Tokamak A Configuration Variable Research Articles

Overview of recent and current research on the TCV tokamak

Through a diverse research programme, the Tokamak à Configuration Variable (TCV) addresses physics issues and develops tools for ITER and for the longer term goals of nuclear fusion, relying especially on its extreme plasma shaping and electron cyclotron resonance heating (ECRH) launching flexibility and preparing for an ECRH and NBI power upgrade. Localized edge heating was unexpectedly found to decrease the period and relative energy loss of edge localized modes (ELMs). Successful ELM pacing has been demonstrated by following individual ELM detection with an ECRH power cut before turning the power back up to trigger the next ELM, the duration of the cut determining the ELM period. Negative triangularity was also seen to reduce the ELM energy release. H-mode studies have focused on the L–H threshold dependence on the main ion species and on the divertor leg length. Both L- and H-modes have been explored in the snowflake configuration with emphasis on edge measurements, revealing that the heat flux to the strike points on the secondary separatrix increases as the X-points approach each other, well before they coalesce. In L-mode, a systematic scan of the auxiliary power deposition profile, with no effect on confinement, has ruled it out as the cause of confinement degradation. An ECRH power absorption observer based on transmitted stray radiation was validated for eventual polarization control. A new profile control methodology was introduced, relying on real-time modelling to supplement diagnostic information; the RAPTOR current transport code in particular has been employed for joint control of the internal inductance and central temperature. An internal inductance controller using the ohmic transformer has also been demonstrated. Fundamental investigations of neoclassical tearing mode (NTM) seed island formation by sawtooth crashes and of NTM destabilization in the absence of a sawtooth trigger were carried out. Both stabilizing and destabilizing agents (electron cyclotron current drive on or inside the q = 1 surface, respectively) were used to pace sawtooth oscillations, permitting precise control of their period. Locking of the sawtooth period to a pre-defined ECRH modulation period was also demonstrated. Sawtooth control has permitted nearly failsafe NTM prevention when combined with backup NTM stabilization by ECRH.

Bifurcated helical core equilibrium states in tokamaks

Tokamaks with weak to moderate reversed central shear in which the minimum inverse rotational transform (safety factor) qmin is in the neighbourhood of unity can trigger bifurcated magnetohydrodynamic equilibrium states, one of which is similar to a saturated ideal internal kink mode. Peaked prescribed pressure profiles reproduce the ‘snake’ structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state. Snake equilibrium structures are computed in simulations of the tokamak à configuration variable (TCV), DIII-D and mega amp spherical torus (MAST) tokamaks. The internal helical deformations only weakly modulate the plasma–vacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.

Understanding the core density profile in TCV H-mode plasmas

Results from a database analysis of H-mode electron density profiles on the Tokamak à Configuration Variable (TCV) under stationary conditions show that the logarithmic electron density gradient increases with collisionality. By contrast, usual observations of H-modes showed that the electron density profiles tend to flatten with increasing collisionality. In this work it is reinforced that the role of collisionality alone, depending on the parameter regime, can be rather weak, and in these dominantly electron heated TCV cases, the electron density gradient is tailored by the underlying turbulence regime, which is mostly determined by the ratio of the electron to ion temperature and that of their gradients. Additionally, mostly in ohmic plasmas, the Ware-pinch can significantly contribute to the density peaking. Qualitative agreement between the predicted density peaking by quasi-linear gyrokinetic simulations and the experimental results is found. Quantitative comparison would necessitate ion temperature measurements, which are lacking in the considered experimental dataset. However, the simulation results show that it is the combination of several effects that influences the density peaking in TCV H-mode plasmas.

Recent results and prospects for correlation ECE measurements on TCV

The Tokamak a Configuration Variable (TCV) has a two channel correlation electron cyclotron emission (CECE) radiometer with a line of sight perpendicular to the magnetic field. The antenna pattern of this radiometer limits resolution to k < 112 m -1 or ks < 0.3 at 500 kHz. It can access the region with minor radius vol < 0.7. A series of measurements has been made of the turbulence spectra at positive and negative triangularity and as a function of collisionality. Also, a series of measurements has been made as a function of poloidal angle, by varying the plasma vertical position with respect to the antenna, the measurements being made on the same flux surface. It is planned to extensively upgrade the diagnostic by integrating four more channels and acquiring a new front-end for the radiometer. This upgrade should reduce the required number of shots for a radial scan by a factor four and improve the signal-to-noise ratio. It is also planned to use a high gain antenna that will extend access to kθ < 174m -1 or kθs < 0.5 at 500 kHz. The present system, measurements and the upgrade are described in this paper.

Dependence of L-mode confinement on the electron cyclotron power deposition profile in the TCV tokamak

Scenarios with different electron cyclotron heating power profile distributions and widths were compared for the first time in experiments on the Tokamak à Configuration Variable (TCV). The heating profile was changed from shot to shot over a wide range from localized on-axis, with normalized minor radius half-width at half maximum σ1/2 ∼ 0.1, up to a widely distributed heating power profile with σ1/2 ∼ 0.4 and finally to a profile peaked far off-axis. The global confinement, MHD activity, density, temperature and electron pressure profile evolution were compared. In particular, the energy confinement properties of discharges with localized on-axis heating and distributed on-axis heating were very similar, with degradation close to that predicted by the ITER L-mode scaling; in the case of off-axis heating, on the other hand, the confinement degradation was even stronger.

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.

Using APCS for Plasma Vertical Control at TCV

Early tokamaks with circular cross-section plasmas were vertically stable in nature and no problems were reported on the control of the vertical position. However, the concept of vertically elongated plasma cross-section, with benefits to the energy confinement time, led to vertical instabilities. To overcome this problem, complex closed feedback loop control systems with a vertical position measurement, signal processing, control algorithm, power supplies and active actuating coils are used. The Tokamak a Configuration Variable (TCV) permits the stabilization of highly elongated plasmas, with a powerful vertical position control system and fast power supplies capable of response times under 0.1 ms. The introduction of the new Advanced Plasma Control System (APCS) in TCV has required the development of a new vertical position observer built to optimally exploit the capabilities of the new system and correct for the changes that were detected by the introduction of signal digitizing in the feedback loop. This paper discusses the need for a new measurement of the plasma vertical position, the method to build the observer and the validation of the measured position by comparing with previous methods and observers. A summary of the ongoing work is provided. We also discuss its aim and the goals that are expected to be achieved by exploring the new digital non-linear control algorithm capabilities of APCS.

Physics design of a saddle coil system for TCV

Abstract The upgrade project for TCV (Tokamak à Configuration Variable) includes the installation of a set of saddle coils, namely the saddle coil system (SCS), located and powered such as to create a helical magnetic perturbation. Using independent power supplies, the toroidal periodicity of this perturbation is tunable, allowing simultaneously edge localized modes (ELM) control through resonant magnetic perturbation (RMP), error field correction and vertical control. Other experimental applications, like resistive wall mode and rotation control, are also in view. In this article, the adequacy of two SCS designs, an in-vessel one and an ex-vessel one, is assessed with respect to the desired experimental applications. The current requirements and the system performances are also characterized. The conducting vessel wall is accounted for in a model used to determine the coupled response functions of the SCS, the screening of the magnetic perturbation by the wall, the induced voltages and currents during a plasma disruption and the maximal magnetic forces exerted on the SCS. A scaling of the SCS parameters with the number of coil turns is presented and the issue of coil heating and cooling is discussed.

Use of webcams as tools for alignment and supervision of a Thomson scattering system in the near infrared

The alignment stability is a major concern for Thomson scattering systems. Even small angular deviations of the laser beams crossing the plasma lead to a loss of the calibration resulting in unreliable measurements of the electron density profile. For the TCV (Tokamak à Configuration Variable) installation, the beam paths from the laser output to the vacuum chamber are ∼25 m long and include several optical components. In order to monitor the alignment on a regular basis, a set of 9 cameras has been installed at several locations along the beam path. They view the actual laser beam pattern by recording the scattered light from an intercepting optical surface (mirror or window) together with the position of markers used for reference. Small “webcams” are used for this purpose; they feature adequate intensity response at the laser wavelength of 1.06 μm, are compact, cheap and several units may be connected to a server PC simultaneously. The real-time images from all the cameras are accessible from a Web browser. This installation has proven to be extremely useful in the early detection of alignment problems and to assist the alignment procedure [1].

Measurement of the magnetic field errors on TCV

A set of 24 saddle loops is used on the Tokamak à Configuration Variable (TCV) to measure the radial magnetic flux at different toroidal and vertical positions. The new system is calibrated together with the standard magnetic diagnostics on TCV. Based on the results of this calibration, the effective current in the poloidal field coils and their position is computed. These corrections are then used to compute the distribution of the error field inside the vacuum vessel for a typical TCV discharge. Since the saddle loops measure the magnetic flux at different toroidal positions, the non-axisymmetric error field is also estimated and correlated to a shift or a tilt of the poloidal field coils.

A 3D multi-mode geometry-independent RMP optimization method and its application to TCV

Resonant magnetic perturbation (RMP) and error field correction (EFC) produced by toroidally and poloidally distributed coil systems can be optimized if each coil is powered with an independent power supply. A 3D multi-mode geometry-independent Lagrange method has been developed and appears to be an efficient way to minimize the parasitic spatial modes of the magnetic perturbation and the coil current requirements while imposing the amplitude and phase of a number of target modes. A figure of merit measuring the quality of a perturbation spectrum with respect to RMP independently of the considered coil system or plasma equilibrium is proposed. To ease the application of the Lagrange method, a spectral characterization of the system, based on a generalized discrete Fourier transform applied in current space, is performed to determine how spectral degeneracy and side-band creation limit the set of simultaneously controllable target modes. This characterization is also useful to quantify the efficiency of the coil system in each toroidal mode number and to know whether optimization is possible for a given number of target modes.The efficiency of the method is demonstrated in the special case of a multi-purpose saddle coil system proposed as part of a future upgrade of Tokamak à Configuration Variable (TCV). This system consists of three rows of eight internal coils, each coil having independent power supplies, and provides simultaneously EFC, RMP and fast vertical position control.

From profile to sawtooth control: developing feedback control using ECRH/ECCD systems on the TCV tokamak

Real time control of heating systems is essential to maximize plasma performance and avoid or neutralize instabilities under changing plasma conditions. Several feedback control algorithms have been developed on the Tokamak a Configuration Variable (TCV) tokamak that use the electron cyclotron (ECRH/ECCD) system to control a wide range of plasma properties, including the plasma current, shape, profiles as well as the sawtooth instability. Controllers have been developed to obtain sawteeth of a pre-determined period, to maximize the sawtooth period using an extremum seeking control algorithm and finally to provide simultaneous control of the plasma emission profile peak and width using multiple independent EC actuators.

Modulation of electron transport during swing ECCD discharges in TCV

Generation of a swing electron cyclotron current drive (swing ECCD), i.e. driving alternated, symmetric, positive or negative local ECCD, during a single discharge and at constant total input EC power, was performed at the Tokamak à Configuration Variable (TCV). The electron temperature is observed to be modulated inside the deposition radius, implying modulation of the electron transport properties. The modulation of ECCD is the only actuator for the observed modifications in the electron transport properties. These exhibit inverted behaviors depending on the deposition location of the co- and counter-ECCD. At more on-axis depositions, swing ECCD results in a higher electron temperature during the co-ECCD phase, whereas at more off-axis depositions, the electron temperature is higher during the counter-ECCD phase. Transport modeling of these discharges shows that the local electron tranport behavior depends on the value of the modulated magnetic shear. The results are transport model independent, confirming the robustness of the magnetic shear modeling, and indicating that the main contribution is due to the ECCD. Moreover, the results are consistent with predictions from gyrokinetic simulations, that the local electron confinement is proportional to the magnetic shear at low shear and inversely at high shear values, s ≳ 1.

Overview of physics research on the TCV tokamak

The Tokamak à Configuration Variable (TCV) tokamak is equipped with high-power (4.5 MW), real-time-controllable EC systems and flexible shaping, and plays an important role in fusion research by broadening the parameter range of reactor relevant regimes, by investigating tokamak physics questions and by developing new control tools. Steady-state discharges are achieved, in which the current is entirely self-generated through the bootstrap mechanism, a fundamental ingredient for ITER steady-state operation. The discharge remains quiescent over several current redistribution times, demonstrating that a self-consistent, ‘bootstrap-aligned’ equilibrium state is possible. Electron internal transport barrier regimes sustained by EC current drive have also been explored. MHD activity is shown to be crucial in scenarios characterized by large and slow oscillations in plasma confinement, which in turn can be modified by small Ohmic current perturbations altering the barrier strength. In studies of the relation between anomalous transport and plasma shape, the observed dependences of the electron thermal diffusivity on triangularity (direct) and collisionality (inverse) are qualitatively reproduced by non-linear gyro-kinetic simulations and shown to be governed by TEM turbulence. Parallel SOL flows are studied for their importance for material migration. Flow profiles are measured using a reciprocating Mach probe by changing from lower to upper single-null diverted equilibria and shifting the plasmas vertically. The dominant, field-direction-dependent Pfirsch–Schlüter component is found to be in good agreement with theoretical predictions. A field-direction-independent component is identified and is consistent with flows generated by transient over-pressure due to ballooning-like interchange turbulence. Initial high-resolution infrared images confirm that ELMs have a filamentary structure, while fast, localized radiation measurements reveal that ELM activity first appears in the X-point region. Real time control techniques are currently being applied to EC multiple independent power supplies and beam launchers, e.g. to control the plasma current in fully non-inductive conditions, and the plasma elongation through current broadening by far-off-axis heating at constant shaping field.

Real time control of plasmas and ECRH systems on TCV

Developments in the real time control hardware on Tokamak à Configuration Variable (TCV) coupled with the flexibility of plasma shaping and electron cyclotron (EC) heating and current drive actuators are opening many opportunities to perform real time experiments and develop algorithms and methods for fusion applications. The ability to control magnetohydrodynamic instabilities is particularly important for achieving high performance fusion plasmas and EC is envisaged as a key actuator in maintaining high performance. We have successfully demonstrated control of the sawtooth instability using the EC launcher injection angle to modify the current profile around the q =1 surface. This paper presents an overview of recent real time control experiments on TCV, developments in the hardware and algorithms together with plans for the future.

Snowflake divertor plasmas on TCV

Starting from a standard single null X-point configuration, a second order null divertor (snowflake (SF)) has been successfully created on the Tokamak à Configuration Variable (TCV) tokamak. The magnetic properties of this innovative configuration have been analysed and compared with a standard X-point configuration. For the SF divertor, the connection length and the flux expansion close to the separatrix exceed those of the standard X-point by more than a factor of 2. The magnetic shear in the plasma edge is also larger for the SF configuration.

Current density evolution in electron internal transport barrier discharges in TCV

Simulations of the plasma current density evolution in electron internal transport barrier discharges on the Tokamak à Configuration Variable (TCV) have been performed, in order to determine the relationship between the safety-factor profile and the electron transport improvement. The results show that the formation of the transport barrier is correlated with the shear reversal in all cases studied, regardless of the different heating and current drive schemes. No indications were found of discrete effects related to low-order rational q surfaces. On the contrary, the increase in confinement along with the negative shear is gradual, but constant, indicating that the transition is smooth, although it can be very fast.

Global plasma oscillations in electron internal transport barriers in TCV

In the Tokamak à Configuration Variable (TCV) (Hofmann F et al1994 Plasma Phys. Control. Fusion 36 B277), global plasma oscillations have been discovered in fully non-inductively driven plasmas featuring electron internal transport barriers (ITB) with strong ECRH/ECCD. These oscillations are linked to the destabilization and stabilization of MHD modes near the foot of the ITB and can lead to large oscillations of the total plasma current and line-averaged density, among others. They are intrinsically related to the fact that ITBs have large pressure gradients in a region of low magnetic shear. Therefore, the ideal MHD limit is relatively low and infernal modes can be unstable. Depending on the proximity to the ideal limit, small crashes or resistive modes can appear which affect the time evolution of the discharge. Being near marginal stability, the modes can self-stabilize due to the modification of the pressure gradient and local q-profile. The plasma recovers good confinement, reverses shear and the ITB builds up, until a new MHD mode is destabilized. TCV results show that this cycling behaviour can be controlled by modifying the current density or the pressure profiles, either with Ohmic current density perturbation or by modifying the ECH/ECCD power. It is demonstrated that many observations such as q ⩾ 2 sawteeth, beta collapses, minor disruptions and oscillation regimes in ITBs can be assigned to the same physics origin: the proximity to the infernal mode stability limit.

Suprathermal electron studies in the TCV tokamak: Design of a tomographic hard-x-ray spectrometer

Electron cyclotron resonance heating and electron cyclotron current drive, disruptive events, and sawtooth activity are all known to produce suprathermal electrons in fusion devices, motivating increasingly detailed studies of the generation and dynamics of this suprathermal population. Measurements have been performed in the past years in the tokamak a configuration variable (TCV) tokamak using a single pinhole hard-x-ray (HXR) camera and electron-cyclotron-emission radiometers, leading, in particular, to the identification of the crucial role of spatial transport in the physics of ECCD. The observation of a poloidal asymmetry in the emitted suprathermal bremsstrahlung radiation motivates the design of a proposed new tomographic HXR spectrometer reported in this paper. The design, which is based on a compact modified Soller collimator concept, is being aided by simulations of tomographic reconstruction. Quantitative criteria have been developed to optimize the design for the greatly variable shapes and positions of TCV plasmas.

Electron thermal transport analysis in Tokamak à Configuration Variable

A Tokamak à Configuration Variable (TCV) [G. Tonetti, A. Heym, F. Hofmann et al., in Proceedings of the 16th Symposium on Fusion Technology, London, U.K., edited by R. Hemsworth (North-Holland, Amsterdam, 1991), p. 587] plasma with high power density (up to 8MW∕m3) core deposited electron cyclotron resonance heating at significant plasma densities (⩽7×1019m−3) is analyzed for the electron thermal transport. The discharge distinguishes itself as it has four distinct high confinement mode (H-mode) phases. An Ohmic H-mode with type III edge localized modes (ELMs), which turns into a type I ELMy H-mode when the ECRH is switched on. The ELMs then vanish, which gives rise to a quasistationary ELM-free H-mode. This ELM-free phase can be divided into two, one without magnetohydrodynamics (MHD) and one with. The MHD mode in the latter case causes the confinement to drop by ∼15%. For all four phases both large-scale trapped electron (TEM) and ion temperature gradient (ITG) modes and small-scale electron temperature gradient (ETG) modes are analyzed. The analytical TEM formulas have difficulty in explaining both the magnitude and the radial profile of the electron thermal flux. Collisionality governs the drive of the TEM, which for the discharge in question implies it can be driven by either the temperature or density gradient. The TEM response function is derived and it is shown to be relatively small and to have sharp resonances in its energy dependence. The ETG turbulence, predicted by the Institute for Fusion Studies electron gyrofluid code, is on the other hand driven solely by the electron temperature gradient. Both trapped and passing electrons add to the ETG instability and turbulent thermal flux. For easy comparison of the results of the above approaches and also with the Weiland model, a dimensionless error measure, the so-called average relative variance is introduced. According to this method the ETG model explains 70% of the variation in the electron heat diffusivity whereas the predictive capabilities of the TEM-ITG models are poor. These results for TCV support the conclusion that the ETG model is able to explain a wide range of anomalous electron transport data, in addition to existing evidence from ASDEX [F. Ryter, F. Leuterer, G. Pereverzev, H.-U. Fahrbach, J. Stober, W. Suttrop, and the ASDEX Upgrade Team, Phys. Rev. Lett. 86, 2325 (2001)], Tore Supra [G. T. Hoang, W. Horton, C. Bourdelle, B. Hu, X. Garbet, and M. Ottaviani, Phys. Plasmas 10, 405 (2003)] and the Frascati Tokamak Upgrade [A. Jacchia, F. D. Luca, S. Cirant, C. Sozzi, G. Bracco, A. Brushi, P. Buratti, S. Podda, and O. Tudisco, Nucl. Fusion 42, 1116 (2002)].