Study of lower hybrid current drive efficiency over a wide range of FTU plasma parameters

Abstract

The key quantities affecting the efficiency of Lower Hybrid (LH) radiofrequency waves in driving non-inductively the toroidal current in a tokamak have been recognized by means of a linear regression analysis over all the data available for the Frascati Tokamak Upgrade. The parameter space is bounded within the following ranges: line averaged plasma density 0.29 × 10 20 n e 1.29 × 10 20 m −3 , central electron temperatures 1.1 T e0 7.4 keV, corresponding to volume averaged temperatures 0.27 T e 1.2 keV, plasma current 0.3 I p 0.7 MA, magnetic field 4 B T0 7.2 T, with a safety factor between 4.7 q a 10.7, LH power 0.4 P LH 2.1 MW and LH parallel refraction index 1.32 N 0 2.42. The experimental current drive (CD) efficiency, reduced to the effective ion charge state Z eff = 1, varies for this data set within 0.12 η * CD 0.34 A W −1 × 10 20 m −2. A linear regression analysis gives a reliable scaling law for η * CD with a correlation coefficient close to 0.9 that points out the importance of the various quantities. The CD efficiency is a significantly increasing function of T e and B T , and a decreasing one of q a and P LH , while N andn e have limited influence. The physical reasons for the observed trend related to the variation of each parameter are recognized and discussed. The main causes are identified in the modification suffered by the N spectrum along the ray trajectory before the power can be absorbed by the electrons and in the interaction with the edge plasma density fluctuations. The analysis also allows putting into evidence the synergy between the LH and electron cyclotron waves, when the latter are absorbed directly on the LH generated suprathermal electron tails and produce the highest values of η * CD .

Analysis of the Chaotic Behavior of the Lower Hybrid Wave Propagation in Magnetised Plasma by Hamiltonian Theory

“…It is worth noting that in the case n n crit , the Hamiltonian of Equation (3) reduces to a much simpler Hamiltonian that corresponds to the electrostatic limit of the dispersion relation. For these simulations, we chose the FTU typical plasma and antenna parameters, that is a central density in the range n e0 = 10 13 ÷ 10 14 cm −3 , inverse aspect ratio = 0.3, toroidal magnetic field on axis B 0 = 4 ÷ 8 T, plasma current I p = 0.3 ÷ 1 MA, corresponding to 3 < q a < 10, LH frequency f = 8 GHz, launched parallel wave number n = 1.32 ÷ 2.42 (following the antenna phasing) (see [41]). …”

“…(being T e0 ≈ 1 − 2 KeV in the LH experiments on FTU [41]). In the reported cases of Figure 4, the time-averaged n is shown for cases of Figure 3a,b (without chaos), n = 3.2, n = 2, respectively, and for cases of Figure 3c,d (in presence of chaos), n = 4.5 and n = 10.4, respectively.…”

“…The fact that the toroidal effects can be considered as perturbations of the integrable system is made possible by the smallness of the inverse aspect ratio , which can be taken as a perturbative parameter. The results of the canonical perturbation theory and the related numerical integration of the rays suggest that the increase in the average value of n is limited to low values, so there must be another mechanism that comes into play to fill the spectral gap and justify the results obtained on Frascati Tokamak Upgrade (FTU) in the Lower Hybrid Current Dive (LHCD) experiments [40,41]. Nevertheless, in our study, we have performed a complete scan in plasma and geometrical parameters (inverse aspect ratio) to establish the transition to the chaotic behavior of the trajectories.…”

Dependence of synergy current driven by lower hybrid wave and electron cyclotron wave on the frequency and parallel refractive index of electron cyclotron wave for Tokamaks

“…D) The current drive efficiency is sensitive to temperature. [17][18][19][20] In Sec. II, the temperature is 3 keV.…”

Integrated numerical design of an innovative Lower Hybrid launcher for Alcator C-Mod

“…Particular attention was also given to the study of the peak fields in the waveguide, the LH2 will be operating at a power density of 5(kW/cm^), just above the weak conditionning limit [4,5].…”