W. Beck

Wave-particle studies in the ion cyclotron and lower hybrid ranges of frequencies in alcator C-mod

Abstract This paper reviews the physics and technology of wave-particle-interaction experiments in the ion cyclotron range of frequencies (ICRF) and the lower hybrid (LH) range of frequencies (LHRF) on the Alcator C-Mod tokamak. Operation of fixed frequency (80 MHz) and tunable (40- to 80-MHz) ICRF transmitters and the associated transmission system is described. Key fabrication issues that were solved in order to operate a four-strap ICRF antenna in the compact environment of C-Mod are discussed in some detail. ICRF heating experiments utilizing the hydrogen (H) and helium-3 (3He) minority heating schemes are described, and data are presented demonstrating an overall heating efficiency of 70 to 90% for the (H) minority scheme and somewhat lower efficiency for (3He) minority heating. Mode conversion electron heating experiments in D(3He), D(H), and H(3He) discharges are also reported as well as simulations of these experiments using an advanced ICRF full-wave solver. Measurements of mode-converted ion cyclotron waves and ion Bernstein waves using a phase contrast imaging diagnostic are presented and compared with the predictions of a synthetic diagnostic code that utilizes wave electric fields from a full-wave solver. The physics basis of the LH current profile control program on Alcator C-Mod is also presented. Computer simulations using a two-dimensional (velocity space) Fokker Planck solver indicate that ~200 kA of LH current can be driven in low-density H-mode discharges on C-Mod with ~3 MW of LHRF power. It is shown that this off-axis LH current drive can be used to create discharges with nonmonotonic profiles of the current density and reversed shear. An advanced tokamak operating regime near the ideal no-wall β limit is described for C-Mod, where ~70% of the current is driven through the bootstrap effect. The LH power is coupled to C-Mod through a waveguide launcher consisting of four rows (vertically) with 24 guides per row (toroidally). A detailed description of the LH launcher fabrication is given in this paper along with initial operation results.

20 years of research on the Alcator C-Mod tokamak

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-modes performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.

Overview of the Alcator C-MOD Research Program

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 (PLH < 900?kW) in preparation for future advanced-tokamak studies have demonstrated fully non-inductive current drive at Ip ~ 1.0?MA with good efficiency, Idrive = 0.4 PLH/neoR (MA, MW, 1020?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 (Bth/B ? R0.68?0.19) is inferred from its measured variation with BT that implies a favourable threshold value for ITER. A phase contrast imaging diagnostic has been used to study the structure of Alfv?n 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 , 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 ? 10 precursor oscillation followed by a rapid primary ejection as the pedestal crashes and then multiple, slower secondary ejections.

Design, and initial experiment results of a novel LH launcher on Alcator C-Mod

The design, construction and initial results of a new lower hybrid current drive (LHCD) launcher on Alcator C-Mod (Hutchinson et al 1994 Phys. Plasmas 1 1511) are presented. The new LHCD launcher (LH2) is based on a novel splitter concept which evenly distributes the microwave power in four ways in the poloidal direction. This design allows for simplification of the feeding structure while keeping the flexibility to vary the peak launched toroidal index of refraction, Ntoroidal, from ?3.8 to 3.8. An integrated model predicts good plasma coupling over a wide range of edge densities, while poloidal variations of the edge density are found to affect the evenness of power splitting in the poloidal direction. The measured transmission loss is about 30% lower than the previous launcher, and a clean Ntoroidal spectrum has been confirmed. Power handling capability exceeding an empirical weak conditioning limit and reliable operation up to 1.1?MW net LHCD power have been achieved. A survey of antenna?plasma coupling shows the existence of a millimetric vacuum gap in front of the launcher. Fully non-inductive, reversed shear plasma operation has been demonstrated and sustained for multiple current diffusion times. The current drive efficiency, ?LH ? neR0Ip/PLH, of these plasmas is (0.2?0.25) ? 1020?m?2A?W?1, which is in agreement with the expected efficiency on the International Thermonuclear Experimental Reactor (ITER).

Alcator C-mod design, engineering, and disruption research

Abstract We describe some of the engineering solutions required to produce a diverted tokamak capable of operation with a toroidal field of 8 T and plasma currents of up to 2 MA. Some design details of the toroidal field magnet, the ohmic heating magnet, the metal plasma-facing components, the rf heating and current drive systems, and the power and liquid nitrogen cooling systems are discussed. Vacuum, vessel bake, boronization, and wall-cleaning systems are also discussed. Finally, disruption research results from Alcator C-Mod are presented.

Lower Hybrid Current Drive on Alcator C-Mod: System Design, Implementation, Protection, Calibration and Performance

A 4.6 GHz 3 MW lower hybrid current drive (LHCD) system has been designed and implemented on Alcator C-Mod. This RF system will allow C-Mod to access advanced tokamak regimes: high confinement, high betan, and high bootstrap fraction and extend them to quasi-steady-state conditions. The LHCD system includes twelve 250 kW klystrons. Power from each klystron is split eight ways using a complex system of waveguides to drive a 96-window coupler array. The amplitude and relative phasing of each klystron is controlled by a computer-based system using I-Q vector modulators and is monitored by I-Q detectors to control the npar spectrum applied to the plasma. Calibration is accomplished using a network analyzer in conjunction with software programs to generate two-dimensional lookup tables that allow compensation for system non-linearities. Forward and reflected powers are monitored to protect the klystrons, waveguides and coupler array from arcing. During the 2006 experimental campaign, nearly 1 MA of current was driven into Alcator C-Mod plasma using 800 kW of coupled RF power.

Integrated numerical design of an innovative Lower Hybrid launcher for Alcator C‐Mod

The new Alcator C‐Mod LHCD system (LH2) is based on the concept of a four way splitter [1] which evenly splits the RF power among the four waveguides that compose one of the 16 columns of the LH grill. In this work several simulation tools have been used to study the LH2 coupling performance and the launched spectra when facing a plasma, numerically verifying the effectiveness of the four way splitter concept and further improving its design. The TOPLHA code has been used for modeling reflections at the antenna/plasma interface. TOPLHA results have been then coupled to the commercial code CST Microwave Studio to efficiently optimize the four way splitter geometry for several plasma scenarios. Subsequently, the COMSOL Multiphysics code has been used to self consistently take into account the electromagnetic‐thermal‐structural interactions. This comprehensive and predictive analysis has proven to be very valuable for understanding the behavior of the system when facing the plasma and has profoundly influenc...

Waveguide Splitter for Lower Hybrid Current Drive

We have developed high power four and eight way splitters for a new Lower Hybrid launcher. The motivation for the new launcher was the need to provide more power and reliability to the launcher structure. In addition there was a desire to simplify and increase the reliability of the implementation of the alumina windows. The launcher consists of 64 waveguide apertures powered by 8 klystrons with maximum power of 250 kW each at 4.6 GHz. Hence, it is necessary to split the power from each Klystron into eight separate waveguides. The outputs of the splitter have a difference in power less than 0.1dB and phase less than 2 degree. The design analysis of the splitter was done with the computer code CST. Structure analysis was performed using Ansys. The splitter is fabricated by machining an open cavity into a thick stainless steel plate creating the specified internal geometry. It is machined to a tight tolerance of +/- 0.005″. A fitted lid is then welded on top of the open cavity using electron beam welding. The excess metal is removed with Electro discharge machining (EDM) creating the external geometry. The waveguides are then butt-welded to the splitter. Welding fixtures/parameters are being developed to achieve the desired tolerances. Two methods for attaching the ceramic windows are being evaluated, brazing and electro-forming.

Design and R&D for the C-Mod Outer Divertor Upgrade

Abstract Operational requirements and research considerations make a high-temperature, toroidally continuous outer divertor an important upgrade to the Alcator C-Mod tokamak. Leading edge melting of tiles, non-uniform heat loads, large electromagnetic forces, and localized impurity sources limit the performance of bulk plasmas. These issues can be addressed by the installation of a well-aligned, toroidally continuous outer divertor. Additionally, future long pulse operation will cause the temperature of the outer divertor to reach bulk temperatures as high as 500 - 600 °C. This future operational requirement combined with the strong temperature dependence of plasma surface interactions (especially fuel retention), makes a controllable, high-temperature outer divertor desirable and necessary. The motivation, criteria, design, and R&D for the upgrade are discussed below.

High Power Water Load for Lower Hybrid Current Drive at 4.6 GHz on Alcator C-Mod

We have developed a high power water load capable of absorbing 250 kW for 5 S at 4.6 GHz. These loads are required for testing and calibrating the lower hybrid current drive systems. The design and analysis of the load was done with the aid of the computer codes CST for RF and Algor and Comsol for water fluid considerations. The load consists of a stainless steel jacket with a Teflon insert. The Teflon is used as a RF impedance matching device, a water seal and is designed to manage water flow over the Teflon-water interface. This design distributes the absorption uniformly at the interface. The flow requirements are 22 gallons/minute with less than 2 psi drop. A shrink fit application to eliminate conventional seals was utilized on the Teflon insert. Initial tests resulted in 300 kW being absorbed for 500 ms with less than -26 db of reflection.