Feedback Coil Research Articles

FLIPEC, an ideal MHD free-boundary axisymmetric equilibrium solver in the presence of macroscopic flows

The most relevant features of FLIPEC (Free fLow Iterative Plasma Equilibrium Code) are presented. This new code iteratively calculates free-boundary, axisymmetric ideal MHD equilibria with arbitrary poloidal and toroidal plasma flows. FLIPEC is a mature code that has emerged from a complete overhaul of a previous version (F-Torija Daza 2022 et al Nucl. Fusion 62 126044). It uses a (inverse) curvilinear coordinate representation for the Grad–Shafranov–Bernoulli equation system, which allows FLIPEC to extend its free-boundary capabilities to arbitrary plasma shapes and removes many limitations with regards to the distance between plasma and external coils. Run-time stabilization of vertical modes has also been implemented by means of artificial feedback coils. Finally, active targeting schemes have also been included. These capabilities are illustrated on two very different cases: the ITER tokamak baseline configuration and a NSTX spherical tokamak equilibrium.

Dual-range TMR current sensor based on magnetic shunt/aggregation effects utilizing single magnetic ring structure

In this paper, we propose and design a novel dual-range tunnel magnetoresistance (TMR) current sensor with a single magnetic ring structure. This design incorporates two distinct magnetic guiding effects, namely magnetic shunt and magnetic aggregation, within the same magnetic ring. By integrating a high-sensitivity TMR sensor chip with a closed-loop feedback circuit, we achieve a TMR current sensor with excellent linearity, high resolution, as well as high frequency response. The magnetic ring structure is first modeled and simulated, establishing a correlation between the distribution of magnetic induction intensity and the parameters of the magnetic ring and feedback coils. Through simulation optimization and theoretical calculations, we determine the optimal positions for TMR sensor chips in the magnetic ring, suitable for both current ranges. When a signal current is present, the TMR sensor chip generates a weak differential voltage signal, which is subsequently amplified, processed, and automatically transmitted to the laptop via a serial port. Furthermore, the sensor allows for automatic switching between the two current ranges. The results demonstrate that our designed dual-range current sensor exhibits outstanding performance characteristics, including a high resolution of 500 μA in the small range, accuracy of 0.10%, excellent linearity of 0.011%, and a fast frequency response of 500 kHz. These features make it highly applicable in various fields such as new energy vehicles and smart grids, indicating promising prospects for its widespread utilization.

A metal object detection system for electric vehicle wireless charging based on LC oscillating circuit and reverse-winding-incorporated detection coils

In magnetically coupled wireless charging systems for electric vehicles, it is common for metal objects to enter the charging system. In this case, the strong magnetic field of wireless charging causes an eddy current effect in the metal object, which generates a large amount of heat. This heat accumulation poses a risk of fire. In this paper, a novel metal object detection system designed for wireless charging of electric vehicles is proposed. The system utilizes the principle that the coil impedance and coupling coefficient change with the presence of metal objects, and a set of composite coils is designed to improve the sensitivity of detection. The detection coil and feedback coil in the composite coil are decoupled by reverse series connection. An LC oscillating circuit based on transformer feedback is used to detect metal objects, and the circuit can make good use of the parameter changes of the composite coil. Finally, an experimental platform for wireless charging metal detection in electric vehicles has been built to validate the proposed system. Through testing, it has been demonstrated that the system can detect the presence of metal objects in a timely and accurate manner.

A Wide-Band Magnetoelectric Sensor Based on a Negative-Feedback Compensated Readout Circuit.

Magnetoelectric (ME) sensors cannot effectively detect broadband magnetic field signals due to their narrow bandwidth, and existing readout circuits are unable to vary the bandwidth of the sensors. To expand the bandwidth, this paper introduces a negative-feedback readout circuit, fabricated by introducing a negative-feedback compensation circuit based on the direct readout circuit of the ME sensor. The negative-feedback compensation circuit contains a current amplifier, a feedback resistor, and a feedback coil. For this purpose, a Metglas/PVDF/Metglas ME sensor was prepared. Experimental measurements show that there is a six-fold difference between the maximum and minimum values of the ME voltage coefficients in the 6-39 kHz frequency band for the ME sensor without the negative-feedback compensation circuit when the sensor operates at the optimal bias magnetic field. However, the ME voltage coefficient in this band remains stable, at 900 V/T, after the charge amplification of the direct-reading circuit and the negative-feedback circuit. In addition, experimental results show that this negative-feedback readout circuit does not increase the equivalent magnetic noise of the sensor, with a noise level of 240 pT/√Hz in the frequency band lower than 25 kHz, 63 pT/√Hz around the resonance frequency of 30 kHz, and 620 pT/√Hz at 39 kHz. This paper proposes a negative-feedback readout circuit based on the direct readout circuit, which greatly increases the bandwidth of ME sensors and promotes the widespread application of ME sensors in the fields of broadband weak magnetic signal detection and DBS electrode positioning.

Equivalent Noise Analysis and Modeling for a Magnetic Tunnel Junction Magnetometer with In Situ Magnetic Feedback

Magnetic tunnel junction (MTJ) sensors have been one of the excellent candidates for magnetic field detection due to their high sensitivity and compact size. In this paper, we design a magnetometer with in situ magnetic feedback consisting of an MTJ sensor. To analyze and evaluate the detectivity of the MTJ magnetometer, a noise model of the MTJ sensor in the magnetometer without magnetic feedback is first developed. Then, the noise model of the MTJ magnetometer with in situ magnetic feedback is also established, including the noises of the MTJ sensor and the signal conditioning circuit, as well as the feedback circuit. The equivalent noise model of the MTJ magnetometer with in situ magnetic feedback is evaluated through nonlinear fitting for the noise voltage spectrum. Although the noise generated by the MTJ sensor is much greater than that of the signal conditioning circuit, the noise introduced by the feedback coils into the MTJ sensor is slightly more than twice that generated by the MTJ sensor itself. The measurement results show that the detectivity of the MTJ magnetometer with in situ magnetic feedback reaches 526 pT/Hz1/2 at 10 Hz. The equivalent noise analysis method presented in this paper is suitable for the detectivity analysis of magnetometers with magnetic feedback.

Drift Mechanism and Gain Stabilization of Digital Closed-Loop Quartz Flexible Accelerometer in Start-Up Performance

In this paper, the performance drift of digital closed-loop quartz flexible accelerometer (QFA) in start-up period is attributed to the heating of the feedback circuit and coil. Compared to the drift compensation by temperature model which is built by statical rule, the gain stabilization of closed-loop channel is an essential and direct method to restrain the drift. In this paper, a high accurate current detection circuit is designed to evaluate the parameters drift and to stabilize the gain of the feedback channel. A stability experiment is conducted to prove the efficiency of the proposed stabilization strategy. The result shows that the stability of output in start-up period improves by 37.6% at most. Therefore, the proposed stabilization strategy can effectively improve the start- up performance of QFA, which could be promoted to other sensors whose performance suffer from the drift of the channel.

Low-noise second-order gradient SQUID current sensors overlap-coupled with input coils of different inductances

In this study, we developed a series of second-order gradient overlap-coupling octagonal superconducting quantum interference device (SQUID) current sensors for transition edge sensor (TES) applications. According to the requirements of TESs, we designed various parameters of SQUID current sensors, including different Josephson junction (JJ) sizes, different hysteresis and screening parameters, input coils with different inductances. The superconducting loop, input coil, and feedback coil of the SQUID current sensors adopt a second-order gradient structure, which can effectively enhance their anti-interference electromagnetic capability. Further, all the coupling structures between the superconducting loop and input/feedback coil adopt overlap-coupling octagonal structures, which can effectively enhance the coefficient of coupling mutual inductance. A series of second-order gradient SQUID current sensors overlap-coupled with input coils of 1.5, 2.5, and 5.5 turns based on Nb/Al–AlO x /Nb JJs were successfully fabricated on 2 inch silicon wafers by optimising the fabrication processes. From the simulation results, a minimal magnetic coupling factor of 0.02 between the feedback coil and input coil is obtained in the overlapped structure of the SQUID current sensor with an input coil of 1.5 turns; thus the magnetic interference between the feedback coil and input coil in this design is negligible. According to the measurement results at liquid–helium temperatures, SQUID current sensors with second-order gradient overlap-coupling octagonal structures have an excellent capability to weaken environmental electromagnetic interference. The SQUID current sensor with an input coil of 5.5 turns has the smallest current sensitivity, 2.4 μA/Φ0 and lowest current noise, 1 pA (√Hz)−1 compared with those of other input coils of different turns. The SQUID current sensor with an input coil of 2.5 turns has the lowest flux noise, 0.3 μΦ0 (√Hz)−1, and the lowest coupled noise energy, 11 h, among all the three different input coils, which are balanced results between the input inductance and the device noise. SQUID current sensors with second-order gradient overlap-coupling octagonal structures show low flux noise, low current noise, and low coupled noise energy, which can be compared with the state-of-art SQUID current sensor worldwide, and can satisfy the requirements of TES applications.

Validation of a NTM model using databases of disruptive plasmas at JET

Pressure-driven instabilities such as the neoclassical tearing modes (NTMs) limit the maximum achievable plasma normalized β N needed for a reliable and efficient tokamak device. For a better understanding of the involved phenomena an almost fully analytic, dynamic, interpretative, multi-mode model describing the evolution of the NTMs is presented. An explicitly derived final formula of the NTM perturbed flux function is provided having a clear time dependent expression. To obtain the latter, the equations the perturbations satisfy are basically solved in the whole space: the ideal plasma perturbed momentum equations, the magnetic island perturbed resistive equations, the perturbed circuit equations in the plasma surrounding resistive wall and coils and finally the perturbed equations in the vacuum between the plasma and the external structures. The external coils play the role of feedback coils and/or error field generating coils. The system of the perturbed equations is completed by all the existing boundary equations across boundaries and the jump equation across the magnetic island that takes into account a heuristic bootstrap term. The assumption of the small perturbations from the equilibrium state is used in order to track the problem analytically, therefore the system of the perturbed equations is a linear one. Obviously the real diagnostic data quantities are not static and cannot be treated as equilibrium quantities from a strict mathematical point of view. This would drive the perturbed system of equations profoundly nonlinear and basically impossible to be analytically treated. Based on the assumption that usually the perturbed quantities have a significantly higher growth rate compared to the diagnostic quantities growth rate, it has been used the approximation that the plasma quantities collected from the diagnostic data behave like equilibrium quantities on the perturbations time scale. This approximation allows us to keep our proposed model linear and to provide a clear, quasi-analytic NTM solution. This solution and therefore the entire model validity have been tested against various discharges at JET.

A second-order gradiometric superconducting quantum interference device current sensor with cross-coupled structure

Superconducting quantum interference device (SQUID) has extremely high magnetic field sensitivity, current sensitivity, and can detect a low-noise weak current signal. The SQUID current sensor has become the only option of the readout of low-noise detector, such as transition-edge sensor (TES). In this paper, a second-order gradiometric cross-coupled SQUID current sensor for TES application is developed. According to the requirements for TES detectors, the structure and various parameters of SQUID current sensor are designed. The SQUID loop, input coil and feedback coil of the SQUID current sensor all use the second-order gradiometric structure. All the couple ways between SQUID loop and input coil or feedback coil adopt cross-coupling mode in different planes, which can effectively weaken the parasitic capacitance. A second-order gradiometric cross-coupled SQUID current sensor based on Nb/Al-AlO<i><sub>x</sub></i>/Nb Josephson junction is successfully fabricated on a silicon wafer by optimizing the process. The properties of the second-order gradiometric cross-coupled SQUID current sensor are measured at liquid helium temperature. The bias current of SQUID is 215 μA when the modulation depth of <i>V</i>-<i>Φ</i> modulation curve is maximum. The maximum modulation peak of SQUID is 31 μV. The flux-to-voltage transfer coefficient of SQUID is 108 μV/<i>Φ</i><sub>0</sub>. The input coil current sensitivity is 17 μA/<i>Φ</i><sub>0</sub>, the mutual inductance between SQUID loop and input coil is 117 pH. The current sensitivity of feedback coil is 86 μA/<i>Φ</i><sub>0</sub>, the mutual inductance between SQUID loop and feedback coil is 23 pH. The second-order gradiometric cross-coupled SQUID current sensor has a white flux noise of 2 μ<i>Φ</i><sub>0</sub>/<inline-formula><tex-math id="M3">\begin{document}$ \sqrt{{\rm{H}}{\rm{z}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M3.png"/></alternatives></inline-formula> and a white current noise of 34 pA/<inline-formula><tex-math id="M4">\begin{document}$ \sqrt{{\rm{H}}{\rm{z}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M4.png"/></alternatives></inline-formula> with 1/<i>f</i> corner frequency around 200 Hz. The result of noise level under the condition without magnetic shielding shows that the SQUID current sensor with second-order gradiometric cross-coupled structure has an excellent capability of weakening the environmental electromagnetic interference. In the future, we will further improve the mutual inductance of the second-order gradiometric cross-coupled SQUID current sensor between SQUID loop and input coil, optimize the size and critical current of Josephson junction, in order to improve the input sensitivity of SQUID device, reduce the current noise level and the 1/<i>f</i> corner frequency, and meet more requirements for TES applications.

Active control of resistive wall mode via modification of external tearing index

Modification of the external tearing index, Δext′, by magnetic feedback is analytically investigated for the purpose of controlling the resistive plasma resistive wall mode (RP-RWM). The matching method is pursued by deriving expressions for the close-loop Δext′ and by linking it to the counterpart from the inner layer. Various feedback coil configurations are found to generally reduce Δext′ and stabilize the RWM, with either proportional or derivative control. Feedback modification of Δext′ is found to be generally independent of the inner layer resistive interchange index DR, confirming that feedback action primarily modifies the solution in the outer ideal region for the RP-RWM. Exception occurs when either the inner layer favorable curvature effect becomes sufficiently large or the feedback action is sufficiently strong to introduce a rotating RP-RWM in the static plasma, leading to complex-valued close-loop Δext′. The perturbed magnetic energy dissipation in the outer region, associated with the eddy current in the resistive wall, is identified as the key physics reason for feedback induced complex Δext′. Similar results are also obtained for active control of the external kink instability, whose open-loop growth rate is significantly reduced by inclusion of the plasma resistivity. Within the single poloidal harmonic approximation, which is most suitable for the matching approach, external active coils combined with poloidal sensors are often found to be more efficient for feedback stabilization of the mode at large proportional gain values. This counter-intuitive result is explained as the lack of (non-resonant) poloidal harmonics for proper description of the feedback coil geometry.

Characterization of Scanning SQUID Probes Based on 3D Nano-Bridge Junctions in Magnetic Field**Supported by the National Key R &D Program of China (Grant Nos. 2017YFF0206105, 2016YFA0301002 and 2017YFA0303000), the Young Investigator Program of CAS (Grant No. 2016217), the Frontier Science Key Programs of the CAS (Grant No. QYZDYSSW-JSC033), and the Strategic Priority Research Program of CAS (Grant No. XDA18000000), the

We develop superconducting quantum interference device (SQUID) probes based on 3D nano-bridge junctions for the scanning SQUID microscopy. The use of these nano-bridge junctions enables imaging in the presence of a high magnetic field. Conventionally, a superconducting ground layer has been employed for better magnetic shielding. In our study, we prepare a number of scanning SQUID probes with and without a ground layer to evaluate their performance in external magnetic fields. The devices show the improved magnetic modulation up to 1.4 T. It is found that the ground layer reduces the inductance, and increases the modulation depth and symmetricity of the gradiometer design in the absence of the field. However, the layer is not compatible with the use of the scanning SQUID probe in the field because it decreases its working field range. Moreover, by adding the layer, the mutual inductance between the feedback coil and the SQUID also decreases linearly as a function of the field.

Two-terminal wide range SQUID amplifier with hysteretic quasi linear transfer characteristics

To implement wide range measurement using direct current Superconducting Quantum Interference Device (dc SQUID), we presented a simple two-terminal SQUID amplifier consists of a shunt resistor and a conventional SQUID in series with feedback coil. Only two terminals are led out with coaxial cable for high speed and low heat loss. Periodically repeated quasi linear flux-to-voltage transfer characteristics are achieved by internal flux feedback. Wide range flux input is modulated by the SQUID amplifier into voltage signal with rapid transient edges. The reconstructed output is the synthesis of measured voltage and counts of flux-quanta counting. Working principles of linearized SQUID amplifier were simulated and experimentally verified. Wide range measurement with error within ±0.1Φ0 was demonstrated in the experiment.

Flux Feedback Amplifier based on Superconducting Quantum Interference Device

We introduced the Flux Feedback Amplifier (FFA) to design simple and easy-to-use Flux-Locked Loop (FLL) used to operate Superconducting Quantum Interference Device (SQUID) for practical applications. The SQUID FFA is a hybrid operational amplifier (op amp) with SQUID as flux input end and feedback coil as flux output end. The room temperature circuit is simplified as a low noise differential amplifier with no more than two op amps, and operated with only one switch. Two SQUID FFA schemes were demonstrated and the dynamic performances were characterized and compared in the experimental results. The scheme with a monolithic op amp achieves one-pole open loop gain with slope of 20 dB/decade and is suited for the wideband FLL. The scheme with two op amps achieves two-pole open loop gain with slope of 40 dB/decade and is suited for the FLL with high slew rate (up to 1Φ0/μs).

HTS-SQUID vector magnetometer with low crosstalk configuration

We propose an HTS-SQUID vector magnetometer with a new configuration reducing the crosstalk between a pickup coil of one SQUID and feedback coils of other SQUIDs. The new configuration is designed to mount three SQUIDs at symmetric positions where the crosstalk should be ideally zero while keeping minimal height of a probe head. In an actual HTS-SQUID magnetometer, however, either pickup coil or feedback coil is inevitably dislocated from the ideal symmetric positions. Numerical simulation of crosstalk revealed that the new configuration has smaller crosstalk than that of a conventional cubic configuration even if the positioning error from the ideal symmetric positions is as small as 1 mm. Three types of SQUID vector magnetometer were fabricated and their crosstalk was measured. The averaged crosstalk of a SQUID vector magnetometer using the new configuration was actually improved by about one order of magnitude as compared with the conventional cubic configuration.

Compact GaN class‐AB Armstrong oscillator for resonant wireless power transfer

In this study, a 4.4 MHz wireless power transfer (WPT) system was designed and implemented using a class-AB Armstrong oscillator that uses a packaged GaN on Si high electron mobility transistor. The oscillator is designed and simulated in advanced design system and then integrated with a magnetically coupled resonant WPT system. The transmitting planar coil of the WPT contains both the main and the feedback coils of the Armstrong oscillator. The whole system was implemented in a printed circuit board and tested. The DC-to-AC conversion efficiency of the simulated Armstrong oscillator is 69%. The maximum measured power transfer efficiency is 40.3% at a 2 cm distance between the transmitting and receiving coils. The input DC power of 37.5 mW provides 15.1 mW of AC power to the load with low distortion, making it suitable for low electromagnetic interference, size, power and cost applications such as biomedical implants.

Noise performance of SQUID magnetometers as a function of critical current value: a statistical-like approach

The critical current value is a key parameter of a Superconducting Quantum Interference Device. In this paper we investigate the effects of different critical current values on the noise performance of SQUID magnetometers. By using a statistical-like approach, we measure the spectral density of magnetic field noise of 200 magnetometers relating the values to the critical current change. The magnetometer consists of a dc-SQUID in a washer shape magnetically coupled to a square superconducting pickup coil. The fully integrated design includes a feedback coil to work in Flux Locked Loop mode and an Additional Positive Feedback circuit to increase its voltage responsivity. The main application field of these sensors is the magnetoencephalography that exploits multichannel systems based on SQUID magnetometers, in which the uniformity of noise performance of the sensors is needed. We found that a best value of critical current is 20 μΑ and also that a value spread ranging from 8 to 40 μm, does not affect in an evident way the sensitivity of such magnetometers. The corresponding noise spectral density remains within the range 2 – 6 fT/⎷Hz.

Flux compensation for SQUID-detected Magnetic Resonance Force Microscopy

One of the major challenges in performing SQUID-detected Magnetic Resonance Force Microscopy (MRFM) at milliKelvin temperatures is the crosstalk between the radiofrequency (RF) pulses used for the spin manipulation and the SQUID-based detection mechanism. Here we present an approach based on balancing the flux crosstalk using an on-chip feedback coil coupled to the SQUID. This approach does not require any additional components near the location of the sample, and can therefore be applied to any SQUID-based detection scheme to cancel predictable RF interference. We demonstrate the effectiveness of our approach by showing that we can almost completely negate flux crosstalk with an amplitude of up to several Φ0. This technical achievement allows for complicated magnetic resonance protocols to be performed at temperatures below 50 mK.

Design and fabrication of low-noise superconducting quantum interference device magnetometer

Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlO<i><sub>x</sub></i>/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ<sub>0</sub> and a white flux noise of 8 μΦ<sub>0</sub>/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.

Effect of critical current spread on the noise performance of SQUID magnetometers: An experimental study

We investigate the noise performance dependence on the critical current spread of a fully integrated magnetometer based on superconducting quantum interference device (SQUID). In particular, we analyze the behavior of spectral density of magnetic field noise carrying out an experimental study on 200 magnetometers. The magnetic sensors consist of a superconducting flux transformer connected to a SQUID in a washer shape and include both a feedback coil for flux locked loop operation and an additional positive feedback circuit to increase the intrinsic voltage responsivity. A damping resistor in parallel to the SQUID inductance has been used in order to prevent both the degradation due to the non-optimal βL value and the washer resonances. Such sensors are often employed in magnetoencephalography or more in general in ultra-high sensitivity applications requiring the magnetic noise spectral density as low as 2 fT/√Hz in the white region.We will show that a SQUID critical current varying from 8 to 40 μA does not cause an evident degradation of the magnetometer sensibility. In addition, the magnetic field noise vs critical current indicates an optimal value of SQUID critical current of 20 μA. It corresponds to an inductance parameter βL of about 2 highlighting the effectiveness of the damping resistor in parallel to the SQUID inductance.

Experimental validation of coil phase parametrisation on ASDEX Upgrade, and extension to ITER

It has been previously demonstrated in Li et al (2016 Nucl. Fusion 56 126007) that the optimum upper/lower coil phase shift ΔΦopt for alignment of RMP coils for ELM mitigation depends sensitively on q95, and other equilibrium plasma parameters. Therefore, ΔΦopt is expected to vary widely during the current ramp of ITER plasmas, with negative implications for ELM mitigation during this period. A previously derived and numerically benchmarked parametrisation of the coil phase for optimal ELM mitigation on ASDEX Upgrade (Ryan et al 2017 Plasma Phys. Control. Fusion 59 024005) is validated against experimental measurements of ΔΦopt, made by observing the changes to the ELM frequency as the coil phase is scanned. It is shown that the parametrisation may predict the optimal coil phase to within 32° of the experimental measurement for n = 2 applied perturbations. It is explained that this agreement is sufficient to ensure that the ELM mitigation is not compromised by poor coil alignment. It is also found that the phase which maximises ELM mitigation is shifted from the phase which maximizes density pump-out, in contrast to theoretical expectations that ELM mitigation and density pump out have the same ΔΦul dependence. A time lag between the ELM frequency response and density response to the RMP is suggested as the cause. The method for numerically deriving the parametrisation is repeated for the ITER coil set, using the baseline scenario as a reference equilibrium, and the parametrisation coefficients given for future use in a feedback coil alignment system. The relative merits of square or sinusoidal toroidal current waveforms for ELM mitigation are briefly discussed.