T. Franke

Status of the new multi-frequency ECRH system for ASDEX Upgrade

Summary form only given. The first two-frequency GYCOM gyrotron Odissey-1 has been installed and put into operation in the new multi-frequency ECRH system at the ASDEX Upgrade tokamak experiment. It works at 105 GHz and 140GHz with output power 610kW and 820kW respectively at a pulse length of 10s. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140GHz. Such gyrotrons will require broadband vacuum windows. Construction and cold tests of a first broadband double-disc toms window are completed. The transmission to the tonis is in normal air, through corrugated aluminum waveguides with I.D.=87mm over a total length of about 70m. Calorimetric measurements gave a total transmission loss of only 12% at 105GHz and 10% at 140GHz. The variable frequency will significantly extend the operating range of the ECRH system, e.g. allow for central heating at different magnetic fields. Other experimental features, like the suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. The stabilization of NTMs requires a very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this reason fast movable launchers have been installed.

Plans for a new ECRH system at ASDEX Upgrade

Abstract A new ECRH system is being constructed for ASDEX Upgrade with a total power of 4 MW, generated by four gyrotrons, and a pulse duration of 10 s. Particular features are the use of gyrotrons which can work at various frequencies in the range 104–140 GHz and correspondingly broad band transmission components. The transmission will be at normal air pressure, and at the torus we will have a tunable double disk vacuum window. A further aim is the installation of fast moveable mirrors for a feedback controlled localized power deposition.

Ion energies and currents of Type I and mitigated ELMs in the ASDEX Upgrade far scrape-off layer

New measurements of ion energies and currents in type I and mitigated ELMs have been carried out in the ASDEX Upgrade far scrape-off layer using a retarding field analyser (RFA). The ion temperature averaged over an ELM, Ti?ELM measured 35?60?mm outside the separatrix (i.e. 15?25?mm in front of the outboard limiter) is in the range 20?200?eV, which is 5?50% of the ion temperature at the pedestal top. Ti?ELM decreases with the separatrix distance with the e-folding length of ~10?mm measured in the far SOL for a particular set of conditions, and increases with the ELM energy WELM. Lowest Ti?ELM is measured during mitigated type I ELMs. Likewise, the ELM-averaged ion current e-folding length increases with WELM, similar to the e-folding length of the heat flux density at the RFA probe head during an ELM, monitored by a fast IR camera. The most plausible explanation of observed trends is that on average the filaments of larger ELMs travel faster radially and have less time to dilute by parallel losses along field lines before reaching the far SOL. These observations provide further evidence that the fraction of the ELM energy deposited on the main chamber plasma-facing components increases with WELM.

Preliminary conceptual design of DEMO EC system

In the framework of EUROfusion Consortium the Work Package Heating and Current Drive addresses the engineering design and R&D for the electron cyclotron, ion cyclotron and neutral beam systems. This paper reports the activities performed in 2014, focusing on the work done regarding the input for the conceptual design of the EC system, particularly for the gyrotron, the transmission line and the launchers.

Fast wave current drive in DEMO

The ability to non-inductively drive a large fraction of the toroidal plasma current in magnetically confined plasmas is an essential requirement for steady state fusion reactors such as DEMO. Besides neutral beam injection (NBI), electron-cyclotron resonance heating (ECRH) and lower hybrid wave heating (LH), ion-cyclotron resonance heating (ICRH) is a promising candidate to drive current, in particular at the high temperatures expected in fusion plasmas. In this paper, the current drive (CD) efficiencies calculated with coupled ICRF wave / CD numerical codes for the DEMO-1 design case (R0=9m, B0=6.8T, ap=2.25m) [1] are presented. It will be shown that although promising CD efficiencies can be obtained in the usual ICRF frequency domain (20-100MHz) by shifting the dominant ion-cyclotron absorption layers to the high-field side, operation at higher frequencies (100-300MHz) has a stronger CD potential, provided the parasitic RF power absorption of the alpha particles can be minimized.

Review of the Innovative H&CD Designs and the Impact of Their Configurations on the Performance of the EU DEMO Fusion Power Plant Reactor

Heating and current drive (H&CD) systems are being investigated for a demonstration fusion power plant DEMO to deliver net electricity for the grid around 2050. Compared to ITER, which has to show the generation of 500-MW thermal power, the target of DEMO is the successful production of 300 to 500 MW electrical power to the grid and to aim for a self-sufficient tritium fuel cycle. Three H&CD systems are under development for DEMO in Europe, the electron cyclotron (EC) system, the neutral beam injection (NBI) system, and the ion cyclotron system. Based on present studies for plasma ramp-up, ramp-down, and flat top phases, to be further validated in more detailed simulations, the assumed total launched power needed from the H&CD system in DEMO is in the range of 50–100 MW, to be provided for plasma heating and control. This paper describes the design and Research and Development status of selected H&CD systems, considered for their deployment in the EU DEMO. It was always considered that different H&CD configurations and design variants will have an impact on the performances for the whole fusion plant. It shall be noted that the basis for the H&CD integrated design and system development is the actual version of the European fusion electricity roadmap. The project also elaborates on H&CD efficiency improvements which will reduce the recirculating power fraction in the future fusion power plants. Different studies under investigation will be discussed such as for NBI the photoneutralization and for EC novel concepts for gyrotron multistage-depressed collector.

Commissioning of the second two-frequency gyrotron in the new multi-frequency ECRH system for ASDEX upgrade

The power deposition in the plasma is primarily determined by the magnetic field B(r). For a single frequency ECRH system this has the consequence that for central heating the magnetic field is no longer a free parameter. However, for plasmas with different plasma currents or different equilibria, the magnetic field should be a free parameter in order to operate at a reasonable edge safety factor q(a). Furthermore, in a plasma with given parameters, some experimental features, like suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. These requests can be satisfied if the gyrotron frequency is variable . In the experiments performed up to now in ASDEX Upgrade, the installed power was only 2 MW, of which 1.6 MW was coupled to the plasma. This imposes a limit for current drive, NTM stabilization or generation of internal transport barriers . The requirement for the new ECRH system is therefore an installed power of 4 MW. Since the current diffusion time in hot plasmas, like those with an internal transport barrier and Te > 10 keV, is several seconds, we need a pulse duration of 10 s compatible with the limit of ASDEX Upgrade flat top discharges. A further requirement is the capability for very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this purpose fast movable mirrors have been installed.

On the present status of the EU demo H&CD systems, technology, functions and mix

Under the umbrella of the EUROfusion Consortium and within the Power Plant Physics and Technology (PPPT) Conceptual Design Activities, the project Heating and Current Drive (H&CD) conducts a number of design activities and developments for a next generation clean and environmental friendly, long pulsed (~2 hours) Demonstration fusion power plant (DEMO). This paper covers the results of the most important state-of-the-art and cutting edge technologies for the H&CD systems, as defined in the European Fusion Roadmap and in more detail specified in the Annual Work Plans (AWPs) in the Work-Package H&CD (WPHCD): (i) Gyrotron developments up to 240 GHz with multi-stage-depressed collector (MSDC) energy recovery for the Electron Cyclotron (EC) system; (ii) Neutral beam (NB) injector investigations with gas or alternatively photo-neutralization in the range of 25 - 35 MW as a modular 1 MeV injector type with reduced Cs consumption sources or alternatively volume-production based non-Cs sources; (iii) Ion Cyclotron (IC) antenna conceptual design for a distributed antenna, representing a new type of design and a transition from the commonly used port plugged antennas.

Multi-frequency ECRH system at ASDEX upgrade

A new ECRH system is currently under construction at the ASDEX Upgrade tokamak. It employs for the first time depressed collector gyrotrons, step-tunable in the range 105–140 GHz. In its final stage it will consist of 4 gyrotrons with a total output power of 4 MW and a pulse length of 10 s. In this paper we describe recent extensions of the system and some experimental results.

Progress with the new multi-frequency ECRH system for ASDEX upgrade

Summary form only given. A multi-frequency ECRH system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step-tunable in the range 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron is in routine operation at ASDEX Upgrade since 2006. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140 GHz. They will be equipped with broadband Brewster windows. Since the transmission of the power from the gyrotron to the tokamak is in normal air, a second broadband vacuum window is required at the tokamak vacuum barrier. These windows must be broadband also for elliptically polarized beams. Therefore tunable double disc CVD diamond windows will be installed at the torus. To condition and test the gyrotrons a new long-pulse dummy load, capable of 1 MW, 10 s and operation in normal air has been operated very reliably. One of the main applications of the new ECRH system will be the suppression of neoclassical tearing modes (NTM). For this reason fast-steerable mirrors have been installed. This capability will allow feedback control of the deposition on the time scale of NTM growth, providing the possibility to validate this scheme for ITER in ASDEX Upgrade. For NTM stabilization experiments a fast modulation capability of the gyrotrons is required and was tested. This is especially important for future experiments like ITER where the width of the driven EC current will be larger than the marginal island size of the NTM leading to a loss of current drive efficiency in the non-modulated case.