Antenna Straps Research Articles

ICRH-related impurity source and control across experiments in H, D, T plasmas at JET-ILW

The experimental and theoretical analysis were focused on experiments conducted to assess the effect of plasma isotopes, protium (H), deuterium (D), and tritium (T) on ion cyclotron resonance heating (ICRH) related plasma wall interactions. Comparison of L-mode discharges with N = 1 3He and N = 1 H minority ICRH heating scenarios were done for different isotopes. For the selected pulses, the behaviour of high-Z, mid-Z and low-Z intrinsic impurity and radiated power behaviour was investigated based on data from VUV, visible spectroscopy, and bolometry diagnostic at Joint European Torus. It was found that for N = 1 3He scenario during radiofrequency antennas operation, core W, Ni content, Be source and the radiated power are higher for π/2 in comparison to dipole antenna phasing. Lowest core Ni, W content and radiated power is clearly observed for H plasmas in comparison to D and T, where for this ICRH scenario behaviour was similar. However, lower Be photon flux is observed for T in comparison to D plasmas. Be sputtering by He particles is responsible for such an effect. Additionally, several computer simulations were conducted using the COREDIV code. The difference in the electron temperature was due to the difference in the isotope masses. Increased temperature in the central plasma in the case of T plasmas leads to higher radiation in the central plasma in comparison to H plasmas. As a result, the power across separatrix is lower and the temperature on the divertor plate decreases with the increase of the isotope mass. At these temperatures on the divertor plate, W is not sputtered by the main plasma ions H, D and T and by He. For the N = 1 H ICRH scenario clear difference between D and T plasma was observed with higher metallic impurity content for T plasma in comparison to D. Impurity content in the plasmas is found to be sensitive to the power balance between the antenna straps. Its minimum is observed for the maximum of P cen/P tot.

Design and operation of a load-tolerant ICRH system in Experimental Advanced Superconducting Tokamak

Ion Cyclotron Resonance Heating (ICRH) has been a dependable tool for sturdy plasma heating with high RF power of several megawatts. However, a sudden increase in the reflected power during ICRH heating experiments is a problem that should be solved for future fusion experimental devices. To solve this issue, a load tolerant matching network has been designed for the ICRH system in EAST. The matching network includes a 3-stub tuner impedance matching system with conjugate-T structure, 30 Ω to 50 Ω transmission line and center grounded antenna strap. By maintaining a low reflection ratio in the network for a wide range of resistance, this matching network can allow sturdy high-power operations without fast impedance matching in EAST. In our matching network, the two arms of a conjugate-T are designed to have a λ/2 length difference which could mitigate current imbalance and antenna poloidal phasing outside of the control problem. The T-point corresponds to the maximum point of the standing wave voltage, which could greatly improve the input impedance of the antenna.

Detection of ion cyclotron emission by using an ion cyclotron range of frequency antennas-based diagnostic system in experimental advanced superconducting tokamak

In the experimental advanced superconducting tokamak (EAST), a novel ion cyclotron range of frequency (ICRF) antenna-based diagnostic system is designed to measure ion cyclotron emission (ICE) driven by high-energy ions. The diagnostic system includes ICRF antenna straps, a three-tune impedance matching system, a coaxial switching system, a direct current block, and a data acquisition and storage system. Using the coaxial switching system, the ICRF antenna can be switched from the heating mode to the coupling mode between two discharges. In the 2023 EAST experiment campaign, core ICE was observed using the ICRF antenna-based diagnostic system during neutron beam injection heating, and the obtained results agreed well with the signal detected by the previous high-frequency B-dot probe-based diagnostic system.

Predictive simulations of NBI ion power load to the ICRH antenna in Wendelstein 7-X

In Wendelstein 7-X (W7-X), a new ion cyclotron resonance heating (ICRH) antenna will be commissioned during the operational campaign OP2.1. The antenna will have to sustain power loads not only from thermal plasma and radiation but also fast ions. Predictive simulations of fast-ion power loads to the antenna components are therefore important to establish safe operational limits. In this work, the fast-ion power loads from the W7-X neutral beam injection (NBI) system to the ICRH antenna was simulated using the ASCOT suite of codes. Five reference magnetic configurations and five antenna positions were considered to provide an overview of power load behavior under various operating conditions. The NBI power load was found to have an exponential dependence on the antenna insertion depth. Differences between magnetic configurations were significant, with the antenna limiter power load varying between 380 W and 100 kW depending on the configuration. Qualitative differences in power load patterns between configurations were also observed, with the low mirror and low iota configurations exhibiting higher loads to the sensitive antenna straps. The local fast-ion power flux to the antenna limiter was also considered and found to exceed the 2.0 MW m−2 steady-state safety limit only in specific cases. The NBI system might thus pose a safety concern to the ICRH antenna during concurrent NBI-ICRH operation, but additional heat propagation simulations of antenna components are needed to establish more realistic operational time limits.

Operational requirements of the ion cyclotron wall conditioning in DTT

The Divertor Tokamak Test facility (DTT), a new superconducting tokamak (B0=6T, Ip=5.5MA, R0=2.19 m, a = 0.7 m, pulse length=100 s) (Ambrosino et al., 2021) under realization at the ENEA Frascati Research centre, needs an efficient first wall conditioning technique suitable for the use in the presence of a permanent magnetic field. The Ion Cyclotron Wall Conditioning (ICWC), based on Radio-Frequency (RF) discharges (Lyssoivan et al., 2012), with waves in the ion cyclotron range of frequencies, is one of the most efficient technique available (between discharges) for routine inter–pulse and overnight conditioning. ICWC will be performed using the first Ion Cyclotron Resonance Heating (ICRH) RF module foreseen during the first phase of DTT and constituted by two Antennas, four or two External Matching Units (EMUs) and four RF solid-state transmitters, operating in the frequency range (60–90) MHz. In the paper by means of the HFSS and CST codes, an eigenvalue analysis of the electromagnetic waves (E/H modes) excited inside the vacuum DTT volume has been carried out. Then full-wave simulations have been run for the entire DTT torus injecting a RF signal with a single and a pair of three–strap antennas, currently in the design phase for DTT, evaluating the S-parameters and the parallel electric field (E//) aligned to the toroidal magnetic field, responsible of the initial gas breakdown. On this basis the most appropriate choice for the RF feeding, in monopole phasing, as well as for the frequency of ICWC discharges in order to maximize E// has been identified and presented in the paper, demonstrating for the first time the usefulness of using an H mode at the frequency of 62.5 MHz instead of the lower frequency of 60 MHz minimizing the RF power required to activate the ionization process. Finally operational parameters for DTT, extrapolated from the analysis of ICWC experiments documented in literature, will be presented.

Estimation of erosion phenomena within helicon plasma sources through a steady-state explicit analytical model

Helicon plasma sources produce high-density discharges without the need of electrodes in direct contact with the plasma, which is thought to provide them with long operational lifetimes. An explicit steady-state analytical model is described with the capability of depicting the 2D plasma density distribution, the sheath potentials and the estimated sputtering and etch rates along the plasma-facing components of the source. The individual constituting submodels are fitted against available experimental data, and the model is used to predict erosion rates within the VX-CR research helicon plasma source. Erosion within these components is dependent on the value of plasma density along the boundaries, the electron temperature and the particular ion-target material combination. The highest erosion rates are found along the upstream system boundary, followed by the regions near the helicon antenna straps where a capacitive RF sheath is formed. The assumptions and limitations of the model are discussed, and future improvements are proposed.

Measurement of the ICRH antenna phasing using antenna strap probe based diagnostic system in EAST tokamak

To operate the ion cyclotron resonance heating (ICRH) antennas in a better heating state and produce relatively low impurities, it is necessary to control the antenna spectrum by changing the antenna phasing. As the electrical length of the antenna feeding transmission lines is changing as a matter of the standing wave pattern at the ceramic supports, 90° elbows, T-connectors and antenna loops, we chose to measure the current at the grounding points of the antenna loops by antenna strap probe. The voltage drops along a small, several millimeter-long paths at the end of the antenna loops give a signal that is proportional to the current in the antenna loop. Through the simulation of the antenna strap probe and the actual measurement of the antenna phasing under vacuum conditions, the reliability of the antenna strap probe based diagnostic system have been successfully proved. Moreover, this system was successfully applied to the ICRH daily experiments in the spring of 2021. In the near future, the active real-time feedback control of the antenna phasing system will be developed based on this diagnostic system in the EAST tokamak.

Flexible Tri-Band Dual-Polarized MIMO Belt Strap Antenna Toward Wearable Applications in Intelligent Internet of Medical Things

This article proposes a novel concept of flexible dual-polarized multiple-input multiple-output (MIMO) belt strap antenna with triple band. It can be integrated into the wearable biomedical network base station (WBNBS) of intelligent Internet of Medical Things (IoMT), setting up a communication link with smart mobile devices, thereby hosting wireless wearable sensors to track the vital signs of the human body. The proposed antenna includes three identical antenna units equidistantly distributed on the belt strap. Each unit is composed of two linearly polarized dipoles placed orthogonally. The goat horn-shaped outer loop and M-shaped inner loop produce the first and second bands, respectively, and their second harmonic jointly forms the third band. The arc-shaped coupled ring is connected with one dipole arm, which is conducive to tune impedance matching. The effects of belt strap bending and antenna radiation on the body are analyzed. Specific absorption rate (SAR) values are reduced by attaching the absorbing-shielding layer. As a proof of concept, the proposed antenna is fabricated for verification. Both measured and simulated results demonstrate the excellent performance with a realized gain of 2.2–3.1 dBi and radiation efficiency of 46.1%–80.1% in −10 dB impedance bandwidths of 2.37–2.7, 2.8–3.22, and 5.41–6.03 GHz. Finally, a smart belt prototype system is manufactured and tested. A link between wearable sensors and smartphones/computers is set up via WBNBS, where vital signs could be digitally displayed and monitored. It could provide a health risk alarm for a serious pandemic.

Development of TWA mock-up for TITAN

The challenge of tokamak auxiliary heating method ICRH is to couple large amount of power through the plasma boundary, where an evanescence layer has to be crossed, without exceeding the voltage standoff at the antenna. Travelling Wave Array antenna has been proposed for ICRH of future fusion reactor such as DEMO in view to decrease the antenna power density. However, so far the voltage standoff of a TWA antenna in the ICRH frequency range (30–60 MHz) has never been demonstrated. A TWA mock-up has been designed and will be tested in the CEA/IRFM testbed TITAN to assess its compatibility to RF high power operation. In this paper, the main characteristics of the TWA mock-up are reported in detail. In addition, some near-field RF analyses are presented, containing reflection characteristics, port voltage, electric field and k// spectrum. Meanwhile, the structural evaluation of the antenna during its lifting process is carried out to check with safety requirements. The assembly strategy of the whole structure is also explained elaborately to ensure the fluency and correct integration of the TWA mock-up. Finally, the RF assessment of antenna straps critical dimensions is briefly described.

Physics basis for the ICRF system of the SPARC tokamak

Ion cyclotron range of frequencies (ICRF) heating will be the sole auxiliary heating method on SPARC for both full-field (Bt0 ~ 12 T) D–T operation and reduced field (Bt0 ~ 8 T) D–D operation. Using the fast wave at ~120 MHz, good wave penetration and strong single-pass absorption is expected for D–T(3He), D(3He), D(H) and4He(H) heating scenarios. The dependences of wave absorption on${k_\parallel }$,3He concentration, resonance location, antenna poloidal location and losses on alpha particles and ash have been studied. The antenna loading has been assessed by comparison with the Alcator C-Mod antennae. An antenna spectrum of${k_\parallel }\sim 15\text{--}18\,{\textrm{m}^{ - 1}}$is shown to be good for both core absorption and edge coupling. For the control of impurity sources, the antenna straps are rotated ~10° to be perpendicular to the B field and the straps can run with different power levels in order to optimize the antenna spectrum and to minimize the image current on the antenna frame. Combining the physics constraints with the SPARC port design, maintenance requirement and contingency against antenna failure during D–T operation, we plan to mount on the inner wall of the vacuum vessel a total of 12 4-strap antennae in 6 ports while keeping 3-strap antennae that are insertable and removable on port plugs as the backup option.

Coaxial and surface mode excitation by an ICRF antenna in large machines like DEMO and ITER

A study of radio frequency (RF) field excitation in the edge of the plasma of DEMO is performed by means of the semi-analytic coupling code ANTITER II. The modeling uses the designed antenna and a reference low coupling density profile of ITER. The results show the existence of coaxial modes propagating between the wall and the Alfvén resonance region where surface modes are excited leading to large standing wave patterns all around the machine. The excitation of these modes can be strongly reduced for a strap current distribution of the antenna array which fulfills the two conditions and (Ii: current of strap i, Si: toroidal strap position). These conditions are satisfied by the triple strap antenna of AUG that has allowed successful ICRH operation in a W coated machine. They are also achieved for the (0ππ0) phasing of the four columns of two poloidal triplets of radiating straps constituting the ITER antenna array. Ways to further optimize the cancellation of these edge modes are also investigated.

Central ion cyclotron emission in the DIII-D tokamak

Collective ion cyclotron emission (ICE) at the ion cyclotron frequency and its harmonics is a potential passive diagnostic of the fast-ion distribution in fusion reactors. ICE is observed in most plasmas in the DIII-D tokamak and is most strongly excited by the fast ions from neutral beam injection. The conventional outboard-edge ICE is detected in H-mode plasmas. However, weaker centrally-localized ICE is measured in L-mode plasmas, including those with negative triangularity shapes. Similar ICE spectra are found with both ICE diagnostics systems, the dedicated magnetic probes and the instrumented antenna straps. Many differences in the behavior of this central ICE are generated by varying the deuterium beam injection angle into deuterium plasmas. The co-current ‘near-perpendicular’ beam excites the most central ICE from the co-current beams, with this emission detected from the fundamental to the fifth ICE harmonic. However, the counter-current ‘near-tangential’ beam destabilizes the highest amounts of centrally-localized ICE. This emission is spectrally broader than that driven by the co-current beams and is observed up to its seventh ICE harmonic. The central ICE excited by this co-current beam correlated strongly on the local electron density and related parameters (plasma current and neutron rate) and increased with deeper fast-ion loss boundaries towards the magnetic axis. This was also the case with second harmonic ICE driven by the counter-current beam but not with its stronger third harmonic emission. The central ICE harmonics destabilized by both beams are observed to have different temporal dynamics. The central ICE amplitude responds rapidly to transient MHD events; it dropped and recovered in less than a millisecond at each sawtooth event. ICE frequency splitting is triggered by both the co-current and counter-current ‘near-tangential’ beams. The data presented in this paper provide opportunities to test and validate models of excitation of ICE by energetic ions.

ICRH antenna S-matrix measurements and plasma coupling characterisation at JET

The paper is dedicated to the characterisation of multi-strap ICRH antenna coupling to plasma. Relevance of traditional concept of coupling resistance to antennas with mutually coupled straps is revised and the importance of antenna port excitation consistency for application of the concept is highlighted. A method of antenna S-matrix measurement in presence of plasma is discussed allowing deeper insight into the problem of antenna-plasma coupling. The method is based entirely on the RF plant hardware and control facilities available at JET and it involves application of variable phasing between the antenna straps during the RF plant operations at >100 kW. Unlike traditional techniques relying on low-power (~10 mW) network analysers, the applied antenna voltage amplitudes are relevant to practical conditions of ICRH operations; crucially, they are high enough to minimise possible effects of antenna loading non-linearity due to the RF sheath effects and other phenomena which could affect low-power measurements. The method has been successfully applied at JET to conventional 4-port ICRH antennas energised at frequencies of 33 MHz, 42 MHz and 51 MHz during L-mode plasma discharges while different gas injection modules (GIMs) were used to maintain comparable plasma densities during the pulses. The S-matrix assessment and its subsequent processing yielding ‘global’ antenna coupling resistances in conditions of equalised port maximum voltages allowed consistent description of antenna coupling to plasma at different strap phasing, operational frequencies and applied GIMs. Comprehensive experimental characterisation of mutually coupled antenna straps in presence of plasma also provided a unique opportunity for in-depth verification of TOPICA computer simulations.

Design and implementation of power and phase feedback control system for ICRH on EAST

Ion cyclotron wave resonance heating (ICRH) is one of the most important auxiliary methods to heat plasma in the Experimental Advanced Superconducting Tokamak (EAST). Several megawatts of power is transmitted through separate coaxial lines and coupled with the plasma through arrays of loop antennas. The parameters of the ICRH system, including the injected power and phasing between antenna straps, are critical to the coupling efficiency of the power as well as the resulting impact on the heating efficiency. In this paper, we present a system for feedback control of the phase between the current straps and the ICRH power on EAST. The feedback control system was tested using both a matched dummy load and a plasma load, and it successfully maintained stable operation in the 2016 EAST campaign. Good control of the injected power and wave phases was achieved during edge-localized mode operation.

A PIC-MCC code RFdinity1d for simulation of discharge initiation by ICRF antenna

Discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current will be used on ITER for (ion cyclotron-) wall conditioning (ICWC, eV, ). In this paper, we present the 1D particle-in-cell Monte Carlo collision (PIC-MCC) RFdinity1d for the study the breakdown phase of ICRF discharges, and its dependency on the RF discharge parameters (i) antenna input power Pi, (ii) RF frequency f, (iii) shape of the electric field and (iv) the neutral gas pressure . The code traces the motion of both electrons and ions in a narrow bundle of magnetic field lines close to the antenna straps. The charged particles are accelerated in the parallel direction with respect to the magnetic field BT by two electric fields: (i) the vacuum RF field of the ICRF antenna and (ii) the electrostatic field determined by the solution of Poisson’s equation.The electron density evolution in simulations follows exponential increase, . The ionization rate varies with increasing electron density as different mechanisms become important. The charged particles are affected solely by the antenna RF field at low electron density ( , ). At higher densities, when the electrostatic field is comparable to the antenna RF field , the ionization frequency reaches the maximum. Plasma oscillations propagating toroidally away from the antenna are observed. The simulated energy distributions of ions and electrons at correspond a power-law Kappa energy distribution. This energy distribution was also observed in NPA measurements at ASDEX Upgrade in ICWC experiments.

Development of an ICRH antenna system at W7-X for plasma heating and wall conditioning

An ICRH antenna system is under construction to be ready for use in the operational phase 1.2 of W7-X. A two strap antenna, with a surface adapted to the 3D shape of the Last Closed Flux Surface (LCFS) of the standard magnetic configuration (m/n=5/5), will be installed in the equatorial plane on the low field side of W7-X. The antenna system is optimised for plasma heating and wall conditioning in presence of the magnetic field in the frequency range 25–38MHz. Each strap is short-circuited on one end, is pre-matched using a tunable capacitor on the other end with connection to the RF power source about halfway in the poloidal direction. To allow optimal coupling to different magnetic configurations in W7-X the antenna can be moved radially over a distance of 350 mm. The cooling circuits in the antenna head are designed to sustain cw W7-X plasma operation in a retracted position and 2MW ICRH pulses with maximum 10s duration every 300s close to the LCFS.

Design of an ICRF system for plasma–wall interactions and RF plasma production studies on TOMAS

Ion cyclotron wall conditioning (ICWC) is being developed for ITER and W7-X as a baseline conditioning technique in which the ion cyclotron heating and current drive system will be employed to produce and sustain the currentless conditioning plasma. The TOMAS project (TOroidal MAgnetized System, operated at the FZ-Juelich, Germany) proposes to explore several key aspects of ICWC.For this purpose we have designed an ICRF system made of a single strap antenna within a metallic box, connected to a feeding port and a pre-matching system. We discuss the design work of the antenna system with the help of the commercial electromagnetic software CST Microwave Studio®. The simulation results for a given geometry provide input impedance matrices for the two-port system. These matrices are afterwards inserted into various circuit models to assess the accessibility of the required frequency range.The sensitivity of the matching system to uncertainties on plasma loading and capacitance values is notably addressed. With a choice of three variable capacitors we show that the system can cope with such uncertainties. We also demonstrate that the system can cope as well with the high reflected power levels during the short breakdown phase of the RF discharge, but at the cost of a significantly reduced coupled power.

A Watch Strap Antenna for the Applications of Wearable Systems

A wearable antenna based on the metal watch strap is proposed. A prototype of a TISSOT classic watch is used for the demonstration. The width of the strap has only a little impact on the radiation patterns. However, the variations of the feeding location would dramatically influence the resonance frequency and the matching performance. The shapes and the graphic patterns of the wristband have contributions on the reflection coefficients as well as the radiation patterns. A fractional bandwidth of 77.8% could be obtained when feeding to the right of the dial at 2.46 GHz, which falls into the ISM (industrial, scientific, and medical) band. The main lobe is fairly wide in both $E$ - and $H$ -planes, which would enable the strap antenna to have a good adaptation for the various postures of the arm. The measurement results show good agreement with that of the simulations, which indicates it a potential option for the applications of wearable systems.

Discharge initiation by ICRF antenna in IShTAR

IShTAR is a linear magnetized plasma test facility dedicated to the investigation of RF wave/plasma interaction. The IShTAR ICRF system consists of a single strap RF antenna. When using the antenna for plasma production without an external plasma source, it is shown that the plasma is either produced in front of the antenna strap or inside the antenna box depending on the antenna parameters. Here, we present experimental and numerical investigation of the plasma initiation parametric dependencies. Detailed pressure and RF power scans were performed in helium at f = 5.22 MHz and f = 42.06 MHz. The experiment shows the parameter ranges for which the plasma is produced in front of the strap, or inside the antenna box. These ranges are validated by simulations with the RFdinity model, and by theoretical predictions.

Advanced ponderomotive description of electron acceleration in ICRF discharge initiation

This contribution proposes a new approach for the ponderomotive description of electron acceleration in ICRF discharge initiation. The motion of electrons in the parallel electric field E z is separated into a fast oscillation and a slower drift around the oscillation centre. Three terms are maintained in the Taylor expansion of the electric field (0th , 1st and 2nd order). The efficiency for electron acceleration by E z (z , t ) is then assessed by comparing the values of these terms at the slow varying coordinate z 0 . When (i) the 0th order term is not significantly larger than 1st order term at the reflection point, or when (ii) the 2nd order term is negative and not sufficiently small compared to the 1st order term at the reflection point, then the electron will gain energy in the reflection.An example for plasma production by the TOMAS ICRF system is given. Following the described conditions it can be derived that plasma production is (i) most efficient close to the antenna straps (few cm's) where the field gradient and amplitude are large, and (ii) that the lower frequency field accelerates electrons more easily for a given antenna voltage.