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While multiple experiments have reported a decrease in intermittent fluctuations in the far Scrape-Off-Layer (SOL) during ion cyclotron resonance heating (Antar et al 2010 Phys. Rev. Lett. 105 165001, Li et al 2022 Nucl. Eng. Technol. 54 207–19, Antar et al 2012 Nucl. Fusion 52 103005), the physical mechanism behind this observation has not been fully established yet. In this work, we demonstrate, for the first time, a direct correlation between the amplitude of RF-induced E×B flows and turbulence suppression in the far SOL. Using the Gas Puff Imaging (GPI) diagnostic on Alcator C-Mod, we show again that Ion Cyclotron Range of Frequencies can significantly alter the flow in the SOL and introduce a shear layer in regions magnetically connected close to the antenna (Cziegler et al 2012 Plasma Phys. Control. Fusion 54 105019). With the 4-strap field-aligned antenna operated in dipole phasing, the ratio of the power coupled by the central two straps to the power coupled by the outer two straps was varied. The resulting RF-induced radial electric field magnitude thus varied, and we show that the impact on the far SOL turbulence correlates with the modified E×B velocity. We then apply a newly-developed blob tracking algorithm (Han et al 2022 Sci. Rep. 12 18142) to higher-resolution GPI videos in order to directly observe the process of blob shearing by RF-induced E×B flows. We show that the radially sheared poloidal flows act as a transport barrier by stretching, stopping, and destroying filaments, which is consistent with the observed difference in turbulence statistics. [ABSTRACT FROM AUTHOR]

The performance of advanced fluid neutral models for the simulation of deuterium atoms in the plasma edge of magnetically confined fusion devices is tested for the first time on cases that include drifts and currents in the plasma model, on a simplified Alcator C-mod case. The unadapted advanced fluid neutral model already gives quantitative agreement, with 5–30% fluid-kinetic discrepancy in the broad vicinity of the strikepoint. A new advective perpendicular transport term is proposed, which further reduces the fluid-kinetic discrepancies for cases with drifts. It is also shown that the ion-neutral current can be calculated consistently for fluid or kinetic neutrals, at least when only atoms are considered. However, its effect on the solution is very small for the studied case.

The scattering of lower hybrid (LH) waves due to scrape-off layer filaments is investigated. It is revealed that scattering can account for the LH spectral gap without any ad hoc modification to the wave-spectrum. This is shown using a multiscale simulation approach which allows, for the first time, the inclusion of full-wave scattering physics in ray-tracing/Fokker–Planck calculations. In this approach, full-wave scattering probabilities are calculated for a wave interacting with a statistical ensemble of filaments. These probabilities are coupled to ray-tracing equations using radiative transfer theory. This allows the modeling of scattering along the entire ray-trajectory, which can be important in the multi-pass regime. Simulations are conducted for LH current drive (LHCD) in Alcator C-Mod, resulting in excellent agreement with experimental current and hard x-ray profiles. A region in filament parameter space is identified in which the impact of scattering on LHCD is saturated. Such a state coincides with experimental LHCD measurements, suggesting saturation indeed occurs in C-Mod, and therefore the exact statistical properties of the filaments are not important.

Real-time fast ferrite tuning (FFT) has been successfully implemented on the ICRF antennas on Alcator C-Mod. The former prototypical FFT system on the E-port 2-strap antenna has been upgraded using new ferrite tuners. A new FFT system with two ferrite tuners and one fixed-length stub has been installed on the transmission line of the D-port 2-strap antenna. These two systems are able to achieve and maintain the reflected power to the transmitters to less than 1% in real time under almost all plasma conditions and help ensure reliable high power operation of the antennas. The loading insensitivity feature vs. plasma conditions of the innovative field-aligned (FA) 4-strap antenna on the J-port allows us to significantly improve the matching by installing a carefully designed stub on each of the two transmission lines. The reduction of the RF voltages in the transmission lines has enabled the J-port FA antenna to deliver 3.7 MW RF power to plasmas out of 4 MW source power. The matching on the J-port antenna can be further improved by adding a single ferrite tuner under real-time control on each transmission line and this scheme will be implemented in the near future. [ABSTRACT FROM AUTHOR]

Active stub tuning with a fast ferrite tuner (FFT) has greatly increased the effectiveness of fusion ion cyclotron range of frequency (ICRF) systems (50–100 MHz) by allowing for the antenna system to respond dynamically to changes in the plasma load impedance such as during the L–H transition or edge localized modes (ELMs). A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system at 4.6 GHz. The amplitude and relative phase shift between adjacent columns of an LHCD antenna are critical for control of the launched n || spectrum. Adding a double-stub tuning network will perturb the phase and amplitude of the forward wave particularly if the unmatched reflection coefficient is high. This effect can be compensated by adjusting the phase of the low power microwave drive for each klystron amplifier. Cross-coupling of the reflected power between columns of the launcher must also be considered. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution is advanced in an iterative manner similar to the time-dependent behavior of the real system. System performance is presented under a range of edge density conditions from under-dense to over-dense and a range of launched n || . Simulations predict power reflection coefficients ( Γ 2 ) of less than 1% with no contamination of the n || spectrum. Instability of the FFT tuning network can be problematic for certain plasma conditions and relative phasings, but reducing the control gain of the FFT network stabilizes the system. [ABSTRACT FROM AUTHOR]

A small robust system has been constructed for in-situ visual inspection of the Alcator C-Mod tokamak. The system consists of a small, light, wide-angle high definition camera and LED package housed in a nacelle on the end of thin, rigid, 3.5 m long support pole. The nacelle has two actuated degrees of freedom allowing the camera to observe nearly 4 π steradians. The support pole has a specific slight curve that allows it to pass to either side of the center column of the tokamak to observe the entirety of the vessel interior, while still fitting through the small aspect ratio Alcator C-Mod vacuum port structure. The support pole and camera can enter the vessel through any horizontal vacuum port with an inner diameter greater than 4 cm, thus a dedicated port is not required. The inspection is typically undertaken during maintenance periods when the vessel is filled with a noble gas near atmospheric pressure thus minimizing the influx of water vapor and the concomitant loss of wall conditioning. The system is operated manually, producing photos and video which are reviewed in near real-time. Nearly the entire vessel, including the plasma facing components, can be carefully inspected in 3–5 h. The system provides improved characterization of the interior components and surfaces of the tokamak with a modest engineering and operational effort. Information gathered from the system has identified damage to plasma facing components that were interfering with tokamak operation, as well as damage to mechanical components which were redesigned during the remainder of the campaign, thereby enhancing program planning. [ABSTRACT FROM AUTHOR]

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/cm^2) at high VSWR (>5) 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. [ABSTRACT FROM PUBLISHER]

An advanced outer divertor is being developed for Alcator C-Mod to study reactor fuel (tritium) retention and plasma wall material interaction physics at reactor temperatures with high power long-pulse discharges. The divertor will be operated at controlled temperature of 600^\circC. To achieve this goal, the divertor will be structurally and electrically continuous along the toroidal direction, requiring it to expand radially as temperature increases. This paper describes the thermal finite element analysis (FEA) and results of the outer divertor. There are four aspects, with focus on the A-Frame assembly. First of all, a one twentieth module of the full divertor is composed of divertor tiles, tile mounting plate, heaters, divertor gusset, A-Frame support, spherical bearings, bracket, halo current shunt, vessel gusset, and so on. By adjusting the power of each of the seven toroidal divertor heaters, the tiles achieve a uniform temperature poloidally with toroidal temperature variation within allowables. The temperature of each component is evaluated, and results are used to support the design changes. Second, radiation simulation on multilayer radiation shields behind divertor plate is presented. Third, radiation simulation of the diverter heater itself is done to understand more details of heat transfer from the heater to the surrounding tiles and support plates. Finally, thermal analysis is completed with a model including a tile and its mounting plate, to predict the effect of plasma heat load on divertor tiles. All the thermal FEA was performed with COMSOL, a commercial FEA software. [ABSTRACT FROM PUBLISHER]

Abstract: As a part of the scheduled expansion of the Alcator C-Mod lower hybrid current drive (LHCD) system from 12 to 16 klystrons to accommodate installation of a second LH antenna, the active control system (ACS) is being redesigned to accommodate the additional klystrons. Digitizers and output modules will be cPCI modules provided by D-tAcq Solutions. The real-time application will run on a standard PC server running Linux. Initially, the new ACS system will be designed to control 8 klystrons on the second LH antenna and the existing ACS will control the remaining 8 klystrons on the existing LH antenna. Experience gained operating the existing LHCD system has given us insight into the design of a more robust, compact, efficient and simple system for the new ACS. The design upgrade will be patterned on the digital plasma control system (DPCS [1]) in use on C-Mod. [Copyright &y& Elsevier]

Abstract: A Surface Science Station (S3) on the Alcator C-Mod tokamak is used to study and optimize the location and rate of boron film deposition in situ during electron cyclotron (EC) discharge plasmas using 2.45GHz radio-frequency (RF) heating and a mixture of helium and diborane (B2D6) gasses. The radial profile of boron deposition is measured with a pair of quartz microbalances (QMB) on S3, the faces of which can be rotated 360° including orientations parallel and perpendicular to the toroidal magnetic field B T ∼0.1T. The plasma electron density is measured with a Langmuir probe, also on S3 in the vicinity of the QMBs, and typical values are ∼1×1016 m−3. A maximum boron deposition rate of 0.82μg/cm2/min is obtained, which corresponds to 3.5nm/min if the film density is that of solid boron. These deposition rates are sufficient for boron film applications between tokamak discharges. However the deposition does not peak at the EC resonance as previously assumed. Rather, deposition peaks near the upper hybrid (UH) resonance, ∼5cm outboard of the EC resonance. This has implications for RF absorption, with the RF waves being no longer damped on the electrons at the EC resonance. The previously inferred radial locations of critical erosion zones in Alcator C-Mod also need to be re-evaluated. The boron deposition profile versus major radius follows the ion flux/density profile, implying that the boron deposition is primarily ionic. The application of a vertical magnetic field (B V ∼0.01T) was found to narrow the plasma density and boron deposition profiles near the UH resonance, thus better localizing the deposition. A Monte Carlo simulation is developed to model the boron deposition on the different QMB/tokamak surfaces. The model requires a relatively high boron ion gyroradius of ∼5mm, indicating a B+1 ion temperature of ∼2eV, to match the deposition on QMB surfaces with different orientation to B T. Additionally, the boron ion trajectories become de-magnetized at high neutral gas throughput (∼0.5Pam3 s−1) and pressure (∼2Pa) when the largest absolute deposition rates are measured, resulting in deposition patterns, which are independent of surface orientation to B T in optimized conditions. [Copyright &y& Elsevier]

Abstract: One of the primary challenges of auxiliary heating of tokamaks in the ion cyclotron range of frequencies (ICRF) is the reduction of impurities associated with ICRF operation. On Alcator C-Mod, a new magnetic field-aligned antenna was optimized for magnetic flux coupling, power handling, and minimized integrated parallel electric field (E ∥). Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E ∥ in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E ∥ is more modest, suggesting 3-D geometrical effects are important. Using finite element method simulations with a 3-D toroidal cold plasma model, multiple antenna phases were analyzed: [0, π, 0, π], [0, 0, π, π], [0, π, π, 0], [0, 0, 0, 0], [0, π/6, 0, π/6], and [0, π/2, π, 3π/2]. In each case, the field-aligned antenna had reduced integrated E ∥ relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing. [Copyright &y& Elsevier]

Abstract: A double-stub fast ferrite tuning (FFT) system has been developed and installed on the Alcator C-Mod tokamak for real-time ion cyclotron range of frequencies (ICRF) antenna matching. The FFT uses a digital feedback control system consisting of a Linux server, a compact PCI digitizer and MDSplus interface. The software is written in IDL, and runs with interrupts disabled. During a plasma discharge, the computer records the forward RF power, reflected power, and phase data on both the antenna side and the transmitter side of the tuning system, calculates the required magnetic coil currents in the ferrite tuners to minimize the reflected power to the antenna, and sends out the current demands in each computation cycle. The FFT system has been demonstrated to achieve good matching to antenna loading in low confinement modes (L-mode), high confinement modes (H-mode), and plasmas with edge-localized modes (ELMs) in Alcator C-Mod. [Copyright &y& Elsevier]

Abstract: We report the results of operation of Alcator C-Mod with all high-Z molybdenum plasma facing component (PFC) surfaces. Without boron-coated PFCs energy confinement was poor (H ITER,89 ∼1) due to high core molybdenum (n Mo/n e ⩽0.1%) and radiation. After applying boron coatings, n Mo/n e was reduced by a factor of 10–20 with H ITER,89 approaching 2. Results of between-discharge boronization, localized at various major radii, point towards important molybdenum source regions being small, outside the divertor, and due to RF-sheath-rectification. Boronization also has a significant effect on the plasma startup phase lowering Z eff, radiation, and lowering the runaway electron damage. The requirement of low-Z coatings over at least a fraction of the Mo PFCs in C-Mod for best performance together with the larger than expected D retention in Mo, give impetus for further high-Z PFC investigations to better predict the performance of un-coated tungsten surfaces in ITER and beyond. [Copyright &y& Elsevier]

Abstract: Characteristics of discrete ELMs produced in Alcator C-Mod discharges of low edge collisionality (0.2< ν ∗ <1) and large lower triangularity (δ lower ∼0.75) are examined. The energy lost per ELM from the H-mode pedestal is ∼10% of the pedestal energy. These ELMs exhibit relatively long-lived precursor oscillations, often with two modes of intermediate toroidal mode number present. At the ELM ‘crash’ multiple plasma filament structures are expelled into the scrape-off-layer. A short-lived high frequency (∼0.5MHz) magnetic oscillation is initiated at the ‘crash’. The initial ELM filaments are large perturbations to the SOL with radial extents of 0.5–1cm and typical radial propagation velocities of 1km/s. Velocities of up to 8km/s have been seen. The poloidal extent of the initial filaments is >4.5cm. The initial filaments are followed (at intervals of ∼100μs) by multiple, less perturbing secondary filaments. [Copyright &y& Elsevier]

Abstract: The range of ‘edge plasma states’ (i.e., the combination of densities, temperatures and gradients in the scrape-off layer near the separatrix) accessible to a tokamak are highly restricted. Experiments in Alcator C-Mod suggest that this restriction is a consequence of electromagnetic fluid drift turbulence (EMFDT), which sets the cross-field transport levels; plasma states are constrained to lie in a narrow region of EMFDT ‘phase-space’ defined by poloidal beta gradient (α MHD) and normalized plasma collisionality. Recent experiments have investigated this behavior over an increased parameter range in lower- and upper-null (LSN/USN) discharges. L-mode edge pressure gradients are found to exhibit clear sensitivity to magnetic topology, favoring higher α MHD for LSN compared to USN. These variations may be caused, in part, by the different edge plasma flow patterns in LSN versus USN. Such flows arise from a ballooning-like transport drive and can produce a topology-dependent modulation in the toroidal plasma rotation. [Copyright &y& Elsevier]

Abstract: In Alcator C-Mod, we have investigated the compatibility of high power ion cyclotron range of frequencies antenna with high performance plasmas and all high-Z plasma facing components, to provide operational information for future devices such as ITER. Boronization appears to be critical achieving low radiated power plasmas but it can be quickly eroded particularly in the presence of ICRF. Here we present circumstantial evidence that suggests RF-enhanced sheaths on flux tubes connected from the antennas to the top of the outer divertor are the most likely erosion mechanism. In addition, antenna operation without a Faraday screen was found to degrade antenna performance through increased impurity sources. [Copyright &y& Elsevier]

Abstract: Alcator C-Mod, is a high-field high-density, diverted, compact tokamak, which, in its present form uses inductive current drive and is heated with 5MW of ICRF auxiliary power. C-Mod is in the process of being upgraded with a 4.6GHz lower hybrid heating and current drive (LHCD) system. The purpose of the experiment is to develop and explore the potential of “Advanced Tokamak Regimes” under quasi-steady-state conditions. In this paper, an overview of the RF transmitter and the controls and protection systems for the Lower Hybrid Project is given. The transmitter will use 12 250kW klystrons operating simultaneously which will result in a total nominal power at the klystrons of nearly 3MW for a planned pulse width of 5s. Active control system vector modulators provide phase and amplitude drive for each klystron, and I-Q detectors are used to monitor phase and amplitude. These feedback signals are used in digital controllers for closed-loop control of klystron phase and amplitude to preset values. An expected upgrade of four additional klystrons will result in a total nominal power of 4MW. The transmitters have been tested to full power, and installation of the lower hybrid current drive experiment on the C-Mod Tokamak is expected in 2004/2005. [Copyright &y& Elsevier]

Abstract: Three-dimensional Monte Carlo neutral transport simulations of gas flow through the Alcator C-Mod sub-divertor yield conductances comparable to those found in dedicated experiments. All are significantly smaller than the conductance found with the previously used axisymmetric geometry. A benchmarking exercise of the code against known conductance values for gas flow through a simple pipe provides a physical basis for interpreting the comparison of the three-dimensional and experimental C-Mod conductances. [Copyright &y& Elsevier]

Abstract: We model the plasma edge of Alcator C-Mod with the B2-Eirene SOLPS5.0 code. The drifts, currents and neutral effects contribute to strong plasma rotation in the edge, as measured experimentally [B. LaBombard, S. Gangadhara, B. Lipschultz, C.S. Pitcher, J. Nucl. Mater. 313–316 (2003) 995; B. LaBombard, N. Smick, B. Lipschultz, J.L. Terry, J. Rice, these Proceedings; N. Smick, B. LaBombard, C.S. Pitcher, these Proceedings]. The plasma flow patterns are a complex superposition of several competing effects, including Pfirsch–Schlüter flows and currents, toroidal momentum generation and transport (with main chamber neutral recycling and drag playing an important role), as well as the E × B and diamagnetic drift flows. In the standard field direction the E × B flow in the core region adds to the toroidal plasma rotation, while in the reverse direction their respective contributions are roughly of the same order of magnitude but with opposite signs in the plasma core. However, in the edge, all terms can be of the same sign over most if not all of the radial extent of the SOL, leading to complex patterns and very strong flows. [Copyright &y& Elsevier]