Recent results on investigations of Alfvén eigenmodes, fast ion confinement and fast ion diagnostics in JT-60U are presented. It was found that toroidicity induced Alfvén eigenmodes (TAEs) were stable in negative shear discharges with a large density gradient at the internal transport barrier (ITB). If the density gradient was small at the ITB, multiple TAEs appeared around the q = 2 surface (pitch minimum) and showed a large frequency chirping (∆f ≈ 80 kHz). In low-q positive shear discharges, the location of the TAEs changed from outside to inside the q = 1 surface, owing to a temporal change of the q profile. A significant depression of the megaelectronvolt ion population was observed only with high-n (n up to 14) multiple TAEs inside the q = 1 surface. Non-circular triangularity induced Alfvén eigenmodes were observed for the first time. Considerable depression of the triton burnup was observed in negative shear discharges. Orbit following Monte Carlo simulations indicated that ripple loss was responsible for the enhanced triton losses. The fast ion stored energies in ICRF heated negative shear discharges were comparable to those of positive shear plasmas. Tail ion temperatures in high (second to fourth) harmonic ICRF heating experiments were first analysed with an MeV neutral particle analyser. The behaviour of MeV ions produced by ICRF heating was studied with gamma ray diagnostics. A scintillating fibre detector system for detecting the 14 MeV neutron emission was developed for the triton burnup studies. Ion cyclotron emission measurements discriminating between parallel and perpendicular components of the electric field were carried out for the first time.

The excitation and stabilization of Alfvén eigenmodes and their impact on energetic ion confinement were investigated with negative ion based neutral beam injection at 330-360 keV into weak or reversed magnetic shear plasmas on JT-60U. Toroidicity induced Alfvén eigenmodes (TAEs) were observed in weak shear plasmas with ⟨βh⟩ ⩾ 0.1% and 0.4 ⩽ vb||/vA ⩽ 1. The stability of TAEs is consistent with predictions by the NOVA-K code. New burst modes and chirping modes were observed in the higher β regime of ⟨βh⟩ ⩾ 0.2%. The effect of TAEs, burst modes and chirping modes on fast ion confinement has been found to be small so far. It was found that a strongly reversed shear plasma with internal transport barrier suppresses AEs.

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

On JT-60U, for the first time in negative shear plasmas, toroidicity induced Alfvén eigenmodes (TAEs) with frequencies in the range of 90 to 110 kHz and toroidal mode numbers n = 5-8 were observed after a broadening of the density profile due to sequential partial collapses during ion cyclotron range of frequency (ICRF) heating. TAEs are more stable in negative shear plasmas than plasmas with a monotonically increasing q profile. The TAE stability was analysed with the NOVA-K code. It was found that the stability is highly sensitive to the gap alignment just inside qmin and that the theoretical results are consistent with the experimental results. In a weak central magnetic shear discharge, modes with very rapid frequency chirping were observed during ICRF heating. The frequencies of these modes rapidly increased from ∼30 kHz up to ∼110 kHz in a period of ∼150 ms. The large frequency chirping is not well understood by the present theory.

Tomidicity-induced Alfvdn eigen (TAE) modes are observed during minority-ion cyclotron resonance heating (ICRH) in the JT-60U. The toroidal mode numben of TAE modes are idenlified as 7, 8, 9, 10 and 1 I from the Doppler shift in the TAE modes with scanning toroidal rotation at a plasma current of 3 MA. The toroidal mode number of TAE modes tends to increase during a giant sawtooth by ICRH with a decreasing safety factor for the central region.The TAE mode number increases with pl3sma current, so that nine TAE modes are observed sequentially during B giant sawtooth al B plasma current of 4 MA. where the maximum toroidal made number is estimated lo be at least 13. There m no Alfvbn continuum gaps for TAE modes in the safety-fanor ranges of i -1/2n c y c i + 1/2n. ( I = 1.2,3, . . .), except for the gaps in ellipticity-induced Alfv6n eigen (EAE) modes, where n is the toroidal mode number of T4E modes. Therefore, control of the y profile pight provide a means of avoiding TAE modes.as long as the pressure gradient of the high-energy ions is Iodized.