A. Kanojia

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).

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.

Development 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 4.6-GHz lower hybrid current drive 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 200 kW of power (20 kW/cm2) at high VSWR for 1-3 s pulses spaced 10-min apart. Breakdown measurements and fabrication considerations will be discussed. In addition, 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.

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.

A Multithreaded Modular Software Toolkit for Control of Complex Experiments

A multithreaded modular software toolkit has been developed for centralized monitoring and control of complex scientific experiments and instruments. The Modular Control Toolkit (MCT) supports Unix-like operating systems and provides a reusable framework for user-developed modules to share data, setup software interlocks, and utilize a dedicated thread for hardware communication.

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.

Attachment of ferrite material used in an active matching network for LHCD on Alcator C-Mod

In order to increase power coupling efficiency of the Lower Hybrid Current Drive system (LHCD) on Alcator C-Mod an active double stub matching network is being developed. One aspect of this development requires application of ferrite material of certain geometry in a section of WR187 waveguide with expected operation in excess of 200kW of microwave power. This has proven to be a challenge due, in large part, to differences in the Coefficient of Thermal Expansion (CTE) between materials at the ferrite-waveguide and ferrite-coating interfaces, the inherent brittleness of ferrite material and the level of homogeneity in the samples. This paper will discuss a method to modify and attach ferrite material to the waveguide. As necessary, materials issues and processes will be discussed as will the effort to develop a set of guidelines to insure process integrity and repeatability across the multichannel LHCD matching network required for C-Mod and other machines.

Transmitter Protection System Upgrade Design for Lower Hybrid Current Drive System on Alcator C-Mod

An upgrade to the transmitter protection system (TPS) is being designed as part of the scheduled expansion of the Alcator C-Mod Lower Hybrid Current Drive (LHCD) transmitter system from 12 to 16 klystrons. The upgrade design is being done as collaborative effort between Alcator C-Mod and Rockfield Research, Inc. as Phases 1 and II of a Small Business Innovative Research (SBIR) grant. A plan is in place to first implement the new design for the cart supporting 4 additional klystrons and then to upgrade the TPS for the existing three carts supporting the 12 existing klystrons. Some parts must be added before longer pulse operation. Experience in operating the existing LHCD system and a study of the klystron design have indicated a need for this upgrade to improve the protection to the klystrons, improve reliability and noise immunity, improve personnel safety and reduce the size of the system.

Upgrade of the ICRF fault and control systems on alcator C-Mod

The Ion Cyclotron RF Transmitter System (ICRF) at Alcator C-Mod comprises four separate transmitters each capable of driving 2 MW of power into plasma loads. Four separate transmission lines guide RF power into three antennas, each mounted in a separate horizontal port, in the C-Mod Tokamak.

Real-time high-field measurements of joint resistance in the alcator C-mod TF coil

The Toroidal Field [TF] coil on Alcator C-Mod is a liquid nitrogen [LN2] cooled copper magnet with 120 turns. It can carry up to 250 kA and generate nominal fields of 8 Tesla. Each turn is made up of four straight segments: outside leg, top arm, inside leg, and bottom arm, 480 segments in all. Sliding felt-metal joints between adjacent segments make it possible to assemble and disassemble the magnet, and allow it to flex during high-field operation. The design of the TF magnet has always included provisions to monitor the voltage drop and resistance along the magnet. Fused voltage taps are installed on each of the outside, top, and bottom segments. One outside leg is split to accommodate the power supply connections. Limited space around the central column precludes the installation of taps on the inside legs. There are, in total, 361 TF voltage taps and 360 measurements. These data can provide valuable information about joint integrity and any local heating or cooling issues, but we have not been able to fully exploit the voltage tap signals. Measuring the TF voltage taps during a magnet pulse is problematic, because the self-inductance of the TF magnet, and coupling to the other C-Mod coils and to the plasma swamps the resistive effects. The high voltage on the TF magnet (on the order of 1 kV) presents isolation issues for the instrumentation as well. Consequently most of the TF voltage tap data has been taken between plasma shots, with a modest 3 kA excitation and a slow reed relay scanner. The new Real-time TF Voltage Tap Monitor must allow simultaneous monitoring of all 360 channels at 12-bit resolution and up to 1 kHz bandwidth during a plasma shot. It should also allow for higher resolution measurements between shots. The large number of channels and modest performance specifications, combined with a 5 kV electrical isolation requirement make this a poor fit for commercial digitizers. A brute force approach, using 360 high-voltage isolation amplifiers, would be expensive and unappealing. This paper describes a novel design using floating V-to-F converters and optical isolators. All the signal processing can be done digitally using counters and registers in a CPLD or FPGA. Finally, there are a number of attractive options for the data interface: inexpensive digital I/O cPCI boards, an Arduino type computer, or even a simple USB connection.