Noncircular Triangularity and Ellipticity-Induced Alfvén Eigenmodes Observed in JT-60U

Abstract

For the first time noncircular triangularity induced Alfvén eigenmodes (NAE) were observed in combined ion cyclotron resonance frequency and neutral beam injection heated plasmas. Ellipticity induced Alfvén eigenmodes (EAE) and toroidicity-induced Alfvén eigenmodes (TAE) were also observed in those plasmas. The threshold beta of the energetic ions for exciting the NAE modes was found to be similar to that for exciting TAE modes. [S0031-9007(98)05662-2] PACS numbers: 52.35. Bj, 52.35.Hr, 52.35.Py, 52.55.Fa Over the last two decades, shear Alfvén eigenmodes (AE) have been studied theoretically ([1-6], and references therein), and experimentally in fusion devices ([7-12], and references therein). Most of that work was concentrated on toroidicity-induced Alfvén eigenmodes (TAE). TAE modes can exist in the lowest gap of the shear Alfvén continuum, where the poloidal harmonics with mode numbers m and m 1 1 couple. The frequency, v, at which the TAE mode appears, is given by v y A jk k j Ӎ y A ͞RjnDm͑͞2m 6 Dm͒j for large aspect ratio tokamaks, with y A B͞ p m 0 r i , the Alfvén velocity (B the magnetic field, r i the ion mass density), n the toroidal mode number, Dm 1, 2, 3 for the TAE, EAE, and NAE modes, respectively, and R the major radius. Higher order AE gaps appear when modes with a larger poloidal mode number difference couple [4]. The ellipticity-induced Alfvén eigenmodes (EAE) reside in the gap created by the coupling of poloidal harmonics with mode numbers m and m 1 2. Their frequency is about twice the TAE frequency [12][13][14]. The next higher gap is created by the coupling of the poloidal harmonics with mode numbers m and m 1 3 and modes residing there are called noncircular triangularity-induced Alfvén eigenmodes (NAE). They are expected at roughly three times the TAE frequency [13]. In the past Wong et al. [7] have detected intermittent oscillations with their Mirnov coil system at about three times the Alfvén frequency that were correlated with sawtooth oscillation but they were unable to determine the precise nature of it.Alfvén Eigenmodes can be excited when the drive overcomes the damping, i.e., there must be a sufficient number of energetic ions that have reached the threshold parallel velocity, y k , which depends on the type of mode [4,15]. TAE modes are excited when y k y A and sideband resonances are present at y k y A ͞3. EAE modes are excited when y k y A ͞2 (there are no sideband resonances for EAE modes). NAE modes are excited when y k 3y A and sideband resonances are present at y k 3y A ͞5.The resonance condition for the trapped particles is given by [6]: v AE v d 1 lv b with v AE the Alfvén eigenmode frequency, v d the precession frequency, v b

Alpha particle physics in a tokamak burning plasma experiment

“…6 Unstable eigenmodes exist in the gap caused by toroidicity, ellipticity, and triangularity. 7 There is also a core-localized version of the TAE 8 and a normal mode caused by finite Larmor radius effects, the KTAE. 9 Other instabilities at higher frequencies include ion-cyclotron emission ͑ICE͒ 1 and compressional Alfvén eigenmodes.…”

Alfvén continuum and Alfvén eigenmodes in the National Compact Stellarator Experiment

“…Instabilities of TAE modes have already been identified in stellarator experiments, 2,3 whereas instabilities of EAE and NAE modes have been observed in tokamaks, 13,14 and there is no reason to deny that they can appear in stellarators, too. The toroidal asymmetry of the stellarators results in additional gaps, where the MAE modes 15,16 (Mirror-induced Alfvén Eigenmodes) and numerous types of HAE modes [15][16][17][18] (Helicity-induced Alfvén Eigenmodes) can reside.…”

High harmonic ion cyclotron heating in DIII-D: Beam ion absorption and sawtooth stabilization

“…The tail population destabilizes toroidicity induced Alfvén eigenmodes (TAEs) [13,14] and other Alfvén modes [15].…”