lack of bootstrap current inside the magnetic island. Analyses of two typical JT-60U discharges with and without NTM show that the polarization current due to the plasma rotation is a plausible candidate to suppress the NTM though it is very sensitive to the value of mode rotation. To establish an advanced control of NTM, a real-time control system, which can identify the island by ECE measurement and control EC ray to the island by a steerable mirror, was prepared in JT-60U, and it successfully stabilized the NTM. Instability due to fast ions is an important subject for sustaining the fusion reaction. In JT-60U, slow frequency sweeping (slow-FS) modes and bursting MHD modes in the frequency range of the toroidicityinduced Alfvén eigenmodes (TAEs) are observed by injections of negativelycharged ion based neutral beam of ∼360 keV. Analyses by the HINST code show that the resonant TAE mode is a candidate of the slow-FS mode. A bursting mode was observed to change its frequency by 10-20 kHz in 1-5 ms (fast FS mode) and evolve explosively in ∼400 µs (abrupt large amplitude event). Analyses of the bursting mode show that the non-linear deformation of the energetic ion distribution may cause the repetition of bursting phenomena and the recovery of lost energetic ions.

Simple modeling of single-mode inhomogeneously broadened laser dynamics

“…This model was proposed by the above authors [31], who demonstrated theoretically that expanding the inhomogeneous medium to two groups of atoms, with different resonant frequencies, brought about qualitative changes as well as a significant reduction in the laser instability threshold.…”

“…Section II is devoted to a review of the Maxwell-Bloch equations with some hints pertaining to the numerical analysis, as well as to a remainder of the dynamic behavior of the system according to the values of the control parameters (excitation parameter C, cavity relaxation rate k, etc.). Section III concerns a rapid review of (a) the Graham and Cho model [28], (b) our four-dimensional (4D) model, which describes the "Casperson instability" [29,30], and (c) the Idiatulin and Uspenskii model [31].…”

Dynamics of an inhomogeneously broadened passively mode-locked laser

“…4. Reconstructed polarisation (top) and population difference (bottom) functions + (ω, t pulse , L)φ0(ω)|, N (ω, t pulse , L)φ0(ω) at the end of the absorber section (z = L, output edge) at the moment of maximal pulse intensity t = t pulse , where P , N approximate solutions of (1)-(3), and φ0 is defined by (5), in a laser with inhomogeneously broadened gain and absorber lines having equal linewidth σ = 1 ps −1 and σ = 5.6 ps −1 . Other parameters are as in Figure 3.…”

“…The effect of inhomogeneous broadening on singlemode [5][6][7][8][9][10][11], multi-longitudinal [12][13][14][15][16][17][18][19] and multitransverse [20,21] mode laser instabilities has been a subject of intense studies during the past decades. In particular, it was shown that inhomogeneous broadening can reduce the so-called second laser threshold as well as the Contribution to the Topical Issue "Non-Linear and Complex Dynamics in Semiconductors and Related Materials", edited by Kathy Lüdge.…”

The influence of the inhomogeneous gain profile on the spatio-temporal dynamics of broad-area class B lasers

“…These theoretical studies are based on a simplified model of the full integro-differential system of Maxwell-Bloch equations for the single-mode inhomogeneously broadened laser. This approach was first introduced by Idiatulin and Uspenskii [19], and relies on a judicious discretization of the inhomogeneous gain profile in the spectral domain. Following this approach, Mezziane [20] studied the temporal dynamics of an inhomogeneously broadened laser and the emergence of self-pulsing behavior.…”