Development of the ion cyclotron emission diagnostic for the W7-X stellarator.

An ion cyclotron emission (ICE) diagnostic is prepared for installation into the W7-X stellarator, with the aim to be operated in the 2022 experimental campaign. The design is based on the successful ICE diagnostic on the ASDEX Upgrade tokamak. The new diagnostic consists of four B-dot probes, mounted about 72° toroidally away (one module) from the neutral beam injector, with an unobstructed plasma view. Two of the B-dot probes are oriented parallel to the local magnetic field, aimed to detect fast magnetosonic waves. The remaining two probes are oriented poloidally, with the aim to detect slow waves. The radio frequency (RF) signals picked up by the probes are transferred via 50 Ω vacuum-compatible coaxial cables to RF detectors. Narrow band notch filters are used to protect the detectors from possible RF waves launched by the W7-X antenna. The signal will be sampled with a four-channel fast analog-to-digital converter with 14 bit depth and 1 GSample/s sampling rate. The diagnostic's phase-frequency characteristic is properly measured in order to allow measuring the wave vectors of the picked up waves.

Development of an experimental facility for the study of microparticle initiated radio frequency vacuum breakdown.

An ongoing objective in the ion cyclotron range of frequencies (ICRF) systems is the improvement of power coupling to the plasma. During the last decade, this goal has been mainly pursued through the study of the coupling resistance, either by optimizing the antenna layout or by tailoring the scrape-off layer profile with gas puffing. Another approach is to increase the voltage handling capability of the ICRF system, limited by breakdown in the launchers or in the transmission lines. This paper describes the design of the ICRF Breakdown EXperiment (IBEX), a device to investigate fundamental aspects of radio frequency arcs under ICRF-relevant conditions. IBEX can achieve a peak voltage of 48 kV at 54 MHz with a 5 kW input power.

Ion temperature measurement techniques using fast sweeping retarding field analyzer (RFA) in strongly intermittent ASDEX Upgrade tokamak plasmas.

This manuscript presents a new method of interpreting the ion temperature (Ti) measurement with a retarding field analyzer (RFA) that accounts for the intermittent/turbulent nature of the scrape off layer (SOL) plasmas in tokamaks. Fast measurements and statistical methods are desirable for an adequate description of random fluctuations caused by such intermittent events as edge localized modes (ELMs) and blobs. We use a RFA that can sweep its current-voltage (I-V) characteristics with up to 10 kHz. The RFA uses an electronics compensation stage to subtract the capacitive pickup due to the finite connecting cable capacitance, which greatly improves the signal-to-noise ratio. In the 10 kHz case, a single I-V characteristic is obtained in time, which is an order of magnitude faster than the ELM cycle. The fast sweeping frequency allows us to reconstruct the Ti probability density function (PDF), which we use as the Ti representation. The boundary conditions that we place on the I-V characteristics when calculating the Ti values impact the resulting Ti PDF. If the boundaries are insensitive to the plasma fluctuations, then the most probable Ti value of the PDF (20 eV-25 eV) is similar to the Ti value obtained via the classical conditional averaging method (20 eV-27 eV). However, if the boundary conditions follow the fluctuations, then the PDF-based method gives a substantially higher most probable Ti value (35 eV-60 eV). Overall, we show that a fast sweeping RFA diagnostic should be used in intermittent SOL plasmas to reconstruct the PDF for accurate Ti measurements.

IShTAR: A test facility to study the interaction between RF wave and edge plasmas.

Existence of high electric fields near an RF antenna launcher causes a number of parasitic phenomena, such as arcing and impurity release, which seriously deteriorate the performance of an Ion Cyclotron Range of Frequencies (ICRF) heating scheme in fusion devices. Limited accessibility of the near antenna region in large-scale fusion experiments significantly complicates the associated experimental studies. The IShTAR test facility has been developed with the requirement to provide a better accessibility and diagnosability of plasmas in the direct vicinity of an ICRF antenna. The purpose of this work is to give a detailed description on the experimental setup and the available diagnostics. Furthermore, the paper will demonstrate the capability of the experiment to study phenomena near an ICRF antenna launcher which are relevant for large-scale fusion ion cyclotron resonance heating systems.

Observations of core ion cyclotron emission on ASDEX Upgrade tokamak.

The B-dot probe diagnostic suite on the ASDEX Upgrade tokamak has recently been upgraded with a new 125 MHz, 14 bit resolution digitizer to study ion cyclotron emission (ICE). While classic edge emission from the low field side plasma is often observed, we also measure waves originating from the core with fast fusion protons or beam injected deuterons being a possible emission driver. Comparing the measured frequency values with ion cyclotron harmonics present in the plasma places the origin of this emission on the magnetic axis, with the fundamental hydrogen/second deuterium cyclotron harmonic matching the observed values. The actual values range from ∼27 MHz at the on-axis toroidal field BT = -1.79 T to ∼40 MHz at BT = -2.62 T. When the magnetic axis position evolves during this emission, the measured frequency values track the changes in the estimated on-axis cyclotron frequency values. Core ICE is usually a transient event lasting ∼100 ms during the neutral beam startup phase. However, in some cases, core emission occurs in steady-state plasmas and lasts for longer than 1 s. These observations suggest an attractive possibility of using a non-perturbing ICE-based diagnostic to passively monitor fusion alpha particles at the location of their birth in the plasma core, in deuterium-tritium burning devices such as ITER and DEMO.

A simple and versatile phase detector for heterodyne interferometers.

The measurement of the relative phase of two sinusoidal electrical signals is a frequently encountered task in heterodyne interferometry, but also occurs in many other applications. Especially in interferometry, multi-radian detectors are often required, which track the temporal evolution of the phase difference and are able to register phase changes that exceed 2π. While a large variety of solutions to this problem is already known, we present an alternative approach, which pre-processes the signals with simple analog circuitry and digitizes two resulting voltages with an analog-to-digital converter (ADC), whose sampling frequency can be far below the frequency of the sinusoidal signals. Phase reconstruction is finally carried out by software. The main advantage of this approach is its simplicity, using only few low-cost hardware components and a standard 2-channel ADC with low performance requirements. We present an application on the two-color interferometer of the ASDEX Upgrade tokamak, where the relative phase of 40 MHz sinusoids is measured.

ICRF wave field measurements in the presence of scrape off layer turbulence on the ASDEX Upgrade tokamak (invited).

A new array of B-dot probes was installed on ASDEX Upgrade. The purpose of the new diagnostic is to study Ion Cyclotron Range-off Frequencies (ICRF) wave field distributions in the evanescent scrape-off layer (SOL) plasma region on the low field side of ASDEX Upgrade. The vacuum measurements (no gas, BT = 0 T) reveal ICRF wave field measurements consistent with the profiles expected from the newly installed 3-strap ICRF antennas outside the antenna box: the shape of the toroidal distribution of both the amplitude and the phase is the same for the case of only the central straps being active, as for the case of only the side straps being active. These profiles become strongly modified during plasma operations. The modifications can be separated into two types: "Inter-edge localized mode (ELM)" and "During-ELM" periods. The phase distribution of the ICRF wave fields remains well-defined during the Inter-ELM period; however, it becomes more spread out over the entire 360° range during ELMs. The observed modulations cannot be explained by the observed changes in the ICRF power, as monitored in the transmission line. However, they are consistent with ICRF coupling changes introduced by plasma filaments: the plasma density perturbations due to the filaments are high enough to change the nature of the fast ICRF wave field from evanescent to propagating. The coverage of the present diagnostic is being expanded to include both the low field side and the high field side probes. Additionally, a manipulator probe head is being developed to measure ICRF wave field radial profiles across the SOL region.

Implementation of the new multichannel X-mode edge density profile reflectometer for the ICRF antenna on ASDEX Upgrade.

A new multichannel frequency modulated continuous-wave reflectometry diagnostic has been successfully installed and commissioned on ASDEX Upgrade to measure the plasma edge electron density profile evolution in front of the Ion Cyclotron Range of Frequencies (ICRF) antenna. The design of the new three-strap ICRF antenna integrates ten pairs (sending and receiving) of microwave reflectometry antennas. The multichannel reflectometer can use three of these to measure the edge electron density profiles up to 2 × 1019 m-3, at different poloidal locations, allowing the direct study of the local plasma layers in front of the ICRF antenna. ICRF power coupling, operational effects, and poloidal variations of the plasma density profile can be consistently studied for the first time. In this work the diagnostic hardware architecture is described and the obtained density profile measurements were used to track outer radial plasma position and plasma shape.

A new B-dot probe-based diagnostic for amplitude, polarization, and wavenumber measurements of ion cyclotron range-of frequency fields on ASDEX Upgrade.

A new B-dot probe-based diagnostic has been installed on an ASDEX Upgrade tokamak to characterize ion cyclotron range-of frequency (ICRF) wave generation and interaction with magnetized plasma. The diagnostic consists of a field-aligned array of B-dot probes, oriented to measure fast and slow ICRF wave fields and their field-aligned wavenumber (k(//)) spectrum on the low field side of ASDEX Upgrade. A thorough description of the diagnostic and the supporting electronics is provided. In order to compare the measured dominant wavenumber of the local ICRF fields with the expected spectrum of the launched ICRF waves, in-air near-field measurements were performed on the newly installed 3-strap ICRF antenna to reconstruct the dominant launched toroidal wavenumbers (k(tor)). Measurements during a strap current phasing scan in tokamak discharges reveal an upshift in k(//) as strap phasing is moved away from the dipole configuration. This result is the opposite of the k(tor) trend expected from in-air near-field measurements; however, the near-field based reconstruction routine does not account for the effect of induced radiofrequency (RF) currents in the passive antenna structures. The measured exponential increase in the local ICRF wave field amplitude is in agreement with the upshifted k(//), as strap phasing moves away from the dipole configuration. An examination of discharges heated with two ICRF antennas simultaneously reveals the existence of beat waves at 1 kHz, as expected from the difference of the two antennas' operating frequencies. Beats are observed on both the fast and the slow wave probes suggesting that the two waves are coupled outside the active antennas. Although the new diagnostic shows consistent trends between the amplitude and the phase measurements in response to changes applied by the ICRF antennas, the disagreement with the in-air near-field measurements remains. An electromagnetic model is currently under development to address this issue.