Thomson scattering on low and high temperature plasmas

Worldwide research is ongoing, to develop and build the tokamak ITER to generate energy based on controlled nuclear fusion. The principle design concept of ITER is a donut-shaped vessel wherein the fusion fuel, a hot plasma of hydrogen isotopes, is contained by high magnetic fields. The fusion power can be produced at a plasma temperature of ~100 million degrees C and density of ~1020 m-3. In order to realize turn key fusion energy plants, a number of issues need to be addressed. Firstly, the control of the bulk plasma to prevent outflow of heat and particles due to instabilities, needs to be improved. Subject of research on many present-day tokamaks is the formation of magnetic islands (so-called tearing modes) due to instabilities in the magnetic field that confines the plasma. Tools to monitor and prevent growth of magnetic islands are therefore very important. Additionally, the mechanisms underlying the occurrence of confinement-friendly internal transport barriers have to be studied. This research can lead to improved plasma performance. The second issue that has to be addressed, is the erosion due to the high power load (10 MW/m2 (continuous) and > 1 GW/m2 (transient) due to Edge Localized Modes (ELMs )) on plasma facing components of the ITER divertor. The linear plasma generators Pilot-PSI and Magnum-PSI have been built to study plasma-wall interaction during these power loads. This research includes the third issue; tritium retention build-up in wall material. This thesis describes the development of Thomson scattering systems to study fast plasma phenomena in tokamaks as well as to study the quasi-continuous plasma of linear plasma generators. Thomson scattering is the most accurate method for measuring the electron temperature (Te) and density (ne) of a plasma: the accuracies of the systems described in this thesis are better than 3 - 4% and 4 - 8% for ne and Te, respectively. Basically, Thomson scattering is the process of acceleration of electrons due to an electromagnetic wave and as a consequence emission of radiation with the same frequency as that of the incoming wave, i.e. the wave is scattered elastically. The re-radiated light is Doppler shifted due to the velocity of the electron. Scattering on an ensemble of electrons results in a spectrum that resembles the electron velocity distribution, from which Te and ne can be retrieved. If the size of the incident wave is larger than the Debye length, then the light is collectively scattered by the electrons bunched in the Debye cloud of an ion. This so-called collective Thomson scattering can be utilized to measure the ion temperature (Ti). The first research challenge was the development of a high repetition rate Thomson scattering system for the TEXTOR tokamak (Jülich, Germany). A so-called double-pass intracavity laser was developed that generates a burst of 30 laser pulses of ~15 J each (with a repetition rate of 5 kHz). The system operates like a laser oscillator with the plasma as part of an 18 m long cavity. A fast detector equipped with CMOS cameras coupled to an image intensifier stage was developed. At a repetition rate of 5 kHz and a density of ne = 2.5×1019 m-3, density and temperature (range: 50 eV – 5 keV) profiles could be measured along the full plasma diameter of 900 mm long, with a spatial resolution of 7.5 mm. Coping with the plasma light background turned out to be the biggest issue: due to the long cavity and the laser pulse is relatively long (~1 ¿s) and a large detector gate window is required, resulting in a much higher plasma light contribution compared to single-pulse Thomson scattering systems. Nevertheless, a combination of high laser pulse energies (~12 J/pulse), careful plasma light monitoring and effective detector gating proved to be the solution to realize a reliable high repetition rate Thomson scattering system. The main step in laser development was the replacement of a high dope ruby rod by one with a low dope (0.03% Cr+), leading to a homogenous absorption of the pumping light from the flash lamps over the ruby rod cross section. The pumping-to-probing efficiency was improved by a factor of 1.5 and a significant minimization of the laser beam divergence, resulting in a better imaging efficiency of the viewing system. The diagnostic system enabled to record rotating magnetic islands during 2.2 ms with a repetition rate of 5 kHz, revealing the detailed density profile evolution inside the islands. Confinement properties of transport barriers were studied by measuring the time evolution of the ne and Te profiles. A second challenge was to develop a Thomson scattering system for the Pilot-PSI linear plasma generator, based on a frequency-doubled Nd:YAG laser. The stray light contribution of the system already existing at Pilot-PSI could be significantly reduced by application of a special carbon aperture system in the vacuum laser beam line, which enabled Thomson scattering measurements at a distance of 17 mm from a target surface exposed to a high power plasma beam. The sensitivity of the detector system was improved by more than a factor of 5 by application of a Generation III image intensifier at the front of the existing ICCD detector. The lower density and temperature limit of the system is 4×1019 m-3 and 0.2 eV, respectively. To achieve these values, the signal from 30 laser pulses (0.35 J/pulse, 10 Hz) needs to be accumulated. Instead of multiple Pilot-PSI discharges, now only one discharge is required to obtain accurate ne and Te profiles. This diagnostic has become a working horse for Pilot-PSI research and revealed different properties of the hydrogen plasma jet such as plasma confinement and indications for ion viscous heating. During ELM simulation experiments, single pulse TS measurements were successfully performed; using only 0.35 J scattering energy the time evolution of the plasma could be measured on shot to shot base. Subsequently, an advanced Thomson scattering system was designed and constructed for Magnum-PSI. This system features a frequency-doubled Nd:YAG laser, equipped with a 35 m long remotely-controllable laser beam line and a high etendue spectrometer based on a transmission grating. The system is designed to measure electron density and temperature profiles of a plasma column of 100 mm in diameter with a spatial resolution of 1.5 mm and features a lower density limit of 9×1018 m-3 (using 30 laser pulses of 0.55 J each, 10 Hz). First measurements at Magnum-PSI show that the design specifications are met and that on virtue of the high light collection power of the detection system even ne and Te profiles of the argon plasma expansion could be measured accurately at densities of 5×1018 m-3 and temperatures below 0.15 eV. In recent years the need arose for an accurate method to determine the ion properties in the plasma jet of the linear plasma generators. Therefore, the author initiated a feasibility study to find out whether CTS can be performed on Magnum-PSI to measure Ti, and moreover the macroscopic velocity of the plasma. It was demonstrated that Ti and the macroscopic velocity can be measured with an accuracy of 10% at ne = 5.0x1020 m-3 (test case: Ti = 2.5 eV, resolution 2.4 mm) and 15%, respectively. This can be achieved by accumulating 10 laser pulses of 1.2 J each, using the fundamental wavelength of a Nd:YAG laser. The proposed system may be used to prove that viscous heating of the ions in the plasma is the main cause for the ion temperature being much higher than the electron temperature in the magnetized plasma jet of Pilot-PSI and Magnum-PSI. Moreover, CTS experiments on Magnum-PSI can possibly prove that this technique is a viable ion temperature determination method for the ITER divertor; presently there are no good candidate techniques for ITER available

Thomson scattering near the high-fluence target surface of the Magnum-PSI linear plasma generator

In the quest to long-term operation of high-power magnetically confined fusion devices, it is crucial to control the particle and heat loads on the wall. In order to predict these loads, understanding of the plasma-wall interaction is important. Near the wall surface, the plasma is accelerated towards the Debye sheath edge. In plasma conditions with high density and low temperature, the interaction between the incoming plasma and recycled neutrals can become important. In this paper, we present incoherent Thomson Scattering (TS) measurements in the near-surface region of the Magnum-PSI linear plasma generator. To enable TS measurements close to the plasma target of Magnum-PSI, a stray light suppression up to a factor 104 was achieved, while retaining high transmission. By incrementally moving the target along the magnetic field, this adapted system was used down to 1.9 mm from the target. In the last 10–15 mm in front of the surface, the electron density as well as temperature were observed to decrease significantly. Under the assumption of constant particle flux in this region, the density drop indicates plasma acceleration. In that case, the measurements can be interpreted to show the plasma presheath, and its lengthscale: ~ 1 cm. The electron cooling indicates an energy loss channel for the electrons near the wall. A reduced electron temperature near the sheath entrance leads to lower estimates of particle and energy flux, as well a

Production of high transient heat and particle fluxes in a linear plasma device

We report on the generation of high transient heat and particle fluxes in a linear plasma device by pulsed operation of the plasma source. A capacitor bank is discharged into the source to transiently increase the discharge current up to 1.7 kA, allowing peak densities and temperature of 70×1020 m-3 and 6 eV corresponding to a surface power density of about 400 MW¿m-2

A differentially pumped argon plasma in the linear plasma generator Magnum-PSI: gas flow and dynamics of the ionized fraction

Magnum-PSI is a linear plasma generator designed to reach the plasma–surface interaction (PSI) regime of ITER and nuclear fusion reactors beyond ITER. To reach this regime, the influx of cold neutrals from the source must be significantly lower than the plasma flux reaching the target. This is achieved by a differential pumping scheme, where the vacuum vessel is divided by skimmers into separate chambers which are individually pumped. The non-magnetized expansion of 5 Pa m3 s-1 (3 slm) argon in a low background pressure was studied in the differentially pumped vacuum vessel fitted with non-cooled flat skimmers. The behavior of the neutral component was studied with direct simulation Monte Carlo simulations and Rayleigh scattering measurements. Thomson scattering and double Langmuir probe measurements were performed on the ionized fraction. It was found that the electrons and neutral particles are not completely coupled in the shock front. The neutral fraction shows clear signs of invasion from hotter background gas, causing the average temperature and density to increase before the shock. This is also shown in the ionization ratio, which has been determined in front of and behind the first skimmer. This study helps us to understand the behavior of the gas flow in the machine and validates our modeling

SolEdge2D-Eirene simulations of Pilot-PSI plasmas

\u3cp\u3eThe exhaust of power is a crucial issue for ITER and next step fusion devices [1]. Predictions for divertor operation are heavily dependent on edge plasma simulations typically utilizing a fluid plasma code in combination with a Monte Carlo code for neutral species. Therefore it is important to validate the codes using well-diagnosed experimental setups. The Pilot-PSI device offers a high density (n\u3csub\u3ee\u3c/sub\u3e ~ 10\u3csup\u3e19\u3c/sup\u3e – 10\u3csup\u3e21\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e), low temperature (T\u3csub\u3ee\u3c/sub\u3e < 5.0 eV) plasma comparable to that expected in the ITER divertor region. In this work, hydrogen plasma discharges in Pilot-PSI have been modelled using the Soledge2D fluid plasma code [2] coupled to the Eirene neutral Monte Carlo code. In the model, the plasma is generated using external volumetric sources of plasma density and power in the region of the cascaded arc plasma source and a constant H\u3csub\u3e2\u3c/sub\u3e gas inflow rate. The external power source is found to be the main control parameter of the simulations and is set in order to match experimental n\u3csub\u3ee\u3c/sub\u3e, T\u3csub\u3ee\u3c/sub\u3e profiles from Thomson scattering (TS) 4 cm downstream of the cascaded arc nozzle. The total injected power is typically 2 – 3 kW. The simulation results are compared to TS measurements 56 cm downstream from the source nozzle (2 cm in front of the Pilot-PSI target) and a Langmuir probe embedded in the target.\u3c/p\u3

Gas temperature in transient CO2 plasma measured by Raman scattering

Rotational Raman scattering on the vibrational ground state of CO2 was performed to determine the gas temperature in narrow-gap dielectric barrier discharges (DBDs). The Raman spectrometer was equipped with a straightforward spectral filtering to mask ca. 30 cm-1 (0.85 nm) centered around the excitation wavelength of 532 nm. Linearisation of the observed transitions (J = 18–42) was applied to retrieve gas temperatures in discharge gaps of 1 mm. The DBD was operated in pure CO2 at atmospheric pressure and non-negligible gas heating of about 160 K was observed at 33 W injected power. Based on a simplified energy balance the gas temperature measurements were extrapolated to a broad range of injected plasma power values (0–60 W)

Self-shielding of a plasma-exposed surface during extreme transient heatloads

The power deposition on a tungsten surface exposed to combined pulsed/continuous high power plasma is studied. A study of the correlation between the plasma parameters and the power deposition on the surface demonstrates the effect of particle recycling in the strongly coupled regime. Upon increasing the input power to the plasma source, the energy density to the target first increases then decreases. We suggest that the sudden outgassing of hydrogen particles from the target and their subsequent ionization causes this. This back-flow of neutrals impedes the power transfer to the target, providing a shielding of the metal surface from the intense plasma flux

Plasma detachment study of high density helium plasmas in the Pilot-PSI device

We have investigated plasma detachment phenomena of high-density helium plasmas in \u3cbr/\u3ethe linear plasma device Pilot-PSI, which can realize a relevant ITER SOL/Divertor plasma \u3cbr/\u3econdition. The experiment clearly indicated plasma detachment features such as drops in the \u3cbr/\u3eplasma pressure and particle flux along the magnetic field lines that were observed under the \u3cbr/\u3econdition of high neutral pressure; a feature of flux drop was parameterized using the degree \u3cbr/\u3eof detachment (DOD) index. Fundamental plasma parameters such as electron temperature (Te) and electron density in the detached recombining plasmas were measured by different \u3cbr/\u3emethods: reciprocating electrostatic probes, Thomson scattering (TS), and optical emission spectroscopy (OES). The Te measured using single and double probes corresponded to the TS measurement. No anomalies in the single probe I – V\u3cbr/\u3echaracteristics, observed in other linear plasma devices [ 16, 17,36], appeared under the present condition in the Pilot-PSI device. \u3cbr/\u3eA possible reason for this difference is discussed by comparing the different linear devices. \u3cbr/\u3eThe OES results are also compared with the simulation results of a collisional radiative (CR) model. Further, we demonstrated more than 90% of parallel particle and heat fluxes were \u3cbr/\u3edissipated in a short length of 0.5 m under the high neutral pressure condition in Pilot-PSI

The occurrence and damage of unipolar arcing on fuzzy tungsten

This research investigated whether unipolar arcing in the divertor of fusion reactors is a potential cause for enhanced wear of the divertor. It was found that 1 µm of nano-fuzz growth is sufficient to initiate arcing, mainly depending on the sheath potential drop and electron density. The average mass loss rate induced by the arc was determined from mass loss measurements and found to be consistent with the value estimated from the arc current. The average arc track erosion depth was estimated by using the measured mass loss and damaged surface area and was found to be one tenth of the fuzzy layer thickness. Due to melting of the fuzzy structures the actual depth is larger and some arc tracks occasionally appeared to even reach the bulk beyond the fuzzy layer. The conclusion of this study is therefore that arcing in the divertor of future tokamaks (e.g. ITER) potentially is an important cause for surface damage and plasma pollution