Archived project

Alcator C-Mod research

Goal: Turbulence measurements (phase contrast imaging and reflectometry)
Lower hybrid control system

Date: 1 November 2002 - 31 July 2005

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Project log

J. Rice
added 2 research items
Experimental observations of driven and intrinsic rotation in tokamak plasmas are reviewed. For momentum sources, there is direct drive from neutral beam injection, lower hybrid and ion cyclotron range of frequencies waves (including mode conversion flow drive), as well as indirect j×B forces from fast ion and electron orbit shifts, and toroidal magnetic field ripple loss. Counteracting rotation drive are sinks, such as from neutral drag and toroidal viscosity. Many of these observations are in agreement with the predictions of neo-classical theory while others are not, and some cases of intrinsic rotation remain puzzling. In contrast to particle and heat fluxes which depend on the relevant diffusivity and convection, there is an additional term in the momentum flux, the residual stress, which can act as the momentum source for intrinsic rotation. This term is independent of the velocity or its gradient, and its divergence constitutes an intrinsic torque. The residual stress, which ultimately responds to the underlying turbulence, depends on the confinement regime and is a complicated function of collisionality, plasma shape, and profiles of density, temperature, pressure and current density. This leads to the rich intrinsic rotation phenomenology. Future areas of study include integration of these many effects, advancement of quantitative explanations for intrinsic rotation and development of strategies for velocity profile control.
X-ray spectra in the wavelength range from 2.70 to 2.76 Å from xenon (Z = 54) in near neon-like charge states have been observed in Alcator C-Mod tokamak plasmas. The 3D (2p⁶ − (2p⁵)3/23d5/2, 2720.4 mÅ) and 3F (2p⁶ − (2p⁵)1/23s1/2, 2729.0 mÅ) transitions from neon-like Xe⁴⁴⁺ have been identified, along with nearby Na-, Mg- and Al-like satellites. The intensity ratio of 3D to the Mg-like satellite near 2.74 Å increases strongly with electron temperature in the range from 3 to 4 keV.
Nils Basse
added 2 research items
The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced Dα (EDA) high (H) confinement mode in Alcator C-Mod [ I. H. Hutchinson, R. Boivin, F. Bombarda et al., Phys. Plasmas 1, 1511 (1994) ] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32 and the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L and EDA H mode and to analyze the quasicoherent (QC) mode associated with EDA H mode. Changes in broadband turbulence at the transition from L to EDA H mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of Dα light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The backtransition from EDA H to L mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC mode at medium density: 2.2×1020 m−3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.
Current profile evolution will be controlled and sustained in the Alcator C-Mod advanced tokamak lower hybrid current drive experiment by use of 3 MW of 4.6 GHz lower hybrid current drive (LHCD) now being installed and tested. LHCD and an existing 5 MW ICRH capability are to be used to develop regimes with high confinement, high β<sub>n</sub> and high bootstrap fraction and extend them to quasi-steady-state conditions. This paper will describe the design, installation and testing of the low power microwave active control system used in the experiment. The LHCD low power microwave active control system uses vector modulators to provide a phase and amplitude controlled driver for each of twelve 4.6 GHz, 250 kW klystrons. Phase and power output of each klystron are monitored by an I-Q detector and the resulting signals are used in digital controllers for closed-loop control of the klystron phase and amplitude to preset values.
Nils Basse
added a research item
Internal transport barriers (ITBs) marked by steep density and pressure profiles and reduction of core transport are obtained in Alcator C-Mod. Transient single barriers are observed at the back-transition from H- to L-mode and also when pellet injection is accompanied by ion cyclotron resonance frequency (ICRF) power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: The ions and electrons are tightly coupled by collisions with Ti/Te = 1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atm have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier region, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long-wavelength drift wave turbulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron modes (TEMs) inside the barrier. The TEM ultimately drives sufficient outgoing particle flux to balance the inward pinch and halt further density rise, which allows control of particle and impurity peaking.
Nils Basse
added 21 research items
Internal transport barriers (ITBs) marked by steep density and pressure profiles and reduction of core trans- port are obtained in Alcator C-Mod. Transient single barriers are observed at the back-transition from H- to L-mode and also when pellet injection is accompanied by ion cyclotron resonance frequency (ICRF) power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: The ions and electrons are tightly coupled by collisions with Ti/Te 1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atm have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier re- gion, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long-wavelength drift wave tur- bulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron modes (TEMs) inside the barrier. The TEM ul- timately drives sufficient outgoing particle flux to bal- ancetheinwardpinchandhaltfurtherdensityrise,which allows control of particle and impurity peaking.
Mode conversion MC of long wavelength fast electromagnetic magnetosonic waves fast wave, or FW into shorter wavelength electrostatic ion-Bernstein, or IBW or slow electromagnetic ion cyclotron, or ICW waves is of great interest in laboratory, magnetic fusion and space physics experiments. Such processes are particularly important in multi-ion species plasmas. In this paper we report recent results from high power ion cyclotron range of frequencies ICRF heating experiments in the Alcator C-Mod tokamak. Mode converted waves near the 3 He–H hybrid layer have been detected by means of phase contrast imaging in H(3 He,D) plasmas E. Nelson-Melby et al., Phys. Rev. Lett. 90, 155004 2003. The measured wave k spectrum and spatial location are in agreement with theoretical predictions F. W. Perkins, Nucl. Fusion 17, 1197 1977, which showed that in a sheared magnetic field, mode-conversion of FW into ICW may dominate over IBW for appropriate ion species i.e., D–T, or equivalently, H– 3 He). Recent modeling with full wave codes, as well as solving the hot plasma dispersion equation in the presence of sheared magnetic fields, verifies the interpretation of such a mode conversion process. Thus, the geometry of the magnetic field, as well as the particular ion species mix, influences the physics of ICRF mode conversion. In this paper, we also report recent results on the study of mode conversion electron heating MCEH in DH plasmas Y. Lin et al., Plasmas Phys. Controlled Fusion 45, 1013 2003. By comparing the experimentally measured MCEH profile with modeling, the study shows that the MC ICW may make a significant contribution to the direct electron heating when the D–H hybrid layer is off axis on the high field side. Preliminary results of mode conversion poloidal plasma flow drive experiments in D(3 He) are also reported. © 2004 American Institute of Physics.
The time evolution of toroidal rotation velocity profiles has been measured in Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas using a tangentially viewing x-ray spectrometer array. The strong co-current toroidal rotation in enhanced Dα (EDA) high confinement mode (H-mode) plasmas is observed to propagate in from the edge on a time scale similar to the energy confinement time. The ensuing steady state rotation velocity profiles in both Ohmic and ion cyclotron range of frequencies (ICRF) heated EDA H modes, which are generated in the absence of any external momentum input, are found to be relatively flat. These profiles may be simulated by a simple diffusion model with the boundary condition of an edge rotation, which appears during the H-mode period. The observed profiles are well matched by the simulations using a momentum diffusivity of ∼ 0.1 m2/s, which is much larger than the calculated neo-classical value, and the momentum transport may be regarded as anomalous. The Alcator C-Mod rotation observations have been compared in detail with the calculations of neo-classical and sub-neo-classical theory, to the predictions from modeling of ICRF wave induced energetic ion orbit shifts, and to estimates from turbulence driven mechanisms. The magnitude and scalings of the observed rotation results are in accord with neo-classical and sub-neo-classical calculations, but the measured momentum diffusivity is higher than the predictions by a large factor. The prediction of rotation reversal with a high magnetic field side resonance location for ICRF wave induced ion orbit shifts has not been observed in the experiments. While the turbulence driven rotation calculations are mostly qualitative, they represent some of the observed features. © 2004 American Institute of Physics.
Nils Basse
added 10 research items
Current ramp experiments with intense ICRF power injected early in the ramp phase in Alcator C-Mod have been carried out. The goal of these experiments is to produce suitable reversed shear (RS) target plasmas for future Advanced Tokamak (AT) plasma research. Future plans call for off-axis injection of Lower Hybrid current drive (LHCD) to maintain the RS plasmas while increasing beta with additional ICRH. In the present experiments evidence of RS q-profiles has been demonstrated in the ramp stage of the discharge with the observation of Alfven-Cascades (or Reversed Shear Alfven-Eigenmodes or RSAE) driven by the energetic ICRF ion tail. The frequencies are in agreement with MHD code predictions. Evidence of sawtooth delay has also been observed by increasing the injected ICRF power.
Summary form only given. Phase contrast imaging diagnostic (PCI) is an internal reference beam interferometric technique which has been used successfully in high temperature tokamak plasma experiments to image line integrated plasma density fluctuations. The PCI technique utilizes a 18 deep grooved "phase plate" which is inserted into an expanded beam path, and with the aid of a detector array one is able to measure wavelengths and correlation lengths of fluctuations propagating perpendicular to the laser beam. In the Alcator C-Mod and DIII-D tokamak PCI experiments, a CO2 laser beam is used to probe low frequency (f = 1 MHz) instabilities and in addition, in C-Mod high power launched ICRF waves (80 MHz) are also monitored. The fluctuations studied in the past in Alcator C-Mod include the so-called "quasi-coherent mode" (a ballooning mode localized to the edge pedestal), semi-coherent TAE-like modes, including Alfven wave cascades, low frequency turbulence, and high power launched ICRF waves. The ICRF waves are detected by a heterodyne technique using optical modulation of the laser beam. The ICRF wave propagation studies have confirmed mode conversion into kinetic ion cyclotron waves (the shear wave branch) and electrostatic ion Bernstein waves. In DIII-D, PCI has been used to study low frequency turbulence during L to H mode transition, ELMs, and coherent edge modes during the quiescent H-mode. Signatures of zonal flows have also been observed in past experiments. While most of the past studies were limited to wavelengths equal or longer than the ion gyro-radius (ki = 1, f = 1 MHz ), new upgrades to the electronics and optics will allow detection of wavelengths and frequencies in the electron gyro-radius regimes (ke = 1, f = 20 MHz). This new capability will allow us to study the electron temperature gradient modes and the trapped electron mode, both being candidates for determining electron transport in magnetically confined plasm- as. While spatial localization of long wavelength modes along the PCI laser beam is usually not possible, in the short wavelength regimes in a sheared magnetic field localization can be achieved by using a rotating masking plate in conjunction with the phase plate
An antenna‐transmitter system for driving current in the LHRF has been installed in Alcator C‐Mod. The antenna is a grill consisting of 4 poloidal rows of waveguides, each with 24 guides in the toroidal direction. Power is supplied by 12 klystrons capable of 250 kW operation at a frequency of 4.6 GHz. Thus the total source power is 3 MW, with about 1.5 MW available to be coupled to the plasma. Power supply and heat throughput limits in C‐Mod limit the pulse length to 5 s, which however represents several current redistribution times. With 90° phasing, the n∥ spectrum is sharply peaked at 2.3 and the range 1.5 < n∥ < 3.5 can be accessed dynamically by varying the phase of the klystrons. The system is in the commissioning phase with klystron power limited to ∼20 kW and pulse length to 10 ms. Early results from plasma operation are discussed. © 2005 American Institute of Physics
Nils Basse
added 2 research items
Phase contrast imaging (PCI) is an internal reference beam interferometry technique which provides a direct image of line integrated plasma density fluctuations. The method has been used with great success to measure waves and turbulence in magnetically confined high temperature plasmas. The principle of PCI was developed in optics in the 1930s by the Dutch physicist Zernike, leading to the development of phase-contrast microscopy. The technique allows one to detect the variation of the index of refraction of a dielectric medium (such as a plasma) due to the presence of waves or turbulent fluctuations. The image produced by the introduction of a phase plate in the beam path, and subsequently imaging the expanded laser beam onto a detector array can be used to calculate wavelengths and correlation lengths of fluctuations in high temperature plasmas. In this paper, the principle of PCI is summarized and examples of measurements from the DIII-D and Alcator C-Mod tokamak plasmas are given.
Nonlinear gyrokinetic simulations of Trapped Electron Mode (TEM) turbulence have repro-duced measured particle fluxes and thermal energy fluxes, within experimental uncertainty, in Alcator C-Mod [1, 2]. This has provided a model for internal transport barrier control with on-axis ICRH in Alcator C-Mod, without adjustable model parameters. The onset of TEM turbulent transport limits the density gradient, preventing radiative collapse. Here we move beyond comparisons of simulated and measured fluxes to a more fundamental and direct comparison with density fluctuation spectra. Using a new synthetic diagnostic, excellent agreement is obtained between wavelength spectra from nonlinear GS2 simulations, and spectra measured by Phase Contrast Imaging. The density fluctuations are associ-ated with the steep density gradient in the C-Mod ITB, which provides spatial localization for the chordal PCI measurement. Gyrokinetic stability analysis shows that Trapped Electron Modes are strongly desta-bilized inside the ITB foot by the addition of on-axis ICRH. Nonlinear GS2 simulations reproduce the relative increase in fluctuation level when on-axis heating is applied. Further, we have extended the GS2 Lorentz collision operator to include classical diffusion associated with the ion finite Larmor radius, and have implemented collisional energy diffusion, together with particle, momentum, and energy conserva-tion terms. Classical diffusion is shown to strongly stabilize trapped electron modes with k θ ρ i > 2 for realistic C-Mod collisionalities. A series of detailed nonlinear gyrokinetic simulations show the nonlin-ear upshift [1, 2] in the TEM critical density gradient increases favorably with collisionality.
Nils Basse
added 2 research items
\Alcator C-MOD has compared plasma performance with plasma-facing components (PFCs) coated with boron to all-metal PFCs to assess projections of energy confinement from current experiments to next-generation burning tokamak plasmas. Low-Z coatings reduce metallic impurity influx and diminish radiative losses leading to higher H-mode pedestal pressure that improves global energy confinement through profile stiffness. RF sheath rectification along flux tubes that intersect the RF antenna is found to be a major cause of localized boron erosion and impurity generation. Initial lower hybrid current drive (LHCD) experiments (P-LH < 900kW) in preparation for future advanced-tokamak studies have demonstrated fully non-inductive current drive at I-p similar to 1.0 MA with good efficiency, I-drive = 0.4P(LH)/n(eo)R (MA, MW, 10(20) m(-3),m). The potential to mitigate disruptions in ITER through massive gas-jet impurity puffing has been extended to significantly higher plasma pressures and shorter disruption times. The fraction of total plasma energy radiated increases with the Z of the impurity gas, reaching 90% for krypton. A positive major-radius scaling of the error field threshold for locked modes (B-th/B alpha R0.68 +/- 0.19) is inferred from its measured variation with B-T that implies a favourable threshold value for ITER. A phase contrast imaging diagnostic has been used to study the structure of Alfven cascades and turbulent density fluctuations in plasmas with an internal transport barrier. Understanding the mechanisms responsible for regulating the H-mode pedestal height is also crucial for projecting performance in ITER. Modelling of H-mode edge fuelling indicates high self-screening to neutrals in the pedestal and scrape-off layer (SOL), and reproduces experimental density pedestal response to changes in neutral source, including a weak variation of pedestal height and constant width. Pressure gradients in the near SOL of Ohmic L-mode plasmas are observed to scale consistently as I-p(2), and show a significant dependence on X-point topology. Fast camera images of intermittent turbulent structures at the plasma edge show they travel coherently through the SOL with a broad radial velocity distribution having a peak at about 1% of the ion sound speed, in qualitative agreement with theoretical models. Fast D,, diagnostics during gas puff imaging show a complex behaviour of discrete ELMs, starting with an n approximate to 10 precursor oscillation followed by a rapid primary ejection as the pedestal crashes and then multiple, slower secondary ejections.
Energetic particle physics is studied in Alcator C-Mod in reactor relevant regimes with high density and equilibrated electron and ion temperatures. Stable Alfven eigenmodes are excited with low-power active magnetohydrodynamic antennas in the absence of a significant energetic particle tail to directly measure the damping rate of the modes. Stable toroidal Alfven eigenmode (TAE) damping rates between 0.5% < gamma/omega < 4.5% have been observed in diverted and limited plasmas. Alfven eigenmodes are destabilized with high-power hydrogen minority ion cyclotron radio frequency (ICRF) heating (P-ICRF < 6 MW) in lower-density plasmas in the current rise and in relatively high-density (<(n)over bar>(e) < 2.5 X 10(20) m(-3)) H-mode plasmas, which creates an energetic hydrogen ion tail with calculated energies up to 400 keV. Low toroidal mode number (n < 4) unstable modes are observed in the current rise with magnetic pickup coils at the wall and phase contrast imaging density fluctuation measurements in the core. Observations of energetic particle modes or TAEs that decrease in frequency and mode number with time up to a large sawtooth collapse indicate that fast particles play a role in stabilizing sawteeth. Alfven eigenmodes can also be used as diagnostics to precisely constrain the q profile and provide a qualitative measure of the fast particle distribution time evolution.
Nils Basse
added 4 research items
The fast magnetosonic wave, mode converted ion cyclotron wave (MC ICW) and mode converted ion Bernstein wave (MC IBW) have all been observed and unambiguously identified in the mode conversion region of Alcator C-Mod. The influences of the species mix, mode conversion location and B pol /B tot have been studied in D(3 He) plasmas at B 0 ∼ 5.4 T (f RF = 50 MHz) and B 0 ∼ 8 T (f RF = 78 MHz). The RF waves were measured by a phase contrast imaging (PCI) system. The experimental observation is compared with the result from a synthetic PCI diagnostic based upon the full wave code TORIC. Good agreement between the observation and modelling has been obtained on the spatial structure of the RF waves. When the mode conversion layer was off axis, both MC ICW and MC IBW were observed. In 5.4 T near-axis mode conversion discharges, the double hump spatial structure of the MC waves was observed experimentally and reproduced by the synthetic PCI. Such a structure is an indication of the up–down asymmetry of the MC ICW. In 8 T near-axis mode conversion discharges, we had the first definitive observation of IBW dominated MC in Alcator C-Mod.
Global and local transport experiments in ohmic, L-mode and H-mode regimes on the Alcator C-Mod tokamak are summarized. For ohmic plasmas, earlier results derived for energy confinement scaling in the Alcator (linear) regime have been confirmed, and the saturated confinement regime has been shown to be equivalent to that of L-mode. For auxiliary heated regimes, C-Mod provided a unique laboratory to test the standard scaling laws that had been previously derived. C-Mod's L-mode performance matches the L-mode scaling laws quite well, but the confinement times in H-mode were about 50% above the existing H-mode scaling laws. This difference was significant and pointed up shortcomings in the range and conditioning of the existing database. H-mode studies emphasize quasi-steady regimes with good energy confinement, no impurity accumulation, and no large edge-localized modes. A new H-mode regime, where the pedestal is regulated by a continuous quasi-coherent mode, has been investigated extensively. The regime is most accessible at higher safety factor, triangularity, and collisionality and at low ion mass, suggesting that the mode is a form of resistive ballooning. Studies on C-Mod first showed the quantitative link between edge temperatures, core temperature gradients, and core confinement. This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ion temperature gradient theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle, and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U, and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard L- and H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro size (1/ρ*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality.
Nils Basse
added 3 research items
An amplitude modulated (AM) reflectometer system operating in O-mode has been used for density profile and fluctuation measurements on Alcator C-Mod. This system consists of five channels, whose frequencies correspond to densities from 0.31 × 10^20 m-3 to 1.5 × 10^20 m-3. The 88 GHz channel has separate upper and lower sideband measurements of the AM waves, resulting in an increased sensitivity to fluctuations. Recently, two additional dedicated fluctuation channels have been brought into operation at 132 and 140 GHz, corresponding to densities of 2.2 × 10^20 m-3 and 2.4 × 10^20 m-3. The new channels allow observations to be made further into the pedestal region and in some cases reach the foot of the internal transport barrier. We will present spectral analysis results from selected channels during confinement transitions in Alcator C-Mod plasmas, e.g. at the L- to H-mode bifurcation. Further, correlation studies will be undertaken between the various channels to elucidate the possible existence of moving and/or overlapping turbulent structures.
A lower hybrid current drive (LHCD) system is being installed on the Alcator C-Mod tokamak. Initially, 12 klystrons operating at 4.6 GHz will deliver a total power of 3 MW to the coupler. The LHCD system will make it possible to modify the current density profile in the outer half of the plasma. This implies that the q-profile can be manipulated, thereby enabling the study of advanced tokamak regimes. In this paper we describe the overall structure of the control and data acquisition system for LHCD. The acquisition setup collects data from the active controller, the transmitter protection and the coupler protection systems. Long pulse tests of the klystrons are presented and a monitor camera is introduced.
At certain values of the edge rotational transform, ι a = 1/q a , the confinement time of plasmas in the Wendelstein 7-AS (W7-AS) stellarator was found to be very sensitive to small modifications of ι a . Since ι a could be changed reproducibly by e.g. a small plasma current, these transitions provided a means to perform systematic investigations of differences in turbulence during 'good' (q a = 2.91) and 'bad' (q a = 2.76) confinement phases [1]. The macroscopic changes of confinement in W7-AS were attributed to the presence of internal transport barriers (ITBs) close to low-order rational ι -surfaces in the plasma and the fact that W7-AS had small magnetic shear [2]. Related empirical and theoretical models on thermal electron transport around low-order rational surfaces can be found in e.g. Refs. [3]. Due to larger shear in the Alcator C-Mod tokamak, confinement transitions associated with low-order rational q-values would therefore be expected to be local instead of global, i.e. a localized temperature flattening. To study the effect of the presence or absence of low-order rational surfaces in C-Mod, we designed discharges where the current was ramped slowly up to lower q a in a controlled fashion. If an ITB is associated with the q = 3 surface, this barrier would be removed from the plasma resulting in a (local) worsening of confinement. Fig. 1 (a) shows flux surfaces of the discharge we analyse in this paper. The L-mode, inner wall limited shot shown had low elongation κ ∼ 1.3, low upper and lower triangularities δ U ∼ δ L ∼ 0.2 and a toroidal magnetic field B φ = 5.5 T. It was heated on-axis by 1.2 MW of ion cyclotron radio frequency power. The bottom trace in Fig. 1 (b) shows the reduction of q a in response to a slow current ramp-up from 1.1 to 1.3 MA. Since global confinement in C-Mod is seen not to be affected by the current ramp, we name the time intervals shown in Fig. 1 (b) 'higher q a ' (HQA) and 'lower q a ' (LQA).
Nils Basse
added a research item
We study internal transport barriers (ITBs) using upgraded fluctuation diagnostics: Phase-contrast imaging (PCI) has improved spatial coverage and fast data acquisition while reflectometry has two high frequency fluctuation channels. Usually, ITBs are created using off-axis ion cyclotron radio frequency (ICRF) heating. The enhanced D-alpha high confinement mode, characterized by a quasi-coherent (QC) mode at the plasma edge, is a prerequisite for ITB formation. However, it has been reported that the QC-mode weakens after the ITB forms. Further, a core mode with a frequency close to that of the QC-mode has been detected by electron cyclotron emission measurements. The impurity accumulation and the continual rise of density during a standard ITB discharge can be halted by the application of additional on-axis ICRF heating. The position of an ITB foot can be varied by changing the total plasma current, indicating that ITBs are influenced by magnetic topology.
Nils Basse
added a project goal
Turbulence measurements (phase contrast imaging and reflectometry)
Lower hybrid control system