TOPLHA: an accurate and efficient numerical tool for analysis and design of LH antennas

This paper presents a self-consistent, integral-equation approach for the analysis of plasma-facing lower hybrid (LH) launchers; the geometry of the waveguide grill structure can be completely arbitrary, including the non-planar mouth of the grill. This work is based on the theoretical approach and code implementation of the TOPICA code, of which it shares the modular structure and constitutes the extension into the LH range. Code results are validated against the literature results and simulations from similar code

Validation of Lower Hybrid coupling codes (Brambilla, GRILL3D-U,TOPLHA) with the FTU conventional grill

design were continuously pursued in view of ITER severe edge conditions. The excitation of LH waves by means of a phased waveguide array was initially proposed in 1974 by Lallia. A systematic study of the coupling problem related to this launching structure was firstly done by Brambilla in 1976 under the so-called grill theory. Subsequently, more detailed analyses were performed, still adopting several approximations in order to simplify the problem, such as the absence of mode conversion and 2D antenna models. The following studies developed more sophisticated models, for instance taking into account new antenna concepts, fast wave excitation, finite poloidal dimensions, etc. At the same time, numerical tools implementing theoretical models were developed; they were essentially based on mode matching technique, single-pass approximation and a slab plasma model. From the antenna viewpoint, the first codes could handle very simple antenna geometries, which then considerably increased in complexity. Lately, due to remarkable developments in computational electromagnetic techniques, new codes, totally or partially based on full-wave antenna solvers, appeared too. This paper validates three LH coupling codes based on different solving techniques: TOPLHA, GRILL3D-U, Brambilla code. They have been run to compute the coupling between the LH waves, launched by a row of the independently phased waveguide array antenna (conventional grill), and some FTU plasmas. TOPLHA and GRILL3D-U have been already compared with another well-established code, i.e. ALOHA, in the frame of the EFDA task HCD-08-03-01, where the LH antenna, proposed for ITER, was used as benchmark. Furthermore a validation of TOPLHA, GRILL3DU and the Brambilla code has been recently done as regards to the FTU conventional grill in terms of average reflection coefficients. In this paper a more detailed analysis is presented by comparing simulated curves with experimental data, that refer to the reflection coefficients effectively measured at the antenna mouth by built-in directional couplers

Design of the Main Transmission Line for the ITER Relevant LHCD System

A very preliminary design of the Lower Hybrid Current Drive system for ITER was developed in the early years of the 2000's. Many parameters of the system were defined on the base of knowledge and technology of those times. Experimental results obtained in the last few years, like the successful tests of a PAM launcher on FTU and on Tore Supra have definitely indicated this concept as a possible candidate for the LHCD launcher for ITER. The successful development of a prototype klystron at 5GHz with a target RF power of 500kW CW has indicated this power as an upper limit for the present high vacuum electron tubes technology at this frequency. From the physics point of view the ITER operational scenarios have been well defined, while the last experiments have pointed out the effectiveness of the LHCD waves at high plasma density. All these results imposed a revision of the existing design of the LHCD system. The RF power limited to a maximum of 500kW per klystron has determined a new modularity of the whole system and of the PAM launcher. In addition a relatively shorter distance between klystrons and ITER machine suggested the possibility of using rectangular oversized waveguides, excited in the fundamental TE01 mode, for the realization of the Main Transmission Lines (MTLs). Therefore all the main microwave components of the system have been revised on these basis and different and more suitable solutions have been investigated. In particular this paper compares the performances of the two different MTLs, based respectively on circular and rectangular waveguides, describes the main components of the proposed rectangular MTL and analyzes their main microwave characteristics. Companion papers in this Conference cover all the other aspects of the updated design of the LHCD system for ITER. The present revision has been carried out in the frame of the EFDA task “HCD-08-03-01: EU Contribution to the ITER LHCD Development Plan (LH4IT)”

RF Modeling of the ITER Lower Hybrid Antenna

In the frame of the EFDA task HCD-08-03-01, a 5 GHz Lower Hybrid system being able to deliver 20 MW CW on ITER and sustains the expected high heat flux has been reviewed. The design of the key RF elements of the antenna such as the Passive-Active Multijunction (PAM), TE10-TE30 mode converter, 3 dB splitter and RF window has been updated. Overall dimensions have been updated from the 2005 design in collaboration with ITER organization. ITER mechanical constraints, such as the port plug size or the rear flange have been taken into account since the initial RF design. The main parallel index has been chosen to be n//=2.0 with a flexibility of [1.8-2.2] in order to maximize the current drive efficiency. In parallel to the RF design, the coupling to the plasma has been studied with the ALOHA and TOPLHA codes and results are in good agreement. The antenna is made of 48 identical modules, 12 on the toroidal direction and 4 on the poloidal direction. Each module is terminated by a RF window located inside the frame and connected to a poloidal 3 dB splitter which feeds 2 TE10-TE30 mode converters. Each of these mode converters divides the power into 3 poloidal rows corresponding to the input of a 4 active waveguides multijunction. An optimization of the multijunction and the mode converter has been made in order to reduce VSWR and total length in order to satisfy the ITER frame constraints. 5 GHz CW RF windows capable of sustaining 500 kW are one of the most challenging RF devices of this antenna, since the ceramic which separates the tokamak vacuum from the pressurized transmission line must handle and evacuate high RF power density flux. Different designs have been proposed based on pill-box geometry and actively water cooled. Efficient designs ensuring low theoretical return loss and acceptable dielectric RF losses for beryllium oxide (BeO) window has been found. All the devices which have been separately studied have been integrated together, allowing a RF characterization of the whole antenna and further optimization for neutron shielding

Bends in Oversized Rectangular Waveguide for the ITER Relevant LHCD System

The present work has been developed within the frame of the EFDA task “HCD-08-03-01: LH4IT, EU contribution to the ITER LHCD Development Plan” The use of rectangular oversized waveguides in the Main Transmission Lines (MTLs) of the Lower Hybrid Current Drive (LHCD) system of ITER, requires to investigate the problem of bends. The high number of involved waveguides (from 24 to 48) must be also taken into account. Thus, it has to consider not only the best choice in terms of curved framework, but also the proper allocation of all the waveguides. In this context, the principal specifications that characterize the design of the bends are: a) to minimize the reflection of the fundamental TE10 mode; b) to maximize the transmission of the fundamental TE10 mode; c) to minimize the coupling between the TE10 mode and other spurious modes that propagate at 5 GHz. This paper presents an overview about the bend options, and it compares the performances of several frameworks analyzed by using the Finite Element Method (FEM) commercial software, HFSS®. First of all, simple circular trajectory curves with different angulations, are considered. Then, the so called Mitre Bends alternatives are deeply analyzed. These curves are studied by several authors in the mono-modal configuration, with different techniques but the propagation in an oversized environment is a topic not much attended in literature. The only design parameter of the simple circular trajectory bend is the bending radius, so that the design is not flexible; the Mitre Bend structure is at least more flexible than the previous one and it is of great interest to study this type of bend to check the possible advantages. Finally an innovative modified Mitre Bend solution based on a cascade of trapezoidal elements is proposed

Benchmark of Coupling Codes (ALOHA, TOPLHA, and GRILL3D) with ITER Lower Hybrid Antenna

In order to assist the design of the future ITER Lower Hybrid launcher, coupling codes ALOHA, from CEA Cadarache, TOPLHA, from Politecnico di Torino, and GRILL3D (Dr.Mikhail Irzak, A.F.Ioffe Physico-Technical Institute, St. Petersburg, Russia), from ENEA Frascati, have been compared with the initial (3 modules with 8 active waveguides per module) and updated (6 modules with 4 active waveguides per module) Passive-Active Multijunction (PAM) Lower Hybrid antennas. Both ALOHA and GRILL3D formulate the problem in terms of rectangular waveguides modes, while TOPLHA is based on boundary-value problem with the adoption of a triangular cell-mesh to represent the relevant waveguides surfaces. Several plasma profiles, with varying edge density and density increase, have been adopted to provide a complete description of the simulated launcher in terms of reflection coefficient, computed at the beginning of each LH module, and of power spectra. Good agreement can be observed among codes for all the simulated profiles

Contribution to the design of the main transmission line for the ITER relevant LHCD system

In the frame of the EFDA Task HCD-08-03-01 (EU contribution to the ITER LHCD Development Plan) the preliminary design of the LHCD system, described in the Detailed Design Description DDD 5.4 LH (2001), has been revised to take into account progresses on technology and on knowledge. In particular a revised layout of the ITER machine and the successful development of a prototype klystron at 5 GHz with a target RF power of 500 kW CW have determined significant changes in the design of the main transmission lines and on the modularity of the launche

Mechanical Design Analysis of ITER-Relevant LHCD Antenna Elements

A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive Active Multijunction (PAM) concept is envisaged on ITER for steady state operation, including the current ramp-up phase. In the frame of an EFDA task, a conceptual design of such a system has been performed. This paper gives an insight of the mechanical analysis, modeling and design carried out on two elements of the antenna: the front face, and the RF windows which constitute the first tritium barrier. The front face of the antenna will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The main issues were to ensure a reasonable margin versus a maximum temperature fixed by safety concerns (650 °C for plasma facing beryllium), and to understand and assess resistance of the assembly while in operation. Analyses were conducted on some variants of this PAM. Manufacturing scenarios considering ITER requirements and involving various techniques (brazing, high isostatic pressure bonding, explosion bonding) were explored, and simulations over subsequent residual stresses were conducted. Analyses showed that surface temperatures do not exceed 500°C, and stresses should not affect the integrity of the antenna mouth. The RF window is a critical component since it also has a safety function. We planed to design a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power. An RF optimization conducted on this concept allowed to combine good RF properties and low dielectric losses. It resulted in a maximum thermal stress in the ceramic of 31 MPa, well below the static fatigue limit of 50 MPa. The residual stresses resulting from the brazing methods involved in the manufacturing of this multi-material assembly (copper, copper alloys and ceramic) were also analyzed. Both studies allow us to move forward and focus on critical issues, such as manufacturing processes (beryllium, bonding techniques…) and R&D associated programs including test of mock-ups

RF modeling of the ITER-relevant lower hybrid antenna

In the frame of the EFDA task HCD-08-03-01, a 5 GHz Lower Hybrid system which should be able to deliver 20 MW CW on ITER and sustain the expected high heat fluxes has been reviewed. The design and overall dimensions of the key RF elements of the launcher and its subsystem has been updated from the 2001 design in collaboration with ITER organization. Modeling of the LH wave propagation and absorption into the plasma shows that the optimal parallel index must be chosen between 1.9 and 2.0 for the ITER steady-state scenario. The present study has been made with n|| = 2.0 but can be adapted for n|| = 1.9. Individual components have been studied separately giving confidence on the global RF design of the whole antenn