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Bursting toroidal Alfvén eigenmodes in KSTAR plasmas

Hole, M J,Qu, Z S,Layden, B,Michael, C A,Woo, M H,Bak, J G,Kim, J,Hezaveh, H IOP 2019 Plasma physics and controlled fusion Vol.61 No.2
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We report on observations of bursty mode activity during early neutral beam heating in KSTAR plasmas, before current flat top while the q profile is still evolving. The magnitude of the activity increases with early beam heating, and reduces with the addition of resonant magnetic perturbation magnetic field coils. A mode analysis yields a toroidal mode number of n?=?2. The mode is observed to be downward chirping in frequency, and exists for the duration of the slowing-down of the beam. Motional Stark effect constrained equilibrium reconstructions are available at adjacent time slices: we have rescaled the total current to the measured value to obtain the q profile during the mode activity. From this we have computed the mode spectrum and identified a number of candidate gap modes. Wave-particle simulations with plausible distribution functions are computed, which demonstrate that the lowest frequency mode satisfies the condition for wave drive <img ALIGN='MIDDLE' ALT='${\omega }_{* }/\omega \gt 1$' SRC='http://ej.iop.org/images/0741-3335/61/2/025016/ppcfaaf40bieqn1.gif'/>, where <img ALIGN='MIDDLE' ALT='${\omega }_{* }$' SRC='http://ej.iop.org/images/0741-3335/61/2/025016/ppcfaaf40bieqn2.gif'/> is the fast ion diamagnetic drift frequency. An interesting finding is the change from exponential growth of the mode above <img ALIGN='MIDDLE' ALT='${n}_{f}/{n}_{i0}\approx 0.6$' SRC='http://ej.iop.org/images/0741-3335/61/2/025016/ppcfaaf40bieqn3.gif'/>, whereby the mode continues to nonlinearly grow at a reduced rate over a period of 100 wave periods up to final saturated amplitude. We believe that this may be because the two spatial resonances at s?=?0.4 and s?=?0.8 overlap for sufficiently high fast ion density, and so the phase-space volume and fast ion density available to drive the mode increases.
2
THE RR LYRAE VARIABLES AND HORIZONTAL BRANCH OF NGC 6656 (M22),

Kunder, Andrea,Stetson, Peter B.,Cassisi, Santi,Layden, Andrew,Bono, Giuseppe,Catelan, Má,rcio,Walker, Alistair R.,Paredes Alvarez, Leonardo,Clem, James L.,Matsunaga, Noriyuki,Salaris, Maurizio American Institute of Physics 2013 The Astronomical journal Vol.146 No.5
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The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC 6656 (M22), with observations spanning a range of 22 years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are ⟨P⟩RR0 = 0.66 ± 0.02 days and ⟨P⟩RR1 = 0.33 ± 0.01 days, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1-type to all RR-type variables is N1/NRR = 0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E( B – – V) = 0.36 ± 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R) = +0.97 ± 0.1 and at least one 'gap' located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.
3
Fourier analysis of non‐Blazhko ab‐type RR Lyrae stars observed with the Kepler space telescope

Nemec, J. M.,Smolec, R.,Benkő,, J. M.,Moskalik, P.,Kolenberg, K.,Szabó,, R.,Kurtz, D. W.,Bryson, S.,Guggenberger, E.,Chadid, M.,Jeon, Y.‐,B.,Kunder, A.,Layden, A. C.,Kinemuchi, K.,Kis Blackwell Publishing Ltd 2011 Monthly notices of the Royal Astronomical Society Vol.417 No.2
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ABSTRACTNineteen of the ∼40 RR Lyr stars in the Kepler field have been identified as candidate non‐Blazhko (or unmodulated) stars. In this paper we present the results of Fourier decomposition of the time‐series photometry of these stars acquired during the first 417 d of operation (Q0–Q5) of the Kepler telescope. Fourier parameters based on ∼18 400 long‐cadence observations per star (and ∼150 000 short‐cadence observations for FN Lyr and for AW Dra) are derived. None of the stars shows the recently discovered ‘period‐doubling’ effect seen in Blazhko variables; however, KIC 7021124 has been found to pulsate simultaneously in the fundamental and second overtone modes with a period ratio P2/P0∼ 0.593 05 and is similar to the double‐mode star V350 Lyr. Period change rates are derived from O − C diagrams spanning, in some cases, over 100 years; these are compared with high‐precision periods derived from the Kepler data alone. Extant Fourier correlations by Kovács, Jurcsik et al. (with minor transformations from the V to the Kp passband) have been used to derive underlying physical characteristics for all the stars. This procedure seems to be validated through comparisons of the Kepler variables with Galactic and Large Magellanic Cloud (LMC) RR Lyr stars. The most metal‐poor star in the sample is NR Lyr, with [Fe/H] =−2.3 dex; and the four most metal‐rich stars have [Fe/H] ranging from −0.6 to +0.1 dex. Pulsational luminosities and masses are found to be systematically smaller than L and <IMG src='/wiley-blackwell_img/equation/MNR_19317_mu1.gif' alt ='inline image'/> values derived from stellar evolution models, and are favoured over the evolutionary values when periods are computed with the Warsaw linear hydrodynamics code. Finally, the Fourier parameters are compared with theoretical values derived using the Warsaw non‐linear convective pulsation code.

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