Study of High-Beta Magnetohydrodynamic Modes and Fast-Ion Losses in PDX

Abstract

levels which might have a significant role in the light shift of the 22p level due to the 1.06-/im laser field are 6s, 7s, Ad, and 5d. These are far from being resonantly coupled to the 22p level, at least 1700 cm" 1 away. Their relative positions are such that their combined effects are partially cancelled* A rough evaluation showed that under these conditions the 5d level, which is expected to be responsible for the largest effect, contributes to the shift of the 22p level an amount of approximately 3xl0" 3 MHz/ MW-cm' 2 . This is at least 4 orders of magnitude less than the measured shift, and is thus completely negligible, With respect to the shift Lv g of the ground state, since it cannot be measured alone the best procedure is to calculate it as carefully and precisely possible. A calculation based on Fig. 1 has been carried out. 6 The result is &v g = -26.3 MHz/MW-cm" 2 . The dashed line in Fig. 3 corresponds to the sum of the two calculated shifts Ai/ e + Ay g , whereas the straight line corresponds to a least-squares fit on the measured shifts. Agreement between experimental and theoretical results is satisfactory.To conclude, this experiment provides clear evidence for the shift of a Rydberg level, due to an intense and strongly nonresonant em field. It is of interest to note that in a pure quantum treat-PACS numbers: 52.55.Gb, 52.35.Py On the PDX tokamak, large-amplitude magnetohydrodynamic (MHD) fluctuations have been observed during plasma heating by injection of high-ment, radiative corrections can be interpreted as the sum of spontaneous and stimulated radiative corrections. The net effect of spontaneous radiative corrections due to vacuum fluctuations is well known to be responsible for the Lamb shift. In the same spirit, the light shifts which have been studied in our experiment can perhaps be viewed as resulting from the stimulated radiative corrections induced by an intense and nonresonant em field.We thank Professor CI. Cohen-Tannoudji for many helpful discussions concerning both the experiment and its interpretation. We are indebted to Dr. M. Aymar and Dr. M. Crance for their calculation of the shift of the ground state.Strong magnetohydrodynamic activity has been observed in PDX neutral-be am-heated discharges. It occurs for fi T q^ 0.045 and is associated with a significant loss of fast ions and a drop in neutron emission. As much as 20%~-40% of the beam heating power may be lost. The instability occurs in repetitive bursts of oscillations of ^ 1 msec duration at 1-6-msec intervals. The magnetohydrodynamic activity has been dubbed the "fishbone instability" from its characteristic signature on the Mirnov coils.

Chapter 5: Physics of energetic ions

“…Fast ions can destabilize particular classes of these modes, primarily via the precession resonance [157,158], and for sufficiently high power density excite a new type of resonant modes [157], such as the fishbones [159]. Low frequency, low mode-number MHDlike modes are analysed in section 6.1, whereas energetic particle effects on higher mode-number kinetic MHD modes, such as kinetic ballooning modes (KBMs) [160] and localized interchange modes [161,162] are considered in section 6.2.…”

Weak effect of ion cyclotron acceleration on rapidly chirping beam-driven instabilities in the National Spherical Torus Experiment

“…The phenomenon of rapid chirping driven by energetic particles is extremely common. It occurs for classic fishbones [5] in many different conventional tokamaks [1]. Chirping in the frequency band between the TAE and classic fishbones was first observed during beam injection into the DIII-D tokamak [6] and was subsequently found in nearly all toroidal devices with intense beam injection including spherical tokamaks [7], conventional tokamaks (called 'bursting modes' in [8,9]), and helical devices [10,11].…”

“…Many different fast-ion instabilities are observed during neutral beam injection [40]. One class of chirping instabilities [41] resemble the classic fishbone [5]: the toroidal mode number n is unity, the frequencies sweep downwards between 10-20 kHz, and the modes occur when q 0 < 1. (q 0 is the central safety factor.)…”

“…The frequencies of these waves can also evolve rapidly if the fast-ion population that determines the wave's dispersion relation is altered by the growing wave. A familiar example of an EPM with rapid frequency sweeping is the classic fishbone [5]; the frequencies of these modes drop by a factor of two in less than a millisecond. In this paper, frequency 'chirping' refers to large changes in frequency on a sub-millisecond timescale.…”

Measurements of fast-ion transport by mode-particle resonances on DIII-D

“…Because of the widely different fast-ion orbits, instabilities with a variety of frequencies 0029-5515/12/103022+10$33.00 1 and mode structures can lead to coherent resonances with energetic ions. On the PDX tokamak, observations of large beamion losses concurrent with bursts of MHD activity dubbed 'fishbones' [2] spurred theoretical investigations as to the cause of the large losses. The traditional fishbone instability appears in high-beta plasmas with a q = 1 surface where a sufficiently large trapped particle population destabilizes the m/n = 1/1 internal kink mode [3].…”