D. Terry

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.

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: research in support of ITER and steps beyond

This paper presents an overview of recent highlights from research on Alcator C-Mod. Significant progress has been made across all research areas over the last two years, with particular emphasis on divertor physics and power handling, plasmamaterial interaction studies, edge localized mode-suppressed pedestal dynamics, core transport and turbulence, and RF heating and current drive utilizing ion cyclotron and lower hybrid tools. Specific results of particular relevance to ITER include: inner wall SOL transport studies that have led, together with results from other experiments, to the change of the detailed shape of the inner wall in ITER; runaway electron studies showing that the critical electric field required for runaway generation is much higher than predicted from collisional theory; core tungsten impurity transport studies reveal that tungsten accumulation is naturally avoided in typical C-Mod conditions.

Development of Fast Ferrite Tuner for Lower Hybrid current drive

We are developing a Fast Ferrite Tuner (FFT) for use in the Lower Hybrid current drive system at 4.6GHz. The FFT consists of two stub tuners mounted on the narrow sides of a WR187 waveguide whose phase lengths are electrically controlled by adjusting the bias magnetic field in ferrite blocks mounted in the stubs. The current in the coils producing the bias magnetic fields will be driven by power supplies which are computer controlled. An algorithm in the computer will change the phase length of the stub tuners to eliminate the reflection from the load. The challenging tasks in the development are obtaining the necessary phase shift with minimum field variation, keeping the losses low (<0.1dB), avoiding resonances in the waveguide during field changes, avoiding breakdown at for high power (>100KW), and developing a power supply that can respond in sub millisecond timescales. This report presents the design concept, the results of measurements on the tasks, and CST simulations of the design.

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.

Operation of a double stub tuner for Alcator C-Mod lower hybrid current drive system

This paper describes the operation of a double stub fast ferrite tuner (FFT) that we have designed for the Alcator C-Mod 4.6GHz Lower Hybrid Current Drive (LHCD) system. This FFT is unique because it uses a single electromagnet coil and permanent magnet on each tuning stub. The ferrite is located on the center of the broad face of the waveguide. The FFT is required to withstand over 200kW of power (20kW/cm2) at high VSWR (>5) for 1-3 second pulses spaced 10 minutes apart. Breakdown measurements and fabrication considerations will be discussed. Also, simulation of thermal conditions will be shown. The FFT will be computer controlled and must react to matching a load in a few hundred microseconds. This puts a severe requirement on power supply response time and its variation. In addition, the calculation time of the controlling software algorithms must be considered as well as the diffusion time of the controlling magnetic field through the waveguide wall. We will discuss these requirements and what we have done to meet them.

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.

The Coupler Protection System Upgrade for Lower Hybrid Current Drive on Alcator C-Mod

The MIT Plasma Science and Fusion Center (PSFC) Alcator C-Mod project has implemented a lower hybrid current drive (LHCD) system that uses 12 VKC-7849 Varian (CPI) klystron transmitters to supply up to 3 MW source power at 4.6 GHz. Power from each transmitter is split eight ways using a complex system of waveguides to drive the 96-channel coupler array. The existing Coupler Protection System (CPS) monitors directional coupler forward and reflected power signals at 60 locations in the system and provides 60 channels of voltage standing wave ratio (VSWR) protection for the coupler array. To further improve coupler protection and provide remote adjustability an upgrade to the CPS is underway. The most efficient approach to this upgrade has been to use an existing fast digitizer board design having MDS+ data system communication capabilities and reserve field programmable gate array (FPGA) circuitry. Working closely with the digitizer board manufacturer and designers, D-TACQ Solutions, PSFC engineers are developing CPS fault protection logic for twelve each of the ten channel, 14 bit, 6 MSPS/channel digitizer boards with associated custom rear transition modules (RTM). These boards will provide the protection and data acquisition functions needed to allow programmable, optimized protection and monitoring for the coupler and accessibility to the existing MIT PSFC MDS+ data acquisition and control system. Details of the CPS upgrade system will be presented.

A Scoping Study for High-Field-Side Launch of Lower Hybrid Waves on ADX MIT

Launching lower hybrid (LH) waves from the high field side (HFS) of a tokamak offers significant advantages over low-field-side (LFS) launch with respect to both wave physics and plasma material interactions (PM!s). The higher magnetic field opens the window between wave accessibility and the condition for strong electron Landau damping, allowing LH waves from the HFS to penetrate into the core of burning plasma, while waves launched from the LFS are restricted to the periphery of the plasma. The lower parallel refractive index (n||) of the waves launched from the HFS yields a higher current drive efficiency as well. The absence of turbulent heat and particle fluxes on the HFS, particularly in double null configuration, makes it the ideal location to minimize PM! damage to the antenna structure. The quiescent scrape off layer (SOL) also eliminates the need to couple LH waves across a long distance to the separatrix, as the antenna can be located close to plasma without risking damage to the structure. The Advanced Divertor eXperiment (ADX) will include an LH launcher located on the HFS. The LH system for ADX will make use of the existing infrastructure from Alcator C-Mod, including sixteen 250-kW klystrons at 4.6 GHz (total source power of 4 MW), high-voltage power supply, and controls. The ADX vacuum vessel design includes dedicated space for waveguide runs, pressure windows, and vacuum feedthroughs for accessing the HFS wall. Compact antenna designs based on proven technologies (e.g., multijunction and four-way splitter antennas) fit within the available space on the HFS of the ADX. Wave coupling simulations of these launchers with HFS SOL density profiles showing good coupling can be obtained by adjusting the distance between the separatrix and the HFS wall. Guard limiters on each side of the LH antenna protect the structure during ramp-up, ramp-down, and off-normal events.