Philippe Marmillod

The control of tokamak configuration variable plasmas

AbstractThe general control of tokamak plasmas has evolved considerably over the past few years with an increase in the plasma pulse length, an increase in the control of additional heating and fueling, and an increase in the degree to which the shape of the plasma can be varied. The Tokamak Configuration Variable (TCV) is specifically designed to explore the operational benefits of plasma shaping over a wide variety of plasma shapes. Consequently, considerable attention has been given to the control of the poloidal field coil currents that impose the desired shape. All aspects of the control of TCV plasmas, from the diagnostic measurements to the power supplies, via particular control algorithms and overall supervision are discussed.

Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)

In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (τEe) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q0/qmin) and the reversed shear volume (ρq-min2) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the...

An overview of results from the TCV tokamak

The Tokamak Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, kappa = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à configuration variable (TCV)

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak a configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi...

Third-harmonic, top-launch, ECRH experiments on TCV tokamak

In the moderate magnetic field of TCV (1.5 T), the recently installed X3 system (3 gyrotrons, 118 GHz, 0.45 MW each, 2 s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system. To compensate for the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows the ray path to maximize along the resonance layer thereby maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation in both density (refraction) and temperature (relativistic shift) a real-time control system on the mirror injection angle, based on a mirror modulation technique (synchronous demodulation), has been developed and successfully tested on TCV. Comparisons of the absorption calculated with the TORAY-GA ray-tracing code and the ECWGB beam-tracing code, which includes diffraction effects, are presented. An experimental study of the X3 absorption versus plasma density in an L-mode plasma shows that with a total injected power of 1.35 MW full single-pass absorption is reached with a significant fraction of the absorbed power associated with the presence of suprathermal electrons. Compared with ohmic/low-power-heating of ELMy H-modes, it has been possible to enter into a different ELMy regime with an injected power of 1.35 MW.

Effect of plasma shape on confinement and MHD behaviour in TCV

Note: 38th Annual Meeting, APS Division of Plasma Physics, Denver, CO, USA, November 1996, Bull. Amer. Phys. Soc. 41(7), 1513 (1996) Reference CRPP-CONF-1996-037 Record created on 2008-05-13, modified on 2016-08-08

Overview of TCV results

Note: Proc. 18th IAEA Fusion Energy Conference, Sorrento, Italy, 4-10 October 2000, IAEA-CN-77 (OV5/1), p. 37 (2000) Reference CRPP-CONF-2000-087 Record created on 2008-05-13, modified on 2017-05-12

Central mass feedback control using the discrete Alfvén wave spectrum

Note: Proc. 16th Europ. Conference on Contr. Fusion and Plasma Physics, Venice, Italy, March 1989, 13B, Part IV, 1461 -1464 (1989) Reference CRPP-CONF-1989-015 Record created on 2008-05-13, modified on 2016-08-08

A radio frequency/high voltage pulse generator for the operation of a planar multipole ion trap/time-of-flight mass spectrometer

We present a radio frequency (RF)/high voltage pulse generator designed to provide suitable waveforms for the operation of a planar multipole ion trap/time-of-flight mass spectrometer. Our generator supplies a RF signal to two pairs of trapping electrodes, allowing ions to be stored in between them. Subsequently, the RF is rapidly switched off and high voltage extraction pulses are applied to the trap electrodes in order to obtain a time-of-flight spectrum of the stored ions. The quenching of the RF and the extraction pulses are synchronized to the RF phase, ensuring well-defined ejection conditions.

High sensitivity field asymmetric ion mobility spectrometer

A high sensitivity field asymmetric ion mobility spectrometer (FAIMS) was designed, fabricated, and tested. The main components of the system are a 10.6 eV UV photoionization source, an ion filter driven by a high voltage/high frequency n-MOS inverter circuit, and a low noise ion detector. The ion filter electronics are capable to generate square waveforms with peak-to-peak voltages up to 1000 V at frequencies up to 1 MHz with adjustable duty cycles. The ion detector current amplifier has a gain up to 1012 V/A with an effective equivalent input noise level down to about 1 fA/Hz1/2 during operation with the ion filter at the maximum voltage and frequency. The FAIMS system was characterized by detecting different standard chemical compounds. Additionally, we investigated the use of a synchronous modulation/demodulation technique to improve the signal-to-noise ratio in FAIMS measurements. In particular, we implemented the modulation of the compensation voltage with the synchronous demodulation of the ion current. The analysis of the measurements at low concentration levels led to an extrapolated limit of detection for acetone of 10 ppt with an averaging time of 1 s.