E. Nelson-Melby

Pedestal profiles and fluctuations in C-Mod enhanced D-alpha H-modes

High resolution measurements on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the “Enhanced Dα” (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2–5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the L-H transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential, and Bpol, with f0∼50–150 kHz and kθ∼4 cm−1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include δ>0.35, q95>3.5, and L-mode target density ne>1.2×1020 m−3. A reduced q95 limit is found for hydrogen discharges.

Mode conversion electron heating in Alcator C-Mod: Theory and experiment

Localized electron heating [full width at half maximum of Δ(r/a)≈0.2] by mode converted ion Bernstein waves (IBW) has been observed in the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. These experiments were performed in D(3He) plasmas at high magnetic field (B0=7.9 T), high-plasma density (ne0⩾1.5×1020 m−3), and for 0.05⩽nHe-3/ne⩽0.30. Electron heating profiles of the mode converted IBW were measured using a break in slope analysis of the electron temperature versus time in the presence of rf (radio frequency) modulation. The peak position of electron heating was found to be well-correlated with 3He concentration, in agreement with the predictions of cold plasma theory. Recently, a toroidal full-wave ion cyclotron range of frequencies (ICRF) code TORIC [M. Brambilla, Nucl. Fusion 38, 1805 (1998)] was modified to include the effects of IBW electron Landau damping at (k⊥ρi)2≫1, This model was used in combination with a 1D (one-dimensional) integral wave equation code METS [...

Investigation of ion cyclotron range of frequencies mode conversion at the ion–ion hybrid layer in Alcator C-Mod

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 3He–H hybrid layer have been detected by means of phase contrast imaging in H(3He,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–3He). Recent mod...

Extension of Alcator C-Mod's ICRF experimental capability

A new 4-strap single-ended ICRF antenna has been added to the Alcator C-Mod tokamak. PPPL designed, fabricated, and tested the antenna up to 45 kV on an rf test stand. It is capable of symmetric phasing for ICRF heating studies, and asymmetric phasing with an improved directed wave spectrum for current drive. Two new 2 MW transmitters, tunable from 40–80 MHz, allow operation in plasma at 43, 60, and 78 MHz to match a variety of toroidal fields and plasma conditions. This addition increases the total available ICRF power to 4 MW at 80 MHz plus 4 MW at 40–80 MHz. Plasma heating and current drive experiments at the extended power levels and new frequencies are planned, and initial system performance will be discussed.

Analysis of ICRF-heated transport barrier experiments in Alcator C-Mod

An internal transport barrier (ITB) mode has been produced in the Alcator C-Mod tokamak via the application of 1–2 MW of off-axis ICRF heating power at 4.5 T and 80 MHz. This mode is characterized by strong peaking of the electron density profile with ne(0) increasing to 6–8×1020 m−3 while the plasma remains H-mode like outside the barrier foot (r/a≃0.5) with ne≃1.5−2.0×1020 m−3. Formation of the density barrier is typically accompanied by a decrease in the toroidal plasma rotation velocity from co-current to zero. Transport analysis of these discharges using the TRANSP code indicates the formation of a transport barrier in χeff(r) at r/a≲0.5. Heating and control of the transport barrier was successfully demonstrated by simultaneously injecting 0.6–1.0 MW of on-axis ICRF heating power at 70 MHz.

Electron power deposition measurements during ion cyclotron range of frequency heating on C-Mod

A 19-channel electron cyclotron emission (ECE) grating polychromator has been added to the existing ECE diagnostics on C-Mod, which include a 9-channel polychromator, heterodyne radiometer and Michelson interferometer. The new instrument can significantly improve the radial resolution of electron power deposition measurements in ICRF experiments on C-Mod. The improved resolution is important for resolving electron power deposition in off-axis mode conversion heating regimes where the mode conversion region can be narrow. The first data from this new instrument were acquired last year during 80 MHz hydrogen minority D-H mode conversion experiments where the H/(H+D) ratio was varied from 0.02 to 0.30 and the toroidal field was varied from 5.1 to 5.7 T. Although complicated by the presence of large sawteeth, some electron power deposition results were obtained from a break-in-slope method. These results, together with results from data acquired during the current C-Mod experimental campaign, will be presente...

Study of Ion Cyclotron Range of Frequencies Mode Conversion in the Alcator C‐Mod Tokamak

ICRF mode conversion (MC) in H(3He, D) and D(H) plasmas have been studied in detail in Alcator C‐Mod. In H(3He, D) plasma, the mode converted ion cyclotron wave (MC ICW) was observed in tokamak plasmas for the first time using a phase contrast imaging system. The MC ICW was observed in the low field side of the ion‐ion hybrid layer, and generally had a wavelength in‐between the fast wave and the MC ion Bernstein wave (IBW). Localized mode conversion electron heating (MCEH) has been clearly observed for the first time in D(H) plasmas with moderate hydrogen concentration in Alcator C‐Mod. Both on‐ and off‐axis (high field side) MCEH have been studied. The MCEH profile was obtained from a break in slope analysis of Te signals in the presence of rf shut‐off. The experimental profiles were qualitatively in agreement with the predictions of the two‐dimensional full‐wave poloidal mode code TORIC. The electron heating contributions from MC ICW and MC IBW are examined from the TORIC simulations.

Phase contrast imaging of ion Bernstein and fast waves in Alcator C-Mod

Direct observation of ICRF (∼80 MHz) waves (k⊥≃0.5 cm−1−10 cm−1) is now possible in Alcator C-Mod using an optical heterodyne technique on the Phase Contrast Imaging (PCI) system, which uses a CO2 laser to observe electron density fluctuations. The PCI observations are vertical chord averages, so the full-wave ICRF code TORIC [1] has been used to simulate the wave fields in these plasmas to aid in interpretation. Mode-converted ion Bernstein waves (IBW) in plasmas composed of H, D and 3He at 6 T have been observed at both high (1 MA) and low (400 kA) current. The fast magnetosonic wave launched from the low-field side has been observed in high density (∼5×1020 m−3) D(H) plasmas with off-axis ICRH at 4.5 T. Comparison between PCI measurements and code results are presented. The measured wave numbers are in good agreement with the local dispersion relations for both types of waves.

Upgrades to the 4-strap ICRF antenna in Alcator C-Mod

A 4-strap ICRF antenna suitable for plasma heating and current drive has been designed and fabricated for the Alcator C-Mod tokamak. Initial operation in plasma was limited by high metallic impurity injection resulting from front surface arcing between protection tiles and from current straps to Faraday shields. Antenna modifications were made in 2/2000, resulting in impurity reduction, but low heating efficiency was observed when the antenna was operated in its 4-strap rather than a 2-strap configuration. Further modifications were made in 7/2000, with the installation of BN plasma-facing tiles and radiofrequency bypassing of the antenna backplane edges and ends to reduce potential leakage coupling to plasma surface modes. Good heating efficiency was now observed in both heating configurations, but coupled power was limited to 2.5 MW in H-mode, 3 MW in L-mode, by plasma-wall interactions. Additional modifications were started in 2/2001 and will be completed by this meeting. All the above upgrades and the...

ICRF heating in Alcator C-Mod: Present status and future prospects

Alcator C-Mod, the high field, high density, diverted, compact tokamak in the world’s portfolio of high performance plasma fusion devices, is heated exclusively with ICRF auxiliary power. In this paper an overview of recent results is summarized, with particular attention given to the importance of RF operation and the flexibility afforded by different heating scenarios. Besides the routine minority heating operation, results in the mode conversion heating regime are also presented (mainly direct electron heating through mode converted ion Bernstein waves). Recent attempts at improving plasma performance by establishing internal transport barriers (ITBs) by various transient profile control techniques (the so-called Advanced Tokamak mode of operation) are also presented. Future improvements in performance afforded by the recent addition of a new 4-strap antenna and 4 MW of tunable (40–80 MHz) ICRF power are also discussed. Mode-conversion current drive (MCCD) and fast wave current drive (FWCD) will be amo...