P. Koert

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).

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.

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.

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...

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.

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.

RF, disruption and thermal analyses of EAST antennas

CAS IPP and MIT PSFC are collaborating on Experimental Advanced Superconducting Tokamak (EAST), the first tokamak with superconducting toroidal and poloidal magnets and a testbed for technologies proposed for the ITER project. Presented in this paper are RF, disruption and thermal analyses of EAST antennas. All were performed by COMSOL commercial software package Version 5. Analyzed are the I port 4 strap and B port 2 × 2 strap antennas, which are currently installed on EAST. RF analysis over the Ion Cyclotron Range of Frequencies (ICRF) gets insight into the coupling mechanism to optimize antenna plasma coupling. A lossy dielectric model was created which loads the antenna. The Scattering parameters (Sparameter) were extracted. Peak electric field parallel to the magnetic field of the straps, coaxes and other components were determined. Parametric analysis of the operation frequencies on the electric field are also performed. Disruption analysis addresses the impact of the magnetic field and plasma. Temporal currents of poloidal field and plasma as well as the spatial toroidal field were imported into the electromagnetic (EM) model. The structural analysis afterwards determined the stress due to antenna loads generated during the disruption. The loads resulted from the reaction of circulating eddy currents in the antennas with the toroidal and poloidal magnetic fields. Thermal analysis, a fluid - heat transfer - structural multiphysics analysis, performed for the strap and Faraday rod by applying heat loads from the plasma, ripple trapped particles and RF heating for steady state, are also presented. Finally, benefits of a future field-aligned 4 strap antenna were discussed.