Index
Introduction
The Research at Rijnhuizen
Results in 2008
Education, Training, Outreach and Public Information
Output
Appendix
website Rijnhuizen
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3.2 | Manipulation of mesoscale structures in hot, magnetized plasmas
Coordinator: Prof. Dr. N.J. Lopes Cardozo
Funding*: FOM Programme 74
Objective: The exploration and development of the concept that hot plasmas have a rich structure on the mesoscale with a significant impact on plasma dynamics and performance. The major objectives are the understanding and control of this structure. The objectives are mainly to be achieved through high-resolution measurements and manipulation of this structure in experiments, along with numerical magneto-fluid simulations and the advancement of relevant theoretical models.
This FOM-programme started in January 2004. The experimental component is carried out at TEXTOR, JET and other (mostly) European fusion devices in a variety of collaborations. The emphasis is on TEXTOR, which with the recent hardware additions (the Dynamic Ergodic Divertor (DED), ECRH and turbulence diagnostics) is particularly suited for this programme. The expertise that Rijnhuizen has built up in ECRH and diagnostics is also employed in studies for ITER systems, carried out in the frame of the ITER-NL consortium and in collaboration with partners in the EU, partly under EFDA contracts.
The following subjects are thus distinguished:
• Turbulence and nonlinear plasma dynamics. In particular: Transport in (DED-induced) stochastic fields, magnetic reconnection, disruption recognition and mitigation.
• Interaction of microwaves with plasmas. In particular: Electron, Cyclotron Resonance Heating (ECRH) and Current Drive (ECCD), stabilisation of (DED-induced) tearing modes, sawtooth control, collective Thomson scattering (with Risø and MIT), development and implementation of a system for real time tracking and suppression of magnetic modes, MHD modeling for mode suppression, many wave processes in plasmas that are triggered by EC-waves and Fokker-Planck calculations on the properties of the EC-beam in ITER relevant conditions.
• Diagnostic development. In particular: high-resolution Thomson scattering, microwave imaging diagnostics, Charge Exchange Recombination spectroscopy (CXRS) and measurements of the current distribution using the Motional Stark Effect (MSE).
• ITER R&D. In particular: the design (and later procurement) of the ECRH upper port launcher system and ITER diagnostics with strong emphasis on the CXRS system.
* supported by the European Fusion Programme (EFP)
In 2008, the programme saw very good progress in all fields, with the demonstration of mode stabilisation with ECRH, studies of transport in and around magnetic islands using modulated ECRH and 2-dimensional imaging diagnostics, design studies and proof of principle for the ITER CXRS system, and the demonstration of the bi-directional steerable ECRH antenna, enabling the measurement of microwave emission from the plasma at the nanowatt level through an antenna carrying hundreds of kilowatts of heating power in the same frequency domain. The following sections summarize the progress in 2008 in the subfields listed above.
3.2.1 Turbulence and nonlinear plasma dynamics
ITER integrated scenario modelling
Under the EU Integrated Tokamak Modelling Task force, an ITER Integrated Scenario (ISM) Working Group was established in 2007. FOM staff members participate in this Working Group, contributing in two subfields of this working group.
The first contribution is in the modelling of the about 100 seconds lasting start-up and ramp-down (current rise and current decay) phases of ITER. This phase is very important, because at the end of this phase the right target profiles of density, electron and ion temperature and current density have to be obtained, as needed for the various ITER scenarios. Moreover, the route through this phase has to avoid instabilities and optimise costs in terms of additional heating and flux consumption. Within the ISM, and in close collaboration with the ITER team, the boundary conditions for the simulations were established, i.e. regarding density, impurity content, transport model and plasma shape. One study investigated the effect of additional heating during the current rise phase. Figure 3.1 shows the effect on the plasma of 10 or 20 MW of Electron Cyclotron Resonance Heating, applied at mid-radius. Apart from increasing the plasma temperature, the heating delays the current penetration, as visualised in a higher magnetic winding number q at the end of the current rise phase. The latter effect is considered beneficial for the stability of the plasma. This contribution has been reported both at the EPS and at the IAEA conference in 2008.
Figure 3.1: Profiles of magnetic winding number (q, left, full lines) and electron and ion temperature (Te, Ti, right, full/dashed lines, respectively) at the start of the current flat-top phase at 100 s for cases with 0 (red), 10 (green) and 20 MW (blue) of ECRH power applied at mid-radius. The left hand panel also shows the self-generated plasma current density, the so-called bootstrap current; a higher bootstrap current helps to save flux consumption and may also help to keep a favourable q profile.
The second contribution is in the predictive modelling of full ITER discharges, as prescribed by given reference scenarios. This is a much more complicated task due to the interactions between the various processes in the plasma. E.g., the fusion reactions create a population of fast particles, which additionally heat the main plasma, but also may drive instabilities. This work is in progress, and results will be reported in 2009.
JET experiments
The strong commitment of FOM with JET was continued in 2008. FOM scientists took part in experiments aiming at the optimisation of the so-called hybrid regime in JET. The hybrid regime is characterized by an edge transport barrier, as in the standard H-mode, plus an optimised current density profile. The latter improves the stability of the discharge. In smaller machines it was found that hybrid discharges have a further improvement of global confinement compared to standard H-mode. In the past this improvement was not found at JET. The 2008 experiments aimed at finding and hopefully curing this difference with smaller machines. Indeed, careful plasma shaping and an adapted initial phase of the discharge have now lead to a 30-40% enhancement over standard H-mode.
Transient heat transport in TEXTOR
The strongly enhanced repetition rate of the Thomson Scattering diagnostic at TEXTOR has made it possible to follow the evolution of electron temperature and density (Te, ne) after switch-on and switch-off of ECRH with unprecedented time and radial resolution. Some experiments with off-axis ECRH were performed. They confirmed the transient reduction of thermal transport during about one confinement time after switch-off of ECRH that was found earlier; however, the improved measurements now allow for better insight in the physical process.
TEXTOR – T-10 - SWIP collaboration
After the mission in November 2007 to the South Western Institute of Physics (SWIP) in Chengdu, China, no new visits took place. The experiments in the HL-2A tokamak at SWIP of November 2007 were analysed and reported at the IAEA conference. A new mission is foreseen in 2009. The HL-2A tokamak at SWIP is similar in size as TEXTOR; due to the presence of 4 independent gyrotrons and Lower Hybrid heating, it offers experimental possibilities beyond the scope of TEXTOR.
3.2.2 Interaction of microwaves with plasma
For fusion reactors, based on the principle of magnetic confinement, it is important to avoid so-called magnetic islands or tearing modes. They reduce confinement and can be the cause of major disruptions. One class of magnetic islands is that of the neoclassical tearing mode (NTMs). NTMs form due to the dynamics between the pressure flattening across the mode, and the associated loss of ‘bootstrap current’.
Control and suppression of this mode can be achieved by means of electron cyclotron waves (ECW), which allow for depositing highly localized power at the island location. The ECW power replenishes the missing bootstrap current by generating a current perturbation either inductively, through a temperature perturbation (ECRH), or non-inductively by direct current drive (ECCD). Both effects (heating and current drive) have been applied successfully to experiments, showing a predominance of heating (ECRH) for medium size limiter tokamaks (such as TEXTOR, in Juelich, Germany) and of current drive (ECCD) effect for mid-to-large size divertor tokamaks (such as ASDEX-UpGrade, Garching, Germany).
The conditions determining their relative importance are still unclear. A numerical study has been applied to isolate the contributions of heating and current drive to the NTMs temporal evolution as described by the modified Rutherford equation. The effects of both heating and current drive can be described by simple analytical expressions in terms of an efficiency fore-factor times a “geometrical” term depending on the power deposition width (wdep), location and modulation. For medium sized tokamaks (TEXTOR, ASDEX-UpGrade, Garching, Germany) the heating and current drive efficiencies are of the same order of magnitude, whereas in a future, large reactor like ITER the current drive efficiency is expected to be significantly larger.
A significant effort has been invested in the developement and testing of a real-time tearing mode track-and-suppress system for TEXTOR. Special emphasis is put on the tearing mode sensor used to determine the radial location and phase of the tearing modes. The feedback signal is obtained from electron cyclotron emission (ECE) measurements taken along the identical line of sight as traced by the incident ECCD millimeter-wave beam, but in reverse direction. Experiments on TEXTOR have demonstrated the proof of principle of this approach.
In some cases, however, the in-line ECE signal is disturbed. The feed-back signal can be processed in real time to eliminate the perturbations. The nature of these perturbations is unclear, but evidence, obtained from a dedicated diagnostic, strongly suggests that a new interaction between the magnetic islands and the high power mm-wave beam has been observed.
Different positions in the plasma correspond to different Electron Cyclotron frequencies, because of the change of magnetic field strength along the radial direction in the torus. By looking at the ECE signal at frequencies on opposite sides of the 140 GHz ECRH frequency, we are in fact looking at positions to the direct left and right of the location where the EC power is deposited. In that way, the response of the plasma to the power injection can be measured, and the feedback control loop can be established.
On the TEXTOR tokamak, a dedicated ECE diagnostic has been implemented for control of the ECRH power deposition on the basis of this principle. First measurements show that the signal allows the clear identification of, for example, the position of a rotating magnetic island, and the sawtooth inversion radius.
3.2.3 Diagnostic development
The Instrumentation Development Group of FOM develops high resolution multi-channel diagnostics with the aim to diagnose small scale structures in hot magnetised plasmas. In the past 10 years, a large fraction of its effort was applied at TEXTOR, but now activities have been initiated at ASDEX-UpGrade, Garching, Germany and JET as well. The goal is to advance novel concepts in optical and microwave diagnostics, as well as expanding and using the available expertise on optical diagnostics for application on ITER and support of the fusion physics programme. The main instrumental results of 2008 were the completion of the multipass Thomson system at TEXTOR, the modification of the ECEI system for ASDEX-UpGrade, Garching, Germany, the beam emission measurements at JET and the pilot test of the coherent imaging MSE system in collaboration with ANU. These diagnostic improvements are highlighted here, while the physics results with these diagnostics are described in the corresponding physics sub-sections.
Thomson scattering system
The Thomson scattering system at TEXTOR has some unique features: it is an intra-cavity system, with an unsurpassed high spatial resolution (< 1 cm) and repetition frequency (5 kHz, up to 45 pulses). Two observation geometries can be used: one in which the full chord through the plasma is imaged and a dedicated system for high resolution observations at the plasma edge, with even superior resolution parameters (down to 2 mm).
The latest extension of the system incorporated the multi-pass capability. The installation of two spherical mirrors in the laser path allowed for multiple passages of the laser light through the plasma. Operation with 12 passes has been accomplished, leading to an effective probing energy of 60 J per pulse and 2.5 kJ per laser burst. With these settings, the experimentally confirmed accuracy in electron temperature and density is typically 2 % and 1 % respectively in each pulse.
With these additions the system is well suited for following fast dynamics in the electron temperature and density. As an illustration, Figure 3.2 shows the small variations in the temperature evolution during a sawtooth crash. Here, both the precursor oscillations (just before the crash) as well as the heat pulses following the crash can be recognized. This first application of the multi-pass intracavity laser probing in a medium sized tokamak proves the high perspectives of this approach in Thomson scattering diagnostics on most fusion plasma devices in the world.
Electron cyclotron emission - imaging diagnostic
In recent years, the flagship of the FOM diagnostics has become the Electron Cyclotron Emission Imaging (ECEI) system, developed in collaboration with the US partners from UC – Davis and Princeton. This system at TEXTOR is now in routine operation, providing unique insights into the temperature evolution inside magnetic structures in the plasma. The system combines the advantages of a wideband radiometer and ‘classical’ ECE systems to provide a true 2D image of Te fluctuations with a total of 128 channels, arranged in a matrix of 8 (horizontal) × 16 (vertical) sample volumes.
Figure 3.2: Result of the Thomson scattering diagnostic during a sawtooth period. Depicted on the left is the averaged temperature profile as well as the variation on this for each laser pulse. In the contour plot on the right of the variations, the precursor oscillations just before the crash (at t=1.507s) are recognized and they are in phase with the ECE-signal depicted in black. Immediately after the crash, both the cooled centre and the outward heat pulse are visible.
Following the success of this system and impact in the MHD field, application of this system on bigger, more ITER-relevant machines is the logical next step. Therefore a project has been initiated to transfer this system to ASDEX-UpGrade, Garching, Germany for the study of edge instabilities (ELMs) and neoclassical tearing modes. In the reporting year, modifications to the TEXTOR system have been undertaken with this transfer in mind. The main improvements to the system are in the optical path, which now allows for a wider field, zoom capability and better focusing, in the process even simplifying the set-up. Moreover, the 16-channel microwave array has been equipped with mini-lenses in front of every single antenna element, resulting in dramatic increases in sensitivity, RF-bandwidth (increased tunable radial coverage) and ECRH-shielding.
The research using the ECEI system over the past period concentrated on the sawtooth instability. To illustrate the capability of the system, snapshots of the pre-cursor oscillation are depicted in Figure 3.3. The question to be answered here is whether the sawtooth instability results from either a resistive kink mode or a more ideal mode like the quasi interchange mode. By observing the shape of the precursor and/or the hot core, this question should
Figure 3.3: Contour distribution of the normalised electron temperature ΔTe, Norm(R,Z) prior and during the sawtooth crash at TEXTOR. The time difference between the frames is Δt = 20 ms. The color scaling goes from green, through yellow up to red. The small solid circle denotes the estimated plasma center around which the hot core rotates. The dashed blue line denotes the estimated inversion radius. The small black crosses denote channels that are evaluated for the analysis. The sawtooth crash occurs in the 10th picture.
in principle be answerable. A problem here is that a full poloidal view is necessary. ECEI can almost provide this. From Figure 3.3 the displacement of the core seems to be due to a kink mode. Whether this is an ideal or a resistive mode is still the subject of investigation.
Active beam spectroscopy
A large fraction of the work in diagnostics is devoted to the application on a Charge Exchange Recombination Spectroscopy (CXRS) system on ITER. A pilot of such a system is operational on TEXTOR and has been extended with the measurement of the beam emission (BES) and the motional Stark (MSE) spectrum, together referred to as Active Beam Spectroscopy. The CXRS system is mainly regarded as a standard diagnostic on TEXTOR to provide the profiles of ion temperature, plasma rotation and impurity content. Apart from that, a research programme has been setup up to prepare for the ITER measurements such as to i) test whether the system is capable of providing additional parameters like the fast ion content ii) validate the assumptions made in the ITER simulations on signal levels and iii) test methods to deduce the impurity concentration with larger accuracy, which is dependent on the measurement of the beam density.
The high throughput of the system allows, besides more accurate measurements, also the observation of hitherto inaccessible features with the TEXTOR CXRS-system. For instance, the slowing down spectrum of the fast hydrogen ions injected by the beam can now be quantitatively resolved. Efforts to deduce the beam density from BES measurements have been extended to JET data as well. Both for TEXTOR and JET discrepancies between code calculations of the attenuation and measurement of the beam density exist and are of the order of 20-40 %.
In collaboration with the group from the Australian National University, an innovative technique has been applied to measure the motional Stark effect: coherence imaging. With this system the polarisation of the beam emission is measured in a 2D field of view, allowing for deduction of the direction of the magnetic field and the related current density in the plasma. This parameter is the basis of nearly all plasma physics phenomena in the confined plasma core, but an accurate measurement of it is problematic, let alone a 2D imaging of it. The ANU system, however, has the potential to provide this information. During a first test campaign promising results have been obtained (see Figure 3.4) and its development will continue further.
Figure 3.4: The hybrid system of the spectra-polarimeter for the coherence imaging of the Motional Stark Effect of the beam emission at the Balmer-alpha wavelength. The quarter wave plate and the FLC constitute the polarimeter. The primary delay plate and the polarizer provide spectral discrimination. The Savart plate imprints a sinusoidal spatial carrier wave in the x-direction. After the focal plane the light is transferred by a coherent fiberbundle to a CCD camera with a broadband interference filter in front.
3.2.4. ITER R&D: ITER-NL, ECRH and Diagnostics
In August 2006, an amount of 15 million Euro was granted for the period 2007 - 2009 from the Dutch Fund for Economic Structure Strengthening (FES) to the programme ‘A frontline Dutch contribution to ITER’. The goal of this programme is to achieve a prominent role for Dutch industry and for Dutch scientists in the ITER project. The programme is executed since 1 January 2007 by the ITER-NL consortium in which FOM, NRG en TNO collaborate.
In 2008, the ITER-NL project has been carried out according to plan and has made good progress. For both the upper port CXRS system (known as the Upper Port Viewer - UPV) and the upper port ECRH system (known as the Upper Port Launcher - UPL), ITER-NL closely collaborates with its future international consortium partners. The formation of the CXRS and ECRH consortia is, however, progressing slower than originally anticipated because the future partners do not always have the same view of what should be arranged in the consortium agreements. Another important reason for delay comes from the fact that Fusion for Energy (F4E) is not yet in a position to discuss, with various consortia that are being established, what should be arranged in consortium agreements and what should be arranged in the contracts between the consortium and F4E.
Good scientific results have been achieved in both the CXRS and ECRH packages. With the pilot-CXRS system at TEXTOR, it was demonstrated that fast ions can be measured, thus demonstrating the potential of this diagnostic for alpha particle measurements in ITER – one of the priority topics within the EFDA programme. The measured data agree well with results from a simulation code and have contributed to obtaining a better knowledge of atomic cross sections. In the CXRS work package attention was also devoted to neutronics calculations, spectrometer design and fibre bundle design. A prototype spectrometer will be procured by ITER-NL in close collaboration with Dutch industry. In the ECRH work package, the in-sightline ECE system, implemented to give a feedback signal to the ECRH launcher and gyrotrons, has been successfully taken into operation at TEXTOR, and has led to interest from IPP-Garching to develop a similar, but in-waveguide system, for ASDEX-UG. First tests of a system for the ASDEX system were successful. In this work package much work has also been focused on remote handling procedures and tools for the upper port plugs and the ECRH testbed.
The results in the ITER-NL technology transfer packages have exceeded expectations. Many Dutch industries have become interested in ITER, thanks to, amongst others, the active public relations-programme executed by ITER-NL. At this moment, six business cases with industry have been started and are on-going, while the seventh is about to be kicked off.
ITER-NL can look back to an extremely successful economic mission to ITER Cadarache in February 2008, under the leadership of the Minister of Economic Affairs, Mrs. Van der Hoeven. The industry has rated the mission very positively. A total of 90 match-making discussions between the 17 Dutch companies that joined the mission and French companies took place. One of them has already led to a substantial spin-off order. Several other companies are still in discussion with their French partners and have good hopes that joint projects will emerge soon. At the end of 2008, ITER-NL was contacted by the French industry platform, who indicated that they are very interested in a return mission in the summer of 2009. As it stands now this mission will also be at the ministerial level.
As a result of the economic mission, but also thanks to industrial exhibitions, the contacts between ITER-NL and F4E are good. The ITER-NL council has made a valuable visit to the director of F4E in July 2008 and the director of F4E, Didier Gambier, has indicated that ITER-NL is an example of how an industrial platform should be set up, also mentioning that the ITER-NL example should be followed by other countries. ITER-NL is now working very closely with the Technical Scientific Attaches stationed at the Dutch Embassies across Europe in order to create an international European industrial network (with Dutch companies in a central role).
A 2-day course in technical and economic issues for European industrialists was organised for Dutch industry with emphasis on the French (nuclear) regulations, both generic as well as specific for ITER. The course was given by French experts from APAVE and AREVA. People from 9 companies attended the course and rated it as being very useful. Additionally courses in CATIA V5 were organised for engineers from Dutch companies.
Figure 3.5: Group photo made during the economic mission under leadership of Minister Maria van der Hoeven in February 2008.
ITER-NL organized industrial exhibits at the ITER Business Forum ‘07 in Marseille, and at the SOFT Conference ‘08 in Rostock. The first meeting led to the start of the economic mission mentioned above. At the SOFT Conference, the Dutch industry gave quite a good appearance. Philips delivered a number of isotropic tungsten blocks to F4E for testing purposes, and various other companies succeeded in forging good business links with foreign companies. On 28th of October ITER-NL, organised the annual industry day, combined this time with the evaluation of the economic mission. The Minister of Economic Affairs, Mrs. Van der Hoeven, gave an inspiring speech and demonstrated satisfaction with the operation of ITER-NL. The industrialists filled out a questionnaire and rated the usefulness and results of ITER-NL thus far with an 8 (on a scale of 1 to 10).
Many items on ITER-NL have appeared in Dutch press (newspapers as well as television and radio). The ITER-NL website has been extended and renewed to make it more accessible for industry. Parts of the website are translated into English, with a possible translation into French under consideration. Calls for tender, calls for grants and calls for expression of interest from F4E and ITER are posted on the website, and are also directly send to companies that might potentially be interested.
3.2.5 Computational magnetohydrodynamics
TEXTOR experiments on the suppression of tearing modes showed that the efficiency of the suppression is dominated by the effect of the island heating and is largely independent of the current driven in the island. This is consistent with the relatively low current drive efficiency under the conditions of the TEXTOR experiments. The wave injection in TEXTOR was not optimised towards achieving the highest possible current drive efficiency, but rather to optimise the driven current density. Further theoretical comparison of the efficiencies of heating and current drive for tearing mode stabilisation was provided in 2008. The equation for the island evolution (the Rutherford equation) has been evaluated numerically. For TEXTOR experiments, it is confirmed that heating is by far dominant. Figure 3.6 compares the dimensionless quantities FH and FCD for a saturated island of width w, assuming a Gaussian power deposition profile. The calculations are static: The EC-beam is centered at the O-point, with a constant deposition width, wdep. No modulation of the EC-power is applied. For small values of w* =w/wdep (in ITER w* ~ 0.5), FH / FCD is about 2.6. This suggests that heating can help to suppress the island below its critical value, in the regime where current drive looses its efficiency.
Figure 3.6: Comparison of the efficiency of tearing mode stabilization due to local current drive (ECCD) and heating (ECH).
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