Table of contents

Energetic alpha particle physics plays an obviously crucial role in burning fusion plasmas. Good confinement of them is required to sustain fusion burn and to avoid damage of the first wall. Because of this importance for nuclear fusion research, Y. Kolesnichenko and the late D. Sigmar initiated a series of IAEA technical (committee) meetings (TCM, since the 8th meeting TM) in order to exchange information on the behaviour of energetic particles in magnetic confinement devices. The role of the TMs has become increasingly important since burning plasma projects such as ITER are in preparation. After every TM, invited speakers are encouraged to publish an adapted and extended version of their contributions to the meeting as an article in a special issue of Nuclear Fusion. An exception was the 8th TM the articles of which were published in a special issue of Plasma Physics and Controlled Fusion (2004 46 S1–118). These special issues attract much interest in the subject.

The 9th IAEA TM of this series was held in Takayama, Japan, 9–11 November 2005, and 53 papers including 16 invited talks were presented. A total of 11 papers based on these invited talks are included in this special issue of Nuclear Fusion and are preceded by a conference summary. Experimental results of energetic ion driven global instabilities such as Alfvén eigenmodes (AEs), energetic particle modes (EPMs) and fishbone instabilities were presented from several tokamaks (JET, JT-60U, DIII-D and ASDEX Upgrade), helical/stellarator devices (LHD and CHS) and spherical tori (NSTX and MAST). Experimental studies from JET and T-10 tokamaks on the interaction of ion cyclotron waves with energetic ions and runaway electrons were also presented. Theoretical works on AEs, EPMs and nonlinear phenomena induced by energetic particles were presented and compared with experimental data. Extensive numerical codes have been developed and applied to obtain predictions of energetic particle behaviour in future ITER plasmas as well as a tool for design study of future machines. The trend in theoretical code work is to become self-consistent and integrated with a suite of predictive codes. The development of various diagnostics for energetic particle measurements was the subject of many presentations, in particular for burning plasma experiments such as ITER.

Alfvén cascade (AC) eigenmodes excited by energetic ions accelerated with ion-cyclotron resonance heating in JET reversed-shear discharges are studied experimentally in high-density plasmas fuelled by neutral beam injection (NBI) and by deuterium pellets. The recently developed O-mode interferometry technique and Mirnov coils are employed for detecting ACs. The spontaneous improvements in plasma confinement (internal transport barrier (ITB) triggering events) and grand ACs are found to correlate within 0.2 s in JET plasmas with densities up to ∼5 × 10¹⁹ m⁻³. Measurements with high time resolution show that ITB triggering events happen before 'grand' ACs in the majority of JET discharges, indicating that this improvement in confinement is likely to be associated with the decrease in the density of rational magnetic surfaces just before q_min(t) passes an integer value. Experimentally observed ACs excited by sub-Alfvénic NBI-produced ions with parallel velocities as low as V_||NBI ≈ 0.2 · V_A are found to be most likely associated with the geodesic acoustic effect that significantly modifies the shear-Alfvén dispersion relation at low frequency. Experiments were performed with a tritium NBI-blip (short time pulse) into JET plasmas with NBI-driven ACs. Although considerable NBI-driven AC activity was present, good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and the TRANSP code modelling based on the Coulomb collision model, indicating the ACs have at most a small effect on fast particle confinement in this case.

Recent upgrades to many of the diagnostic systems on DIII-D (Luxon J.L. 2002 Nucl. Fusion42 614) such as the CO₂ interferometer, far-infrared scattering, beam-emission spectroscopy (BES), and quadrature reflectometer have significantly extended their capabilities and made possible the experimental study of Alfvén eigenmodes (AEs) through observation of the AE induced density perturbation. Measurements have revealed the presence of several different classes of AEs in DIII-D discharges including the toroidal Alfvén eigenmode (TAE), reverse shear AE (RSAE or Alfvén cascade) and ellipticity induced Alfvén eigenmode. Based on a simple model for the RSAE frequency, a sensitive diagnostic for the evolution of the minimum magnetic safety factor (q_min) is presented and results are compared with motional Stark effect (MSE) measurements. Strong localization of high toroidal mode number RSAEs to regions near the minimum of the magnetic safety factor is exhibited on the CO₂ interferometer and BES measurements. Based on this observation, a method for providing constraints on the radial location of q_min is demonstrated and a favourable comparison to MSE measurements is made. Detailed measurements of TAEs using a new all-digital large bandwidth two-colour CO₂ interferometer system show a strong asymmetry between vertical and radial viewing interferometer chords confirming previously reported results. Additionally, effects related to line-integrated observations are clearly illustrated by comparison to local BES measurements and potential issues related to this are discussed.

Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) are observed in the JET tokamak when energetic ions, in the range of several hundred keV, are created by high field side ion cyclotron resonance frequency heating. Fokker–Planck calculations demonstrate that the heating method enables the formation of an energetically inverted ion distribution which supplies the free energy for the ions to excite a mode related to the geodesic acoustic mode. The large frequency shifts of this mode are attributed to the formation of phase space structures whose frequencies, which are locked to an ion orbit bounce resonance frequency, are forced to continually shift so that energetic particle energy can be released to counterbalance the energy dissipation present in the background plasma.

Confinement degradation of energetic ions due to Alfvén eigenmodes (AEs) has been investigated with negative ion based neutral beam injection at ∼370 keV into JT-60U weak shear plasmas. AEs with a rapid frequency sweeping and then saturation as the minimum value of the safety factor decreases have been observed. This frequency behaviour can be explained by the reversed-shear induced AE (RSAE) and the transition from RSAEs to the toroidicity-induced AEs (TAEs). The associated change in the charge exchange neutral particle flux suggests energetic ion transport due to these AEs. The total neutron emission reduction rate in the presence of these AEs is evaluated by calculation using the orbit following Monte-Carlo code taking into account the change in bulk plasmas. The evaluation indicates confinement of energetic ions is degraded due to these AEs. In particular, it is found that the confinement degradation of energetic ions is maximum during the transition from RSAEs to TAEs and the maximum reduction rate (ΔSn/Sn)_MAX is estimated to be ∼45%.

The confinement of fast particles is of crucial importance for the success of future burning plasma experiments. On JET, the confinement of ion cyclotron resonant frequency (ICRF) accelerated fast hydrogen ions with energies exceeding 5 MeV has been measured using the characteristic γ-rays emitted through their inelastic scattering with carbon impurities, ¹²C(p,p'γ)¹²C. Recent experiments have shown a significant decrease in this γ-ray emission (by a factor of 2) during so-called tornado mode activity (core-localized toroidal Alfvén eigenmodes (TAEs) within the q = 1 surface) in sawtoothing plasmas. This is indicative of a significant loss or extensive re-distribution of these (>5 MeV) particles from the plasma core. In this paper, mechanisms responsible for the radial transport and loss of these fast ions are investigated and identified using the HAGIS code, which describes the interaction of the fast ions and the TAE observed. The calculations show that the overlap of wave-particle resonances in phase-space leads to an enhanced radial transport and loss. On both JET and ASDEX Upgrade, new fast ion loss detectors have been installed to further investigate the loss of such particles. On JET, fast ion loss detectors based around an array of Faraday cups and a scintillator probe have been installed as part of a suite of diagnostic enhancements. On ASDEX Upgrade, a new fast ion loss detector has been mounted on the mid-plane manipulator allowing high resolution measurements in pitch angle, energy and time. This has enabled the direct observation of fast ion losses during various magnetohydrodynamics (MHD) phenomena to be studied in detail. Edge localised mode (ELM) induced fast ion losses have been directly observed along with the enhancement of fast ion losses from specific areas of phase-space in the presence of neoclassical tearing modes (NTMs) and TAEs.

Clump and hole creations are observed with TAE bursts in energetic neutral spectra at low-magnetic field configurations of the LHD. Energy slowing down of the clump and the hole are also observed, experimentally. From the slowing down time analysis of the clump and/or hole, the location of each orbit is identified. The drift surface of each orbit has its maximum or second maximum close to the gap location of the TAE burst. The simultaneous observations of clump and hole creations in the energetic spectra reveal the enhanced radial transport of energetic particles by TAE bursts on the LHD.

The purpose of this work is to reveal the effects of the energetic particle mode (EPM) on fast-ion transport and consequent fast-ion loss in the compact helical system (CHS). For this purpose, fast particle diagnostics capable of following fast events originating from the EPM (f < 100 kHz) and from the toroidicity-induced Alfvén eigenmode (TAE) (f = 100–200 kHz) are employed in CHS. Experiments show that the EPM excited by co-circulating fast ions in an outward-shifted configuration is identified as a mode of m/n = 3/2 and can enhance fast-ion loss when its magnetic fluctuation amplitude exceeds ∼4 × 10⁻⁵ T at the magnetic probe position. The lost fast-ion probe (LIP) located at the outboard side of the torus indicates that bursting EPMs lead to periodically enhanced losses of co-going fast ions having smaller pitch angles in addition to losses of marginally co-passing fast ions. Coinciding with EPM bursts, the Hα light detector viewing the peripheral region at the outboard side also shows large pulsed increases similar to that of the LIP whereas the detector viewing the peripheral region at the inboard side does not. This is also evidence that fast ions are expelled to the outboard side due to the EPM. The charge-exchange neutral particle analyser indicates that only fast ions whose energy is close to the beam injection energy E_b are strongly affected by EPM, suggesting in turn that observed EPMs are excited by fast ions having energy close to E_b.

Super-thermal fast ions provide a source of free energy to excite instabilities, which in turn can enhance the loss of fast ions. It has been proposed that when multiple modes with resonances closely spaced in phase space reach sufficient amplitude so that the fast ion trajectories overlap, very rapid non-linear growth can occur. The modification of the fast ion distribution by this loss may in turn excite additional, otherwise stable, modes, leading to an 'avalanche' effect greatly enhancing the transport of fast ions (Berk H.L., Breizman B.N., Fitzpatrick J. and Wong H.V. 1995 Nucl. Fusion35 1661). It has been proposed that in ITER (ITER Physics Basis Editors et al1999 Nucl. Fusion39 2137), the transport of fast ions will be through a similar interaction of many modes. In NSTX (Ono M. et al2000 Nucl. Fusion40 557) bursts of multiple TAE-like instabilities are correlated with fast ion losses in a manner which qualitatively resembles avalanche behaviour.

The spectrum of compressional Alfvén eigenmodes (CAE) is analysed and shown to be discrete in tokamaks with low aspect ratio, such as the National Spherical Torus Experiment (NSTX), as well as in conventional tokamaks, such as DIII-D. The study is focused on recent similarity experiments on NSTX and DIII-D in which sub-cyclotron frequency instabilities of CAEs were observed at similar plasma conditions (W.W. Heidbrink et al2006 Nucl. Fusion46 324). The global ideal MHD code NOVA recovers the main properties of these modes predicted by theory and observed in both devices. The discrete spectrum of CAEs is characterized by three quantum mode numbers for each eigenmode, (M, S and n), where M, S and n are poloidal, radial and toroidal mode numbers, respectively. The expected mode frequency splitting corresponding to each of these mode numbers seems to be observed in experiments and is consistent with our numerical analysis. The polarization of the observed magnetic field oscillations in NSTX was measured and is also consistent with the numerical analysis, which helps to identify them as CAE activity. CAE mode structure was obtained and shown to be localized in both radial and poloidal directions with typical radial localization toward the plasma edge and poloidal localization at the low field side of the plasma cross section.

Neutral beam heated plasmas in spherical tokamaks exhibit a wide variety of Alfvén eigenmodes (AEs) and so provide important information on the physics of highly energetic ions and their associated instabilities, which is one of the most important issues for the fusion programme. Measurements of the internal AE mode amplitude provide input for simulations of potential ion and alpha particle losses due to energetic particle driven modes in ITER and other next step devices. New observations and results of the analysis of perturbative and non-perturbative modes on MAST and START spherical tokamaks are presented in the paper together with prospects for diagnostic applications. Comparison of the mode amplitude measured with Mirnov coils with that estimated from the frequency sweep speed of chirping modes shows good correlation for all EP driven modes analysed, both perturbative and non-perturbative.

Experiments using ion cyclotron current drive (ICCD) to control sawteeth are presented. In particular, discharges demonstrating shortening of fast ion induced long sawteeth reported in (Eriksson et al 2004 Phys. Rev. Lett.92 235004) by ICCD have been analysed in detail. Numerical simulations of the ICCD driven currents are shown to be consistent with the experimental observations. They support the hypothesis that an increase in the magnetic shear, due to the driven current, at the surface where the safety factor is unity was the critical factor for the shortening of the sawteeth. In view of the potential utility of ICCD, the mechanisms for the current drive have been further investigated experimentally. This includes the influence of the averaged energy of the resonating ions carrying the current and the spectrum of the launched waves. The results of these experiments are discussed in the light of theoretical considerations.

Active feedback control for regulation of the safety factor (q) profile at the start of the high stored energy phase of an advanced tokamak discharge has been demonstrated in the DIII-D tokamak. The time evolution of the on-axis or minimum value of q is controlled during and just following the period of ramp-up of the plasma current using electron heating to modify the rate of relaxation of the current profile. In L-mode and H-mode discharges, feedback control of q is effective with the appropriate choice of either off-axis electron cyclotron heating or neutral beam heating as the actuator. The q profile is calculated in real time from a complete equilibrium reconstruction fitted to external magnetic field and flux measurements and internal poloidal field measurements from the motional Stark effect diagnostic.

Hydrogen retention in graphite tiles exposed to hydrogen discharges at the JT-60 open divertor has been investigated by means of thermal desorption spectroscopy (TDS). Most of the plasma facing area was covered with re-deposited layers of maximum thickness of about 70 µm appearing at the inner divertor region. Major parts of retained hydrogen were thermally desorbed as hydrogen molecules with a peak temperature of around 970 K. Almost all the hydrogen atoms were retained homogeneously in the re-deposited layers with an averaged hydrogen concentration of ∼0.03 in H/C, which is much smaller than the saturated hydrogen concentration (H/C = 0.4–1.0). Since the saturated hydrogen concentration in carbon materials decreases with increasing temperature, the re-deposited carbon layers are very likely subjected to higher temperatures during the discharges, which are supported by the higher release temperature of hydrogen in TDS. This result suggests that hydrogen retention can be significantly reduced with higher wall temperatures.

Systematic and statistical studies have been conducted in order to develop an understanding of the parametric dependences of both the global and thermal energy confinement times at low aspect ratio in high power National Spherical Torus Experiment discharges. The global and thermal confinement times of both L- and H-mode discharges can exceed values given by H-mode scalings developed for conventional aspect ratio. Results of systematic scans in the H-mode indicate that the confinement times exhibit a nearly linear dependence on plasma current and a power degradation weaker than that observed at conventional aspect ratio. In addition, the dependence on the toroidal magnetic field is stronger than that seen in conventional aspect ratio tokamaks. This latter trend is also evident in statistical analyses of the available dataset. These statistical studies also indicate a weaker parametric dependence on plasma current than found in the systematic scans, due to correlations among the predictor variables. Regressions based on engineering variables, when transformed to dimensionless physics variables, indicate that the dependence of Bτ_E on β_t can range from being negative to null. Regressions based directly on the dimensionless physics variables are inexact because of large correlations among these variables. Scatter in the confinement data, at otherwise fixed operating parameters, is found to be due to variations in ELM activity, low frequency density fluctuations and plasma shaping.

A 3-D field line integration code, TRIP3D, has been modified to model stochastic magnetic perturbation produced by a resistive wall mode, error field (RWMEF) coil in the NSTX tokamak with very low aspect ratio. Multiple field lines with a uniform poloidal angle interval on each flux surface are automatically traced for the first time to follow the lines with large elongation plasmas. Each RWMEF coil can be configured to produce perturbation fields with dominant toroidal mode numbers of n = 1 or 3. In this study, it is found that the strongest stochastic layer is produced by the n = 3 configuration rather than n = 1 for the same coil current. Two NSTX divertor discharges, a lower single null and a double null have been modelled with different RWMEF-coil currents and toroidal modes. RWMEF currents of 2 kAt are sufficient to produce a strong stochastic field and significantly perturb the plasma boundary due to weak toroidal field in the spherical tokamak. The edge electron thermal diffusivity due to stochastic magnetic field is estimated to be 1 m² s⁻¹ with a 2 kAt current, which is comparable to that in DIII-D with an 8 kAt C-coil current. Currents of this magnitude, when used in the DIII-D I-coil configured for n = 3 perturbations suppress large edge localized modes (ELMs) and thus may have an impact on ELMs in NSTX. The result has been verified by the initial experiments.

In the context of the French Laser-Mégajoule (LMJ) fusion-research programme, the neutron and photon emissions of a high-gain direct-drive target are characterized. The neutron spectrum accounting for primary, secondary, and tertiary nuclear reactions is determined by means of the post-processor Diane of 1D calculations. Photon sources are considered over a wide range of energy from soft x-rays to gamma. Several energy windows of neutron and photon spectra are also identified in order to measure the target areal density.

Common research topics that are being studied in small, medium and large devices such as H-mode like or improved confinement, turbulence and transport are reported. These included modelling and diagnostic developments for edge and core, to characterize plasma density, temperature, electric potential, plasma flows, turbulence scale, etc. Innovative diagnostic methods were designed and implemented which could be used to develop experiments in small devices (in some cases not possible in large devices due to higher power deposition) to allow a better understanding of plasma edge and core properties.

Reports are given addressing research in linear devices that can be used to study particular plasma physics topics relevant for other magnetic confinement devices such as the radial transport and the modelling of self-organized plasma jets involved in spheromak-like plasma formation. Some aspects of the work presented are of interest to the astrophysics community since they are believed to shed light on the basis of the physics of stellar jets. On the dense magnetized plasmas (DMP) topic, the present status of research, operation of new devices, plasma dynamics modelling and diagnostic developments is reported. The main devices presented belong to the class of Z-pinches, mostly plasma foci, and several papers were presented under this topic. The physics of DMP is important both for the main-stream fusion investigations as well as for providing the basis for elaboration of new concepts. New high-current technology introduced in the DMP devices design and construction make these devices nowadays more reliably fitted to various applications and give the possibility to widen the energy range used by them in both directions—to the multi-MJ level facilities and down to miniature plasma focus devices with energy of just a few J.