Table of contents

There is considerable interest in the development of low voltage startup scenarios for large tokamaks since it is proposed that in ITER the electric field which will be applied for ionization and plasma current ramp-up will be limited to values of E ≤ 0.3 V/m. Studies of low voltage startup have been carried out in DIII-D with and without electron cyclotron preionization and preheating. Successful Ohmic startup has been achieved with E ∼ 0.25 V/m by paying careful attention to error fields and prefill pressure, while electron cyclotron heating (ECH) assisted startup with E ∼ 0.15 V/m has been demonstrated. ECH assisted startup gives improved reliability at such low electric fields and permits operation over an extended range of prefill pressures and error magnetic fields. Using ECH, startup at E = 0.3 V/m with |B_⊥| > 50 G over most of the vessel cross-section has been demonstrated. Such an error field represents an increase by more than a factor of two over the highest value for which Ohmic startup was achieved at the same electric field. During low voltage Ohmic startup with extreme values of prefill pressure and/or error magnetic fields, excessive breakdown delays are observed. The experimental data agree well with theoretical predictions based on the Townsend avalanche theory. ECH assisted startup is always prompt. The primary effect of ECH during the plasma current ramp-up is a decrease of the resistive component of the loop voltage V_rcs. A significant reduction (∼30%) in V_res is achieved for low ECH powers (P_RF ∼ 300-400 kW), but a further large increase in P_RF results in only a modest additional decrease in V_res. ECH was not applied over the whole ramp-up phase in these experiments and produced a reduction in volt-second consumption up to the current flat-top (I_p ∼ 1 MA) of ⪅ 10%. These experiments confirm that the low electric fields specified in the ITER design are acceptable and demonstrate the substantial benefits which accrue from the use of ECH assisted startup.

The scaling of the non-linear reconnection process associated with the m = 1 internal kink instability is studied in cylindrical geometry, using a three-dimensional numerical code with a full set of resistive MHD equations. In the presence of an ideal unstable kink mode, the non-linear evolution of the instability shows a transition from a purely resistive kink mode for high resistivity to Kadomtsev reconnection driven by the ideal kink mode for low resistivity. For the case of low resistivity, the assumptions of the Kadomtsev reconnection model have been checked, and the results confirm Kadomtsev's estimations of a scaling law of η^1/2. A model is proposed to understand the transition and to compare the studies with previous numerical results obtained for different plasma parameters.

Impurity ion dynamics are incorporated into a numerical stability analysis of electrostatic drift waves in a sheared slab model. Using a simple iδ model for the electron branch, it is shown that a significant increase in growth rate, proportional to the primary ion mass at moderate Z_eff, results for T_e ≫ T_i. For the ion branch, the ion temperature gradient (ITG) mode, it is found that in hydrogen and deuterium plasmas the growth rates are inversely proportional to the primary ion mass when Z_eff < 2; for Z_eff ≥ 2, the ITG mode is generally stable. Using quasilinear theory, it is also shown that for ITG mode turbulence in hydrogen and deuterium plasmas in the low Z_eff range (typical of high density Ohmic plasmas) the energy confinement time scaling with atomic mass number A of the primary ion species is τ_E ∝ A^β, with β ≃ 0.6. Conversely, the present analysis indicates that confinement in helium plasmas is similar to that obtained for hydrogen. It is suggested that a similar result obtains for electron drift wave turbulence in low density Ohmic plasmas where T_c ≫ T_i when Z_eff ≫ 1.

A new type of non-resonant or near-resonant resistive pressure driven mode localized near the magnetic axis is found in heliotron and torsatron configurations. This mode has a ballooning-like structure and becomes unstable in highly resistive plasmas (magnetic Reynolds number S ≲ 10⁵) when the pressure profile is sufficiently peaked in the almost shear free region around the magnetic axis. As S increases, the radial mode structure becomes narrower and the peak position approaches the magnetic axis. The numerically obtained growth rates are proportional to S^−0.33. When S ≳ 10⁵, a transition occurs to an unstable resistive interchange mode localized at a resonant surface. A non-resonant resistive mode with properties similar to those of the above mentioned mode is also observed in straight cylindrical configurations. The dispersion relation for these configurations is derived analytically in the electrostatic approximation, which gives the growth rate proportional to S^−1/3. The non-resonant resistive mode may suppress highly peaked pressure profiles when the resistive interchange mode is marginal.

The energy distribution of neutral particles is detected from spheromak plasmas by a time of flight neutral atom spectrometer. These particles originate in charge exchange interactions within the bulk plasma and carry the temperature of the majority ion component. This is the first direct measurement of temperatures of the main ion species in spheromaks. The data are found to be in the range of 500 eV during the sustainment and late decay phases and are in agreement with impurity Doppler temperatures for species expected to be localized to the plasma centre.

The transport of pellet fuelled plasmas in JT-60 has been investigated with a predictive tokamak transport code. Peaked density profiles have been observed in pellet fuelled plasmas during the sawtooth free phase. Simulation analysis shows that an inward pinch of ∼0.2 m/s at the plasma half-radius and a reduced particle diffusion coefficient of 0.1 m²/s in the central region are necessary to explain the peaked density profile. These particle transport properties yield improved energy confinement in a plasma with a strong particle source in the central region under sawtooth free conditions, i.e. in a plasma fuelled by pellet injection. The dependence of the improved stored energy on the plasma current can be explained by reduced particle diffusion within the q = 1 surface.

The paper presents a study of the effect of charge exchange and elastic scattering collisions between ions and neutrals on the plasma flow along the divergent magnetic field lines in the divertor region of a stellarator. In the analysis, which is kinetic, an indirect solution to the kinetic equation for the ions is obtained using a Monte Carlo description for the ion dynamics together with the assumption of Boltzmann electrons and quasi-neutrality. Ion-neutral collisions are studied in a three-dimensional velocity space using a Monte Carlo algorithm. The results show that charge exchange and elastic scattering collisions play a significant role in reducing the drop of the electrostatic potential in the divertor chamber below that obtained from collisionless calculations. This, in turn, reduces the electrostatic barrier, which may be useful in retarding the flow of high Z impurity ions from the target plate towards the confined plasma. The calculations for the ion energy distribution suggest that, owing to ion-neutral collisions, most of the impact energy of ions on the divertor plate is gained while they cross the sheath region of the plate.

A subset of the ITER L-mode and H-mode confinement data is tested against the constraints imposed by various theoretical models for thermal plasma transport. Matrix algebra is used to facilitate such tests. A new result obtained is that the fundamental constraint imposed by the high-β collisional Fokker-Planck equation (Kadomtsev) is satisfied by the data. An additional constraint on the characteristic scale length associated with thermal diffusion can also be satisfied by the data. Dimensionally correct empirical scaling laws embodying theoretical constraints can thus be derived.

Spatially resolved VUV emissions from low-Z intrinsic impurities have been used to measure the effects of an ergodic magnetic limiter (EML) on the edge region of the TEXT plasma. Using previously established techniques for determining the electron density ne and the electron temperature T_e from line pair ratios within the beryllium-like O V ion, poloidal modulations in n_e, and T_e are found corresponding to EML induced magnetic islands. In general, increases of 50% in both n_e and T_e are found near the poloidal locations of X-points and decreases of the same order are found near O-points. Modulations and radial profile broadening of intrinsic carbon and oxygen ions are also found, indicating enhancement of edge impurity transport during EML perturbation.

The paper presents a study of the effects of a simulated L- to H-mode transition for the proposed BPX (Burning Plasma Experiment) device on the coupling of a recessed coil antenna in the ion cyclotron range of frequencies. Two computer codes are used: ANTIMP, a code developed by the authors, which uses a Runge-Kutta method of solution, and a finite element code, developed by Brambilla, which has been modified to examine fourfeed antenna coils. The radiation resistance is calculated for antenna parameters consistent with a design proposed by the Oak Ridge National Laboratory. Both finite cavity size and finite phase velocity along the current straps are modelled. The plasma density is simulated using both piecewise linear profiles and parabolic profiles. For anticipated BPX values of the density gradient at the separatrix, the radiation resistance decreases by a factor of two to three during an L- to H-mode transition. The reduction of the value of the radiation resistance due to finite phase velocity can be minimized by using a four-feeder scheme.

An analytical investigation of the effect of radial diffusion on the burnup of fusion produced 1 MeV tritons in hot deuterium plasmas is presented. A simple and useful expression is obtained for the anomalous burnup ratio, defined as the burnup fraction with radial diffusion effects to that without radial diffusion effects. When applied to recent experimental results from JET and TFTR showing anomalously reduced burnup, a diffusion constant of 0.1-0.3 m²/s is inferred for the high energy tritons.

The free-boundary toroidal kink mode is compared with the internal kink mode for a finite beta, circular crosssection toroidal equilibrium. The toroidal kink is more unstable than the ideal internal kink in the sense that the unstable parameter range of the toroidal kink is more extensive. Moreover, when both modes are unstable, the growth rate of the toroidal kink is generally several times larger than that of the ideal internal kink.

Microwave transmission measurements in TORE SUPRA exhibit low frequency oscillations of the transmitted power, associated with a saturated m = 1, n = 1 mode, as observed by soft X-ray diagnostics. It is shown that these oscillations are related with refraction effects and specifically with modulations of the electron density profile due to a rotating magnetic island. An analytical solution of the ray equations modified by a rotating density perturbation is obtained which explains the frequency spectrum of the oscillations.

The radial sheath structure in ignited plasmas is studied with regard to self-consistent effects of the guiding centre orbits of alpha particles and E × B induced toroidal rotation. The distribution of slowing down alpha particles determines the structure of the radial sheath through finite Larmor radius effects.

The role of an enhanced electron tail on the emission and absorption of synchrotron radiation in a tokamak reactor is investigated. It is shown that the radiofrequency power loss at ω ≫ ω_c in a Maxwellian plasma is radiated by electrons with velocities significantly higher than the thermal velocity. Therefore, the synchrotron radiation loss will be enhanced when the tail distribution develops a superthermal feature. The enhanced loss with respect to Trubnikov's result has been computed for a homogeneous plasma.

This report is intended as a review for readers who are not necessarily specialists in the particular plasma diagnostic technique of LIDAR Thomson scattering discussed at the meeting.

The workshop on problems of helium transport and exhaust in tokamaks was held under the auspices of the International Energy Agency and was sponsored by the Oak Ridge National Laboratory (ORNL) and the Forschungszentrum Jülich (KFA). Representatives from major tokamak laboratories attended, and 24 talks were given on the experimental status of He transport and exhaust investigations, on modelling studies and reactor issues, and on plans for future studies.

The 18th European Conference on Controlled Fusion and Plasma Physics was held in the former East Berlin, within walking distance of many historical sites such as the Unter den Linden and the Brandenburg Gate. The joy and the pain of German reunification were manifest and highlighted in the welcoming speech on behalf of the governing body of the united city. Few speakers could resist the temptation to refer to the fusion of the two Germanies; hopefully, the resultant energy release will be controlled and peaceful.