Localized Power Deposition Research Articles

ECRH on ASDEX Upgrade - System Status, Feed-Back Control, Plasma Physics Results -

The ASDEX Upgrade (AUG) ECRH system now delivers a total of 3.9 MW to the plasma at 140 GHz. Three new units are capable of 2-frequency operation and may heat the plasma alternatively with 2.1 MW at 105 GHz. The system is routinely used with X2, O2, and X3 schemes. For B t = 3.2 T also an ITER-like O1-scheme can be run using 105 GHz. The new launchers are capable of fast poloidal movements necessary for real-time control of the location of power deposition. Here real-time control of NTMs is summarized, which requires a fast analysis of massive data streams (ECE and Mirnov correlation) and extensive calculations (equilibria, ray-tracing). These were implemented at AUG using a modular concept of standardized real-time diagnostics. The new realtime capabilities have also been used during O2 heating to keep the first reflection of the non-absorbed beam fraction on the holographic reflector tile which ensures a well defined second pass of the beam through the central plasma. Sensors for the beam position are fast thermocouples at the edge of the reflector tile. The enhanced ECRH power was used for several physics studies related to the unique feature of pure electron heating without fueling and without momentum input. As an example the effect of the variation of the heating mix in moderately heated H-modes is demonstrated using the three available heating systems, i.e. ECRH, ICRH and NBI. Keeping the total input power constant, strong effects are seen on the rotation, but none on the pedestal parameters. Also global quantities as the stored energy are hardly modified. Still it is found that the central ion temperature drops as the ECRH fraction exceeds a certain threshold.

Comparison between eight‐ and sixteen‐channel TEM transceive arrays for body imaging at 7 T

Eight- and sixteen-channel transceive stripline/TEM body arrays were compared at 7 T (297 MHz) both in simulation and experiment. Despite previous demonstrations of similar arrays for use in body applications, a quantitative comparison of the two configurations has not been undertaken to date. Results were obtained on a male pelvis for assessing transmit, signal to noise ratio, and parallel imaging performance and to evaluate local power deposition versus transmit B(1) (B(1) (+) ). All measurements and simulations were conducted after performing local B(1) (+) phase shimming in the region of the prostate. Despite the additional challenges of decoupling immediately adjacent coils, the sixteen-channel array demonstrated improved or nearly equivalent performance to the eight-channel array based on the evaluation criteria. Experimentally, transmit performance and signal to noise ratio were 22% higher for the sixteen-channel array while significantly increased reduction factors were achievable in the left-right direction for parallel imaging. Finite difference time domain simulations demonstrated similar results with respect to transmit and parallel imaging performance, however, a higher transmit efficiency advantage of 33% was predicted. Simulations at both 3 and 7 T verified the expected parallel imaging improvements with increasing field strength and showed that, for a specific B(1) (+) shimming strategy used, the sixteen-channel array exhibited lower local and global specific absorption rate for a given B(1) (+) .

Local SAR in parallel transmission pulse design.

The management of local and global power deposition in human subjects (specific absorption rate, SAR) is a fundamental constraint to the application of parallel transmission (pTx) systems. Even though the pTx and single channel have to meet the same SAR requirements, the complex behavior of the spatial distribution of local SAR for transmission arrays poses problems that are not encountered in conventional single-channel systems and places additional requirements on pTx radio frequency pulse design. We propose a pTx pulse design method which builds on recent work to capture the spatial distribution of local SAR in numerical tissue models in a compressed parameterization in order to incorporate local SAR constraints within computation times that accommodate pTx pulse design during an in vivo magnetic resonance imaging scan. Additionally, the algorithm yields a protocol-specific ultimate peak in local SAR, which is shown to bound the achievable peak local SAR for a given excitation profile fidelity. The performance of the approach was demonstrated using a numerical human head model and a 7 Tesla eight-channel transmit array. The method reduced peak local 10 g SAR by 14-66% for slice-selective pTx excitations and 2D selective pTx excitations compared to a pTx pulse design constrained only by global SAR. The primary tradeoff incurred for reducing peak local SAR was an increase in global SAR, up to 34% for the evaluated examples, which is favorable in cases where local SAR constraints dominate the pulse applications.

Multiple electron cyclotron power deposition location tracking by break-in-slope analysis in TCV plasmas

Modulation of the amplitude of externally injected electron cyclotron (EC) power is a frequent method used to determine the radial power deposition profile in fusion plasmas. There are many tools to analyze the plasma response to the power modulations under quasi-stationary conditions. This paper focuses on the unique ability of the break-in-slope (BIS) method to retrieve a quasi-instantaneous estimate of the power deposition profile at each power step in the modulation, an outcome particularly relevant to track the power deposition location under non-stationary conditions. Here, the BIS analysis method is applied to the signals of a fast and high radial resolution wire-chamber soft x-ray camera in the Tokamak à Configuration Variable (TCV) where the plasma magnetic configuration and thus the EC resonance location are varied during the plasma discharge. As a step to validate this technique before real-time control experiments, the time-varying EC power deposition location of a single beam is successfully monitored by off-line BIS analysis. Simultaneous tracking of deposition locations of two EC beams gives promising results.

Calculations of lower hybrid current drive in ITER

A detailed study of lower hybrid current drive (LHCD) in ITER is provided, focusing on the wave propagation and current drive mechanisms. A combination of ray-tracing and Fokker–Planck calculations are presented for various plasma scenarios, wave frequency and polarization. The dependence of the driven current and the location of power deposition upon the coupled wave spectrum is systematically determined, in order to set objectives for the antenna design. The respective effects of finite-power levels, magnetic trapping, and detailed antenna spectra are accounted for and quantitatively estimated. The sensitivity of LHCD to density and temperature profiles is calculated. From the simulation results, an optimum value for the parallel index of refraction is proposed as a compromise between efficiency and robustness with respect to those profile variations. The corresponding current drive efficiency is found to be similar for the two frequencies generally considered for ITER, f = 3.7 GHz and f = 5.0 GHz.

Suppression of MHD instabilities by ion Bernstein wave (IBW) heating in the HT-7 tokamak

In the HT-7 tokamak, the m = 2/n = 1 tearing mode can be effectively suppressed by the ion Bernstein wave (IBW) when the location of power deposition is near the q = 2 rational surface. Off-axis electron heating and electron density broadening have been observed. In the meantime, the particle confinement is improved with an increase in electron density and a drop in Dα emission. The induced large electron density gradients and electron pressure gradients can be spatially correlated with the IBW deposition profile via theoretical calculation. It is suggested that off-axis IBW heating modified the electron pressure profile and the current density profile can be redistributed, thus resulting in the suppression of the magnetohydrodynamics (MHD) instability. It provides an integrated way for both the stabilization of tearing modes and controlling pressure profiles.

Thermal characteristics of thermobrachytherapy surface applicators for treating chest wall recurrence

The aim of this study was to investigate temperature and thermal dose distributions of thermobrachytherapy surface applicators (TBSAs) developed for concurrent or sequential high dose rate (HDR) brachytherapy and microwave hyperthermia treatment of chest wall recurrence and other superficial diseases. A steady-state thermodynamics model coupled with the fluid dynamics of a water bolus and electromagnetic radiation of the hyperthermia applicator is used to characterize the temperature distributions achievable with TBSAs in an elliptical phantom model of the human torso. Power deposited by 915 MHz conformal microwave array (CMA) applicators is used to assess the specific absorption rate (SAR) distributions of rectangular (500 cm2) and L-shaped (875 cm2) TBSAs. The SAR distribution in tissue and fluid flow distribution inside the dual-input dual-output (DIDO) water bolus are coupled to solve the steady-state temperature and thermal dose distributions of the rectangular TBSA (R-TBSA) for superficial tumor targets extending 10–15 mm beneath the skin surface. Thermal simulations are carried out for a range of bolus inlet temperature (Tb = 38–43 °C), water flow rate (Qb = 2–4 L min−1) and tumor blood perfusion (ωb = 2–5 kg m−3 s−1) to characterize their influence on thermal dosimetry. Steady-state SAR patterns of the R- and L-TBSA demonstrate the ability to produce conformal and localized power deposition inside the tumor target sparing surrounding normal tissues and nearby critical organs. Acceptably low variation in tissue surface cooling and surface temperature homogeneity was observed for the new DIDO bolus at a 2 L min−1 water flow rate. Temperature depth profiles and thermal dose volume histograms indicate bolus inlet temperature (Tb) to be the most influential factor on thermal dosimetry. A 42 °C water bolus was observed to be the optimal choice for superficial tumors extending 10–15 mm from the surface even under significant blood perfusion. Lower bolus temperature may be chosen to reduce the thermal enhancement ratio (TER) in the most sensitive skin where maximum radiation dose is delivered and to extend the thermal enhancement of radiation dose deeper. This computational study indicates that well-localized elevation of tumor target temperature to 40–44 °C can be accomplished by large surface-conforming TBSAs using appropriate selection of coupling bolus temperature.

Generation of multiple analog pulses with different duty cycles within VME control system for ICRH Aditya system

Ion Cyclotron Resonance Heating (ICRH) is a promising heating method for a fusion device due to its localized power deposition profile, a direct ion heating at high density, and established technology for high RF power generation and transmission at low cost. Multiple analog pulse with different duty cycle in master of digital pulse for Data acquisition and Control system for steady state RF ICRH System(RF ICRH DAC) to be used for operating of RF Generator in Aditya to produce pre ionization and second analog pulse will produce heating.The control system software is based upon single digital pulse operation for RF source. It is planned to integrate multiple analog pulses with different duty cycle in master of digital pulse for Data acquisition and Control system for RF ICRH System(RF ICRH DAC) to be used for operating of RF Generator in Aditya tokamak. The task of RF ICRH DAC is to control and acquisition of all ICRH system operation with all control loop and acquisition for post analysis of data with java based tool. For pre ionization startup as well as heating experiments using multiple RF Power of different powers and duration. The experiment based upon the idea of using single RF generator to energize antenna inside the tokamak to radiate power twise, out of which first analog pulse will produce pre ionization and second analog pulse will produce heating.The whole system is based on standard client server technology using tcp/ip protocol. DAC Software is based on linux operating system for highly reliable, secure and stable system operation in failsafe manner. Client system is based on tcl/tk like toolkit for user interface with c/c++ like environment which is reliable programming languages widely used on stand alone system operation with server as vxWorks real time operating system like environment. The paper is focused on the Data acquisition and monitoring system software on Aditya RF ICRH System with analog pulses in slave mode with digital pulse in master mode for control acquisition and monitoring and interlocking.

Conceptual design of automation of ICRH vacuum system on Aditya Tokamak

Ion Cyclotron Resonance Heating (ICRH) is a successful heating method for a fusion device due to a localized power deposition profile and is an established technology for raising temperature of ion species. The ADITYA-ICRH system consists of RF generator, pressurisable transmission line, a matching network, vacuum transmission line section (VTL) and antenna. The intermediate VTL provides vacuum isolation from that of ADITYA at one end and also separates the pressurisable transmission line at the other end. The ICRH vacuum system consists of VTL, a turbo molecular pump (TMP), pneumatic gate valve, ionization gauge, one vacuum window at antenna side and a gas barrier towards other side.During ADITYA-ICRH operation the ICRH vacuum system should be operated remotely with all the necessary safety interlocks, controls and status at the RF control room.In this paper the schematics of automated vacuum system with interlocks, sequence of automation with flow chart and related results will be discussed.

Automatic impedance matching network for ICRH-RF experiments on SST-1

Ion Cyclotron Resonance Heating (ICRH) is a promising heating method for a fusion device due to its localized power deposition profile, a direct ion heating at high density, and established technology for high power RF generation and transmission low cost. For the same reason 1.5-Megawatt ICRH system is developed indigenously for steady state super-conducting tokamak (SST-1).Since plasma-loading impedance is generally small as compared to the characteristic impedance of the transmission system, a significant amount of power will be reflected from the antenna back towards the generator giving very high reflections to SWR which can damage the high power RF tube. Hence matching network is used to match the total transmission line system to the antenna impedance so that the RF generator sees a matched load and can operate at high efficiency. In the ICRH system, coarse matching network and the on-line automatic matching network designed for the impedance matching of the system to transfer maximum power to the tokamak plasma during 1000 seconds operation. The plasma impedance varies in time on milliseconds scale and hence on-line impedance matching has to match the impedance on a faster scale to avoid the reflections as well as to transfer maximum power to the plasma for heating.There are two transmission lines connected with Hybrid coupler sourced by RF Generator (RF Power) 45.6 MHz frequency. Automatic matching network is connected with each transmission line, which offers on-line fine matching along with coarse matching system connected with transmission line. The matching system includes stubs, phase shifters and the automatic matching system consists of motorized vacuum variable capacitors connected to 9" transmission line.The method includes the detection of reflected power in transmission line with the help of probes, give a right signal to VME based data acquisition and control system to do calculations and generate the signal to vary the capacitance of motorized vacuum variable capacitors within few milliseconds with the help of the controller of the motor of the variable capacitor.Initially automatic matching system was tested with individual transmission lines and then both the lines were matched simultaneously. In order to deliver power to both the lines from a single RF generator, hybrid coupler was used which also protects RF generator from reflections.Here we present the details of the on-line matching system and its testing results. The significant result is that we could match the variable load impedance with the generator impedance within 40-50 milliseconds with d1600 count of motor controller program.

Development of density-inclination plasmas for analysis of plasma nano-processes via combinatorial method

We have developed a plasma-process analyzer based on the “combinatorial method”, in which process examinations with a continuous variation of sample-preparation conditions can be carried out in one execution of experiment via placing substrates on a substrate holder with an inclined distribution of process parameters (ion flux and radical flux) and the distributions of particle fluxes are finely controlled and characterized via particle diagnostics. In the present study, plasma-fluid simulations have been performed to show the feasibility of the combinatorial plasma-process analyzer, in which density inclinations of the plasma parameters (ion density, radical density) are obtained via sustaining plasmas by localized deposition of discharge power using low-inductance antenna modules. The simulation results showed that density-inclination plasmas were feasible by localized power deposition for sustaining plasmas, indicating that a variety of process conditions can be efficiently analyzed via placing substrates on a substrate holder, along which process parameters are inclined.

Disruption control on FTU and ASDEX upgrade with ECRH

The use of ECRH has been investigated as a promising technique to avoid or postpone disruptions in dedicated experiments in FTU and ASDEX Upgrade. Disruptions have been produced by injecting Mo through laser blow-off (FTU) or by puffing deuterium gas above the Greenwald limit (FTU and ASDEX Upgrade). The toroidal magnetic field is kept fixed and the ECRH launching mirrors have been steered before every discharge in order to change the deposition radius. The loop voltage signal is used as disruption precursor to trigger the ECRH power before the plasma current quench. In the FTU experiments (Ip = 0.35–0.5 MA, Bt = 5.3 T, PECRH = 0.4–1.2 MW) it is found that the application of ECRH modifies the current quench starting time depending on the power deposition location. A scan in deposition location has shown that the direct heating of one of the magnetic islands produced by magnetohydrodynamic (MHD) resistive instabilities (either m/n = 3/2, 2/1 or 3/1) prevents its further growth and also produces the stabilization of the other coupled modes and the delay of the current quench or its full avoidance. Disruption avoidance and complete discharge recovery are obtained when the ECRH power is applied on rational surfaces. The modes involved in the disruption are found to be tearing modes stabilized by a strong local ECRH heating. The Rutherford equation has been used to reproduce the evolution of the MHD modes. In the ASDEX Upgrade experiments L-mode plasmas (Ip = 0.6 MA, Bt = 2.5 T, PECRH = 0.6 MW ∼ POHM) the injection of ECRH close to q = 2 significantly delays the 2/1 onset and prolongs the duration of the discharge: during this phase the density continues to increase. No delay in the onset of the 2/1 mode is observed when the injected power is reduced to 0.35 MW.

Design of a new ECRH launcher for FTU tokamak

The present ECRH launcher installed on a single equatorial port of FTU tokamak was designed to inject four beams independently steered continuously in poloidal direction and in a set of predetermined toroidal angles. The launching mirrors can be moved only shot by shot. New experimental programmes for control of MHD instabilities with ECH/ECCD and heating of over-dense plasmas with ECBW require new launcher respectively with fast-steerable mirrors and increased toroidal capabilities. The required scanning speed for tracking the rational surfaces in the FTU plasma is 1 cm in 10 ms in poloidal direction, while the maximum toroidal angle needed for O–X–B heating scheme is around ±40°. Two ECRH lines, feeding the old launcher, will be switched to the new launcher, located in a different equatorial position, capable of launching two independent beams from small movable mirrors in the plasma proximity. A control on the power deposition width will be achieved by changing the beam radius in the plasma using an optical system composed by two mirrors (zooming range 17–28 mm). Place has been reserved for future arrangements of additional components, e.g. a remote steering waveguide. A dedicated feedback control for the poloidal motion of the launching mirrors is being developed, in order to adapt the tracking of the power deposition location to the dynamic changes of magnetic surfaces in real-time. The maximum toroidal angle impacts strongly on the movable mirror design; dimensions (height around 90 mm, width around 54 mm) are limited by the port width (=80 mm) and the need to preserve the maximum steering angle. Since the mirrors will not be actively cooled, temperature control will be achieved by covering the backside with a high emissivity coating, to obtain an efficient radiative cooling. A detailed description of the launcher is presented in the paper.

Detonation initiation on the microsecond time scale: DDTs

Spatially resolved, thermal power deposition of limited duration into a finite volume of reactive gas is the initiator for a deflagration-to-detonation transition (DDT) on the microsecond time scale. The reactive Euler equations with one-step Arrhenius kinetics are used to derive a novel formula for the gas velocity supporting the lead shock in a detonation. Numerical solutions of the reactive Euler equations are used to describe the detailed sequence of reactive gasdynamic transients leading to a planar detonation, characterised by unusually large power output, far from the power deposition location. Results are presented for deposition into a region isolated from the planar boundary of the reactive gas as well as for that adjacent to the boundary. The quantitative dependences of DDT evolution on the location and magnitude of thermal power deposition and activation energy are identified.

Present Status of the New Multifrequency ECRH System for ASDEX Upgrade

A new multifrequency electron cyclotron resonance heating system is under construction for the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak experiment. For the first time in a fusion device, this system employs multifrequency gyrotrons that are step-tunable in the range 105-140 GHz. In its final stage the system will consist of four gyrotrons with a total power of 4 MW and a pulselength of 10 s. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. Transmission line elements such as corrugated waveguides, polarizer mirrors and vacuum windows are designed to cope with this frequency band. The system includes fast steerable launchers at the front end that will allow for localized feedback-controlled power deposition in the plasma.

Electron Bernstein wave heating of over-dense H-mode plasmas in the TCV tokamak via O-X-B double mode conversion

This paper reports on the first demonstration of electron Bernstein wave heating (EBWH) by double mode conversion from ordinary (O-) to Bernstein (B-) via the extraordinary (X-) mode in an over-dense tokamak plasma, using low field side launch, achieved in the TCV tokamak H-mode, making use of its naturally generated steep density gradient. This technique offers the possibility of overcoming the upper density limit of conventional EC microwave heating. The sensitive dependence of the O-X mode conversion on the microwave launching direction has been verified experimentally. Localized power deposition, consistent with theoretical predictions, has been observed at densities well above the conventional cut-off. Central heating has been achieved, at powers up to two megawatts. This demonstrates the potential of EBW in tokamak H-modes, the intended mode of operation for a reactor such as ITER.

Electron Bernstein Wave Heating and Emission in the TCV Tokamak

Electron cyclotron resonance heating (ECRH) of high-density tokamak plasmas is limited because of reflections of the waves at so-called wave cutoffs. Electron Bernstein wave (EBW) heating (EBWH) via a double mode conversion process from ordinary (O)-mode, launched from the low field side, to extraordinary (X)-mode and finally to Bernstein (B)-mode offers the possibility of overcoming these density limits.In this paper, the O-X mode conversion dependence on the microwave injection angle is demonstrated experimentally. The dependence on the injection angle is studied in high-density plasmas in H-mode, in the presence of magnetohydrodynamic activity, edge-localized modes, and sawteeth. The results of localized heat deposition at an overdense location are presented, demonstrating EBWH for the first time via the O-X-B mode conversion process in a standard aspect-ratio tokamak. The results of global and local power deposition are compared with ray-tracing calculations. Moreover, a temperature increase due to EBWH is observed.Initial EBW emission measurements with a newly installed ECRH reception launcher are presented. The inverse double mode conversion process B-X-O is observed by measuring the emission for several frequencies at an optimum angle.

Active MHD control at high currents in RFX-mod

The modified RFX is a very flexible device used for a variety of control schemes for MHD instabilities and for advanced reverse field pinch scenarios. Relative to the previous machine, RFX-mod has a thin Cu shell with vertical field penetration time τS, lowered from 450 to 50 ms and shell/plasma proximity from b/a = 1.24–1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide better Bϕ control. Newly designed graphite tiles protect the vessel from localized power deposition. A mesh of 192 external saddle coils, supervised by a digital feedback system, controls radial fields due to field errors and MHD modes. The paper presents an overview of the very encouraging results obtained using both new and ‘standard’ advanced operational modes in the current range 0.3–1 MA. A dramatic improvement of plasma performance was obtained by using the saddle coils to cancel all the radial field components, an operation mode dubbed virtual shell (VS). The toroidal voltage was lowered by more than 25% and the pulse length was tripled, up to 7 times the τS. Steady-state RFP pulses are now limited only by the applied volt-seconds. The improved magnetic boundary also has an effect on the tearing modes underlying the sustainment of the RFP, whose core amplitude is more than halved. The VS combined with new schemes for the active rotation of the MHD dynamo modes has allowed us to obtain reliable and well-controlled long RFP pulses in the MA current range. This results in a 100% increase in the particle and energy confinement time relative to the previous experiment and opens the possibility of exploring the machine performance in the 2 MA design range.

Design of the critical components in the ITER ECH upper launcher steering mechanism

The purpose of the ITER electron cyclotron resonance heating (ECRH) antennae located in the upper port launcher will be to stabilize the neoclassical tearing modes (NTM) by driving currents locally inside either the q = 3 / 2 or 2 island. The ITER reference design uses a front steering (FS) concept with the moveable mirror close to the plasma. The FS launcher is capable of steering eight 2 MW beams via two sets of steering mirrors [R. Chavan, M. Henderson, R. Bertizzolo, J.-D. Landis, F. Sanchez, H. Shidara, The ECH front steering launcher for the ITER upper port, this conference. [8]]. In the adverse invessel operating conditions, reliable operation is required to guarantee the availability of the mirror steering mechanism during the 20-year lifetime of ITER. The dynamic performance and steering accuracy of the mirror system are essential for the localized beam power deposition. In order to increase the steering reliability and accuracy, traditional bearing and push–pull rods are avoided, which tend to introduce backlash or grip. The proposed frictionless and backlash free mechanism design uses elastically compliant structural components to guide and drive the rotating mirror. The actuator system is based on a pressure controlled helium filled bellows array working against a set of preloaded compressive springs. The radiation and temperature sensitive pressure control servovalves are located in the port duct behind the closure plate. Resistance to cyclic fatigue is the critical design requirement for compliant systems. In order to achieve the high reliability, four critical components are identified and described in detail. The bellows, the flexure pivots, the return springs and the spiral cooling tubes are the core elements of the mechanism allowing the controlled rotation of the mirror. Prototypes of the critical components are being tested. Corresponding analytical and cyclic fatigue numerical models of the compliant components are to be validated by fatigue bench tests. The detailed design and the principal tests of the critical components used in the FS mechanism will be presented.

Rigorous Characterization of Resonant Hot Spot Conditions in a Stratified Tissue Model

A unified approach for determining the lossy resonance (hot-spot) conditions in a lossy stratified biological tissue model is proposed. These conditions may lead to a significant enhancement of local electromagnetic power deposition in a single layer as compared to the power dissipated in the background. Rigorous analysis of electromagnetic wave power absorption in a planar stratified tissue model renders a closed-form characterization of six possible asymptotic cases and the associated conditions and bounds on the optimal absorption in the particular layer as a function of its normalized thickness and parameters of the surrounding layers. It is shown that, even very thin and low attenuating layers (sites) of biological tissue, are capable of dissipating a very substantial amount of the incident power, subject to specific lossy resonance conditions. From a dosimetric point of view, the results obtained allow for prediction of naturally occurring spatial resonances in biological tissues on both macroscopic and microscopic scales. On the other hand, they also provide an effective mean for design and synthesis of optimally absorbing materials and tissues in therapeutic applications