Retarding Field Analyzer Research Articles

Latest investigations on fluctuations, ELM filaments and turbulent transport in the SOL of ASDEX Upgrade

This paper presents turbulence investigations in the scrape-off layer (SOL) of ASDEX Upgrade in ohmic, L-mode and H-mode discharges using electrostatic and electromagnetic probes. Detailed studies are performed on small scale turbulence and on ELM filaments. Simultaneous measurements of floating and plasma potential fluctuations revealed significant differences between these quantities. Large errors can occur when the electric field is extracted from floating potential measurements, even in ohmic discharges. Turbulence studies in ohmic plasmas show the existence of density holes inside the separatrix and blobs outside. Close to the separatrix a reversal of the poloidal blob propagation velocity occurs. Investigations on the Reynolds stress in the scrape-off layer (SOL) show its importance for the momentum transport in L-mode while its impact for momentum transport during ELMs in H-mode is rather small. In the far SOL the electron density and temperature were measured during type-I ELMy H-mode at ASDEX Upgrade resolving ELM filaments. Strong density peaks and temperatures of several 10 eV were detected during the ELM events. Additional investigations on the ions in ELM filaments by a retarding field analyser indicate ion temperatures of 50–80 eV. ELMs also expel current concentrated in filaments into the SOL. Furthermore, discharges with small ELMs were studied. In N2 seeded discharges the type-I ELM frequency rises and the ELM duration decreases. For discharges with small type-II ELMs the mean turbulent radial particle flux is increased over the mean particle flux in type-I ELM discharges at otherwise similar plasma parameters.

COLDDIAG: A Cold Vacuum Chamber for Diagnostics

One of the still open issues for the development of superconducting insertion devices is the understanding of the heat load induced by the beam passage. With the aim of measuring the beam heat load to a cold bore and in order to gain a deeper understanding in the beam heat load mechanisms, a cold vacuum chamber for diagnostics is under construction. We plan to have access with the same set-up to a number of different diagnostics, so we are implementing: i) retarding field analysers to measure the electron flux, ii) temperature sensors to measure the total heat load, iii) pressure gauges, iv) and mass spectrometers to measure the gas content. The inner vacuum chamber will be removable in order to test different geometries and materials. COLDDIAG is built to fit in a short straight section at ANKA, but we are proposing its installation in different synchrotron light sources with different energies and beam characteristics. A first installation in DIAMOND is planned in June 2011. Here we describe the technical design report of this device and the planned measurements with beam.

Theory of fast particle generation in front of LH grills

During tokamak operation with lower hybrid (LH) power a few per cent of the launched power is absorbed by the scrape-off layer plasma in magnetic flux tubes in front of the LH grill. At strike points of these flux tubes, intense plasma–wall interaction is seen in visible and infrared wavelengths, and local wall damage can occur. The parallel power flux within these ‘hot spots’ is estimated to be up to about 10 MW m−2 by infrared imagery. Recent experimental results from retarding field analyzer measurements on Tore Supra as well as JET IR camera measurements have shown the existence of fast electrons as far as a few centimeters from the grill mouth. This finding cannot be explained by the standard theory. We present therefore in this paper a novel theory explaining the fast electron generation in a several cm wide layer in front of the LH grill by taking into account LH wave propagation features closely connected with the blob character of edge turbulence. We demonstrate that the computed power-flows then essentially agree with data from infrared diagnostics. An alternative theoretical explanation considers plasma density modulations due to ponderomotive force effects in front of the LH grill.

Ion temperature fluctuation measurements using a retarding field analyzer

The retarding field analyzer (RFA) is a widely used diagnostic tool for the ion temperature measurement in the scrape-off-layer (SOL) of the thermonuclear plasma devices. However, the temporal resolution in the standard RFA application is restricted to the ms timescale. In this paper, a dc operation of the RFA is considered, which allows for the measurement of the plasma ion temperature fluctuations. The method is based on the relation for the RFA current-voltage (I-V) characteristic resulted from a common RFA model of shifted Maxwellian distribution of the analyzed ions, and the measurements of two points on the exponentially decaying region of the I-V characteristic with two differently dc biased RFA electrodes. The method has been tested and compared with conventional RFA measurements of the ion temperature in the tokamak ISTTOK SOL plasma. An ion temperature of T(i) = 17 eV is obtained near the limiter position. The agreement between the results of the two methods is within ∼25%. The amplitude of the ion temperature fluctuations is found to be around 5 eV at this location. The method has been validated by taking into account the effect of fluctuations in the plasma potential and the noise contamination, proving the reliability of the results obtained. Finally, constrains to the method application are discussed that include a negligible electron emission from the RFA grids and the restriction to operate in the exponentially decaying region of the I-V characteristic.

First measurements of edge localized mode ion energies in the ASDEX Upgrade far scrape-off layer

Using the retarding field analyzer technique, ion energies carried by edge localized mode (ELM) filaments have been measured for the first time in the far scrape-off layer (SOL) of the ASDEX Upgrade tokamak. Energies, Ei ELM, exceeding 160 eV have been found, 5–6 cm outside the separatrix, with a decay length of about 2 cm. The measured ELM particle ion temperature in the far SOL is in the range Ti ELM ≈ 50–80 eV, in good agreement with the predictions from two simple collisionless models of ELM parallel transport. In between ELMs, Ti ≈ Te ≈ 10 eV is observed in the far SOL, consistent with relatively strong ion–electron thermal coupling in this region.

Quantification of the deuterium ion fluxes from a plasma source

We present an electron-cyclotron-resonance plasma source with a biased sample holder that is well suited for the implantation of deuterium ions into tungsten. Achievable energies range from several electronvolts up to 200 eV per deuteron. The deuterium ion fluxes from this plasma source were thoroughly quantified with a retarding field analyser and a plasma monitor, i.e. an energy resolving mass spectrometer. We present the results of these measurements for different microwave input powers, D2 gas pressures and sample bias settings. Typical achievable deuteron fluxes are of the order of 1019 to 1020 D m−2 s−1 and are predominantly carried by ions. Additionally, an attempt to quantify the flux of neutral deuterium atoms to the sample surface is presented.

Ion energy measurements in the scrape-off layer of MAST using a Retarding Field Analyzer

We present first ion energy measurements obtained in the scrape-off layer (SOL) of the MAST tokamak using a Retarding Field Analyzer (RFA). Results from two sets of experiments are reported. First, ion temperature (Ti) profiles were measured in L-mode discharges. Ti is lower than 20eV and larger than the electron temperature with a ratio in the range 1–2.6, even in beam-heated discharges. This is consistent with a relatively high degree of ion–electron thermal coupling. Second, the RFA was used to estimate the ion energy in ELMs. Two different scenarios were studied and gave very different results. In the first one, a type-I ELMy H-mode, ions with energies exceeding 500eV were found as far as 20cm away from the separatrix. More statistics would be needed to calculate Ti in ELM filaments. In the second one, featuring type-III ELMs, no ions with energy larger than 200eV were detected 10cm outside the separatrix.

Numerical Analysis of the Measurement of Near-Beam Electron Cloud Density in Field-Free Region at KEK B-Factory Low-Energy Ring

A large number of electrons in the so-called electron cloud are accumulated in beam chambers in positron storage rings. These electrons interact with the beam and can make the beam unstable. The density information of the electron cloud near the beam is therefore fundamental for studying beam instability and mitigation techniques related to the electron cloud. Recently, a method to measure the density of the electron cloud near the beam has been proposed by Kanazawa et al. The method enables the measurement of high-energy electrons selectively using a retarding field analyzer located on a chamber wall, noting that the electrons near the beam receive a strong kick by the beam. They calculated the density of the electron cloud simply assuming that the electrons that receive a kick are stationary. We examined the measurement technique in detail using a new computer code developed by us. The analysis showed that the volume near the beam occupied by the detected electrons, i.e., the observed volume, was strongly deformed owing to the horizontal velocity of the electrons; nevertheless this volume calculated assuming that the stationary electrons can still be used for calculating the density of the electron cloud in their measurement conditions.

Three-dimensional energy profile measurement of a molecular ion beam by coincidence momentum imaging compared to a retarding field analyzer

We demonstrate a novel method using coincidence momentum imaging to measure the three-dimensional energy profile of a molecular ion beam. This involves determining the laboratory center-of-mass momentum for each fragmentation event and accumulating an energy profile over many events. This method is compared with a conventional retarding field analyzer and the two independent methods show reasonable agreement.

Ion Temperature Measurements in the Tore Supra Scrape‐Off Layer Using a Retarding Field Analyzer

AbstractThe retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measuring the ion tem‐perature Ti in the tokamak scrape‐off layer. An overview of the outstanding RFA performance over ten years of operation in Tore Supra tokamak is given and the validation of Ti measurements is addressed. The RFA measurements in Tore Supra are found to be well reproducible. The ion‐to‐electron temperature ratio is higher than one at low‐to‐moderate ion‐electron collisionality regime and converges to unity at high collisionality regime (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fundamental aspects of substrate biasing: ion velocity distributions and nonlinear effects

Ion bombardment of the substrate is a significant parameter in plasma processing such as dry etching or thin film deposition. The ion bombardment is described by ion velocity distribution functions (IVDFs), which were here measured quantitatively at a sinusoidally and non-sinusoidally biased electrode. The electrode voltage was monitored and controlled in the frequency domain using fast Fourier transformation. IVDF measurements were performed by a floating retarding field analyzer. A full modulation of the IVDF by arbitrary bias waveforms is only achieved if sufficiently high sheath voltages are used. If the applied sheath voltages become too low, the IVDFs are only partly determined by the RF bias waveforms and the system response becomes nonlinear. An analytical sheath model is derived from the experimental data, which accounts for arbitrary bias waveforms as well as for collisional and nonlinear effects in the sheath. It is shown that a combined DC and RF biasing of the electrode is required to gain full control over the ion bombardment of the substrate.

Time evolution of electronegativity in a pulsed inductively coupled oxygen plasma

The electronegativity in a continuous wave (CW) and pulsed mode plasmas was calculated using the measured results of both the single Langmuir probe and the retarding field analyzer. For the pulsed mode measurement, both of the measurements were performed in a time-resolved method using a boxcar sampling technique. For the conversion of the retarding field analyzer measurement results into absolute positive ion densities, argon plasma was used as a reference. The pulsed oxygen plasma was generated using the inductively coupled antenna and modulated at a repetition rate of 5 kHz and the duty ratio of 50%. The gas pressure was changed from 5 to 30 mTorr. The time evolution of the electronegativity shows that there is a pressure regime where the electron attachment reaction during the RF on-time is very active, indicating that the negative ion density reaches its maximum value during the RF on-time. Compared to the CW oxygen plasma, the electronegativity of the pulsed oxygen plasma varies within a wider range of values.

Calibration of a miniaturized retarding field analyzer for low-temperature plasmas: geometrical transparency and collisional effects

Retarding field analyzers (RFAs) are important diagnostics to measure fluxes and energies of ions impinging onto the wall of a plasma reactor. Any quantitative use of the data requires a proper calibration, which is here performed for a miniaturized RFA. The calibration accounts for the transparencies of the RFA grids as well as for collisions inside the RFA. An analytical model is derived which covers both geometrical and collisional effects. The model is calibrated and experimentally verified using a Langmuir probe. We find that the transparency of an RFA is a random variable which depends on the individual alignment of the RFA grids. Collisions inside the RFA limit the ion current transfer through the RFA at higher pressures. A simple method is presented which allows one to remove these artefacts from the RFA data and to obtain quantitative ion velocity distributions.

Ion energy distribution analysis of the TVA plasma ignited in carbon vapours using RFA

In order to understand plasma processes and to obtain technological control in thin film deposition, the study of surface-plasma interactions is essential. Apart from the type and flux of the impinging ions/neutral atoms on the surface, the ion energy distribution (IED) is an important parameter in understanding surface modification due to the plasma. In this paper, results of ion energy analysis of the Thermionic Vacuum Arc (TVA) plasma ignited in carbon vapours are presented. An in-house, computer-controlled retarding field analyzer was used for determining experimentally ion energy distributions of the carbon ions arriving at the substrate. The correlation of the carbon IED with the applied arc voltage in the TVA plasma was put in evidence for the first time.

Monte Carlo simulation for Multi-Mode Elastic Peak Electron Spectroscopy of crystalline materials: Effects of surface structure and excitation

The Multi-Mode Elastic Peak Electron Spectroscopy (MM-EPES) analysis is confined to incoherent electron elastic scattering and the use of variable primary energy. This experimental method is very sensitive to the surface region of the sample. However, for quantitative interpretation, the MM-EPES method needs jointly a Monte Carlo (MC) computer simulation of electron trajectories in the solid. In the present work, we proposed a new approach to calculate the percentage η e of elastic reflected electrons by the surface of a sample. This simulation takes into account the surface effects (i.e. surface plasmon), and the atoms arrangement in the substrate. The concept of the surface excitation parameter (SEP) is also presented. Computer simulations were performed on the three low index single crystals of Cu, Au, Si and Ag. The results confirm that the distribution of substrate atoms, according to the crystallographic structure, influences the intensity measured by EPES. A simple prediction formula was proposed to calculate η e for elastic electrons entering in a Retarding Field Analyzer (RFA) spectrometer which is the apparatus giving experimentally numerical values of the percentage η e .

Time- and energy resolved photoemission electron microscopy-imaging of photoelectron time-of-flight analysis by means of pulsed excitations

The present work enlightens the developments in time- and energy resolved photoemission electron microscopy over the past few years. We describe basic principles of the technique and demonstrate different applications. An energy- and time-filtering photoemission electron microscopy (PEEM) for real-time spectroscopic imaging can be realized either by a retarding field or hemispherical energy analyzer or by using time-of-flight optics with a delay line detector. The latter method has the advantage of no data loss at all as all randomly incoming particles are measured not only by position but also by time. This is of particular interest for pump–probe experiments in the femtosecond and attosecond time scale where space charge processes drastically limit the maximum number of photoemitted electrons per laser pulse. This work focuses particularly on time-of-flight analysis using a novel delay line detector. Time and energy resolved PEEM instruments with delay line detectors enable 4D imaging ( x, y, Δ t, E Kin) on a true counting basis. This allows a broad range of applications from real-time observation of dynamic phenomena at surfaces to fs time-of-flight spectro-microscopy and even aberration correction. By now, these time-of-flight analysis instruments achieve intrinsic time resolutions of 108 ps absolute and 13.5 ps relative. Very high permanent measurement speeds of more than 4 million events per second in random detection regimes have been realized using a standard USB2.0 interface. By means of this performance, the time-resolved PEEM technique enables to display evolutions of spatially resolved (<25 nm) and temporal sliced images life on any modern computer. The method allows dynamics investigations of variable electrical, magnetic, and optical near fields at surfaces and great prospects in dynamical adaptive photoelectron optics. For dynamical processes in the ps time scale such as magnetic domain wall movements, the time resolution of the delay line detectors even allows to be used directly for real-time experiments as well.

Retarding Field Analyzer for Ion Energy Distribution Measurement Through a Radio‐Frequency or Pulsed Biased Sheath

AbstractA compact, floating retarding field energy analyzer for measurement of ion energy distributions impacting an electrode through a radio‐frequency or pulsed bias sheath in a plasma discharge is presented. The analyzer is designed to sit on the electrode surface, in place of the substrate, and wide‐band low pass filters allow it to float at the electrode potential. This avoids the need for modification of the electrode. The capabilities of the analyzer are demonstrated through ion energy distribution and electron energy distribution measurements at the electrode surface in an inductively coupled plasma reactor. For a sinusoidal radio‐frequency driving signal applied to the electrode the analyzer is shown to resolve ions with different mass. When the radio‐frequency power to the plasma pulsed the analyzer is used to resolve the ion energy distributions at different times in the pulse. The high energy tail of the electron energy distribution reaching the electrode surface is also measured. A comparison with a Langmuir probe shows exceptional agreement in the energy region where both devices overlap.

Auger electron spectroscopy for the advanced student laboratory

This paper presents Auger electron spectroscopy with a retarding field analyser designed for an advanced physics experiment carried out in ‘Physics Laboratory II’ at the Institute of Experimental Physics, University of Wroclaw, Poland. The authors discuss the process of setting up the experiment and the results of the measurement of Auger spectra. The advantages and disadvantages of the apparatus are discussed along with its implementation in the teaching process.

Measurement of lower hybrid hot spots using a retarding field analyzer in Tore Supra

A retarding field analyzer was used to provide direct measurements of the particle and power fluxes of suprathermal electrons in hot spots that are magnetically connected to the region in front of the waveguides of a lower hybrid antenna in Tore Supra. Hot spots can be divided into two regions according to the temporal evolution of the local suprathermal electron flux. Adjacent to each waveguide row is a layer of strong, dc current on which is superimposed strong fluctuations. Deeper into the plasma a second type of electron flux is identified: highly intermittent (∼10 kHz) bursts separated by brief periods of zero flux. Electron bursts occur at least up to 40–50 mm in front of the grill mouth. These observations contradict the standard theory that predicts that hot spots should be at most ∼5 mm wide.

Directional Auger Electron Spectroscopy - Possibilities, Limitations and Proposal of Improvements

Physical foundations of directional auger electron spectroscopy (DAES) and calculation of DAES profiles in single scattering cluster approach are presented. Limitations of this method (application only to investigation of the crystalline structure of homogeneous samples) is shown and explained as the result of participation of inelastically scattered electrons in the Auger signal generation. To extend the DAES application for the interface structure, the use of as low as possible energy of primary electrons is proposed because in such a case the participation of inelastically scattered electrons becomes negligible and single scattering cluster calculation should describe correctly the DAES profiles for interfaces. Besides, the extension of single scattering cluster calculations to the second elastic scattering is recommended. To check the technical possibility of DAES use in the proposed version, the Auger spectrum of Cu LMM peaks was recorded for a Cu3Au(001) sample with the use of a retarding field analyzer with the primary beam energy 1200 eV. Quality of this spectrum seems to be good enough for using in DAES.