F. Bombarda

First results from Alcator-C-MOD

Early operation of the Alcator‐C‐MOD tokamak [I.H. Hutchinson, Proceedings of IEEE 13th Symposium on Fusion Engineering, Knoxville, TN, edited by M. Lubell, M. Nestor, and S. Vaughan (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] is surveyed. Reliable operation, with plasma current up to 1 MA, has been obtained, despite the massive conducting superstructure and the associated error fields. However, vertical disruptions are not slowed by the long vessel time constant. With pellet fueling, peak densities up to 9×1020 m−3 have been attained and ‘‘snakes’’ are often seen. Initial characterization of divertor and scrape‐off layer is presented and indicates approximately Bohm diffusion. The edge plasma shows a wealth of marfe‐like phenomena, including a transition to detachment from the divertor plates with accompanying radiative divertor regions. Energy confinement generally appears to exceed the expectations of neo‐Alcator scaling. A transition to Ohmic H mode has been observed. Ion cyclotron heating experiments have demonstrated good power coupling, in agreement with theory.

H mode confinement in Alcator C-Mod

A series of experiments, examining the confinement properties of ion cyclotron range of frequencies (ICRF) heated H mode plasmas, has been carried out on the Alcator C-Mod tokamak. Alcator C-Mod is a compact tokamak that operates at high particle, power and current densities at toroidal fields up to 8 T. Under these conditions the plasma is essentially thermal with very little contribution to the stored energy from energetic ions (typically no more than 5%) and with Ti~Te. Most of the data were taken with the machine in a single null `closed divertor configuration with the plasma facing components clad in molybdenum tiles. The data include those taken both before and after the first wall surfaces were coated with boron, with emphasis on the latter. H modes obtained from plasmas run on boronized walls typically had a lower impurity content and radiated power and attained a higher stored energy than those run on bare molybdenum. Confinement enhancement, the energy confinement time normalized to L mode scaling, for discharges with boronized walls, ranged from 1.6 to 2.4. The unique operating regime of the Alcator C-Mod device provided a means for extending the tests of global scaling laws to parameter ranges not previously accessible. For example, the Alcator C-Mod edge localized mode (ELM)-free data were found to be 1.1 to 1.6 times the ITERH93 scaling and the ELMy data almost 2.0 to 2.8 times the ITERH92 ELMy scaling law, suggesting that the size scaling in both scalings may be too strong. While both ELM-free and ELMy discharges were produced, the ELM characteristics were not easily compared with observations on other devices. No large, low frequency ELMs were seen despite the very high edge pressure and temperature gradients that were attained. For all of our H mode discharges, a clear linear relationship between the edge temperature pedestal and the temperature gradient in the core plasma was observed; the discharges with the `best transport barriers also showing the greatest improvement in core c

Scaling and transport analysis of divertor conditions on the Alcator C-Mod tokamak

Detailed measurements and transport analysis of divertor conditions in Alcator C‐Mod [Phys. Plasmas 1, 1511 (1994)] are presented for a range of line‐averaged densities, 0.7<ne<2.2×1020 m−3. Three parallel heat transport regimes are evident in the scrape‐off layer: sheath‐limited conduction, high‐recycling divertor, and detached divertor, which can coexist in the same discharge. Local cross‐field pressure gradients are found to scale simply with a local electron temperature. This scaling is consistent with classical electron parallel conduction being balanced by anomalous cross‐field transport (χ⊥∼0.2 m2u2009s−1) proportional to the local pressure gradient. A 60%–80% of divertor power is radiated in attached discharges, approaching 100% in detached discharges. Detachment occurs when the heat flux to the plate is low and the plasma pressure is high (Te∼5 eV). High neutral pressures in the divertor are nearly always present (1–20 mTorr), sufficient to remove parallel momentum via ion–neutral collisions.

Radiofrequency-heated enhanced confinement modes in the Alcator C-Mod tokamak

Enhanced confinement modes up to a toroidal field of BT=8u2009T have been studied with up to 3.5 MW of radiofrequency (rf) heating power in the ion cyclotron range of frequencies (ICRF) at 80 MHz. H-mode is observed when the edge temperature exceeds a threshold value. The high confinement mode (H-mode) with higher confinement enhancement factors (H) and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement (H=2.0), high normalized beta (βN=1.5), low radiated power fraction (Pradmain/Ploss=0.3), and low effective charge (Zeff=1.5) has been obtained in Enhanced Dα H-mode. This type of H-mode has enhanced levels of continuous Dα emission and very little or no edge localized mode (ELM) activity, and reduced core particle confinement time relative to ELM-free H-mode. The pellet enhanced performance (PEP) mode is obtained by combining core fueling with pellet injection and core heating. A highly peaked pressure profile with...

X-ray observations of central toroidal rotation in ohmic Alcator C-Mod Plasmas

Impurity toroidal rotation has been observed in the centre of ohmic plasmas in the Alcator C-Mod tokamak from the Doppler shifts of argon and molybdenum X-ray lines. The rotation is highest (~6 × 106 cm/s) in the early portion of the discharges, when the loop voltage is highest and the electron density is lowest, and then typically settles to values ≤ 2 × 106 cm/s during the steady state period. The impurity rotation is in the same direction as the electron toroidal drift, opposite to the plasma current, and reverses direction when the plasma current direction is reversed. Molybdenum and argon ions rotate with the same velocity. These observations are in qualitative agreement with neoclassical theory

Comparison of detached and radiative divertor operation in Alcator C-Mod

The divertor of the Alcator C‐Mod tokamak [Phys. Plasmas 1, 1511 (1994)] routinely radiates a large fraction of the power entering the scrape‐off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MWu2009m−3 in ion cyclotron range of frequency (ICRF) heated discharges, have been measured using bolometer arrays. An analysis of both Ohmic and ICRF‐heated discharges has demonstrated some of the relative merits of detached divertor operation versus high‐recycling divertor operation. An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases.

Observations of Alcator C‐Mod plasmas from a five chord high energy resolution x‐ray spectrometer array

X‐ray spectra from Alcator C‐Mod plasmas have been collected using a high wavelength resolution, five spectrometer array during a wide range of operating conditions, providing a large variety of diagnostic information. Each independently scannable von Hamos type spectrometer has a wavelength range of 2.8–4.0 A, and the complete Rydberg series of helium‐ and hydrogenlike argon have been observed. Spectra of Δn=1 ground state transitions and satellites taken along different chords have been simulated using the results from a collisional‐radiative model and the mist transport code. Line ratios are very sensitive to the electron temperature and deduced profiles are in good agreement with ECE profiles. Line intensities have been utilized to obtain absolute argon densities. The widths of the strongest lines have been used to deduce ion temperature profiles. Transitions from around n=9 to the ground state are populated by charge exchange in the outer regions of the plasma and these line intensities have been use...

Electron heating via mode converted ion Bernstein waves in the Alcator C-Mod tokamak

Highly localized direct electron heating [full width at half-maximum (FWHM)≅0.2a] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center (r/a⩾0.3) has been observed in H(3He) discharges at B0=(6.0–6.5)u2009T and ne(0)≅1.8×1020u2009m−3. [Here, the minority ion species is indicated parenthetically.] Off-axis heating (r/a⩾0.5) has also been observed in D(3He) plasmas at B0=7.9u2009T. The concentration of 3He in these experiments was in the range of n3He/ne≅(0.2–0.3) and the locations of the mode conversion layer and electron heating peak could be controlled by changing the 3He concentration or toroidal magnetic field (B0). The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to ...

X ray observations of up-down impurity density asymmetries in alcator C-Mod plasmas .

A five chord, high resolution X-ray spectrometer array has been used to measure vertical brightness profiles of helium-like argon emission from Alcator C-Mod plasmas, out to the last closed flux surface. During standard Alcator C-Mod operation, with the X point and the ion B* Del |B| drift downward, the helium-like argon brightness is a factor of ~8 larger at the top of the plasma than at the bottom, near the plasma edge. In these edge regions, where the electron temperature is low, the upper levels of observed transitions are populated by radiative recombination of hydrogen-like argon. This up-down brightness asymmetry can be explained by a factor of ~8 enhancement of hydrogen-like argon at the top of the machine at r/a=0.9. This implies a vertical impurity drift from inside the plasma, since the hydrogen-like argon is born near the plasma centre. With the ion B* Del |B| drift upward, the enhancement switched to the bottom of the machine, but did not change when the X point was moved up. This asymmetry agrees qualitatively with the predictions of neoclassical parallel impurity transport

Impurity transport in the divertor of the Alcator C-Mod tokamak

Abstract Argon has been injected into the private flux zone of the Alcator C-Mod divertor and its transport into the confined plasma studied. The injected gas pulse lasts for about 200 ms, but the impurity concentration increases to an equilibrium level and remains there for ∼ 200 ms. It is clear that the argon is acting as a recycling impurity. Nevertheless it has been observed that less than 0.5% of the injected atoms reach the confined plasma and that the percentage decreases with increasing plasma density. The good impurity screening is discussed in terms of the ionisation mean free path in the divertor.