ICRH operations during the JET tritium and DTE2 campaigns

The JET-ILW pure tritium and deuterium–tritium (DTE2) experimental campaigns took place in 2021–2022. Tritium (T) and deuterium–tritium (D–T) operations present challenges not encountered in present day tokamaks (Horton et al 2016 Fusion Eng. Des. 109–111 925–36). This contribution focuses on ion cyclotron resonance heating (ICRH) operations in tritium and deuterium–tritium plasmas, starting with a summary of the program of improvements to the ICRH system which spanned a few years prior to these experiments. Procedures were implemented to address specific constraints from tritium and deuterium–tritium operations (tritium safety and reduced access to the RF generator area) and increase the system reliability and power availability during plasma pulses. Operation of the upgraded real time RF power control system that maximises the launched power while taking into account limitations from the system or antenna coupling is described. We also report on the result from dedicated pulses performed to assess the potential harmful impact of the 2nd harmonic tritium resonance in the plasma, close to the inner wall, when using the standard central hydrogen minority ICRH scheme. During DTE2, the ITER-like antenna was not used because water leaked from an in-vessel capacitor into the vessel on day-2 of the experimental campaign. The lessons learnt from this incident are highlighted. Finally, the ICRH plant adjustments required to safely perform ion cyclotron wall cleaning discharges are described.

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Introduction
Experimental campaigns with pure tritium and deuteriumtritium plasmas were run on JET-ITER-Like Wall (JET-ILW) in 2021-2022.This was the result of years of technical preparation [1] to ensure readiness of JET systems for these experiments key for the fusion community.These campaigns were a unique opportunity to address essential questions for magnetic fusion development, and in particular, to prepare for ITER operations and research [2][3][4].To deliver these objectives, ICRH was used in most JET-DTE2 and tritium plasmas to provide electron heating for central impurities chase out and discharges stationarity [5].ICRH was also used for bulk heating of fuel ions and boost fusion power [6][7][8].Specific experiments were performed to study the impact of isotopes on RF induced plasma wall interactions [9].The demonstration of ITER D-T ICRH scenarios was one of the one of the highlevel objectives of the JET DTE2 program [10,11].Finally, ion cyclotron wall cleaning (ICWC) was one of the tools used for tritium recovery after the JET-DTE2 campaign [12].
This paper describes the operation of the ICRH system during these experimental campaigns and is organised as follows: in section 2, the JET ICRH system is described with an emphasis on aspects related specifically to the preparation for JET D-T operations 4 .In section 3, some key points implemented to maximise ICRH system reliability and performance during D-T experiments are highlighted.The real-time procedure which aims at maximising the launched RF power during a JET plasma pulse is also briefly described.In section 4, more details are given on (ω = ω c,H ) hydrogen minority ICRH with the presence of the 2nd harmonic tritium resonance on the inner wall.At the beginning of the JET-DTE2 campaign, a water leak developed between the ITER-like antenna (ILA) and the JET vessel; the incident is described, with a reflection on lessons learnt, in section 5. Constraints specific to the operation of the ICRH system for ICWC are described in section 6. Concluding remarks are in section 7.

The JET ICRH system, and requirements for JET D-T operations
A description of the ICRH system can be found in [13,14]; a simplified representation of the system with RF generators, transmission line and an antenna is shown in figure 1.A top view of JET with the auxiliary heating systems is shown in figure 2. The A2 antennas [15] A, B, C and D were used during JET-DTE2.These antennas were installed in 1993, and they were designed to be compatible with tritium operations (the A2 antennas were operated during the JET DTE1 campaign in 1997).In particular, the double conical feedthrough (DCF) design [16] is used for the vacuum windows, providing extra safety for tritium containment.When the JET plasma facing components were changed from carbon to beryllium and tungsten [17], the antenna guard limiters were also replaced (CFC tiles changed to Be tiles) contributing to the overall reduction of gas retention in JET [18].The A2 antennas transmission line network was modified in the 2000s to provide ELM resilience; A&B are fed via a network of four 3 dB hybrid couplers [13], while C&D are fed via four T-junctions, forming the external conjugate-T network [14].Antennas C&D can also be fed independently.Experiments where maximum ICRH power was needed used midplane gas injection close to the antennas to improve coupling [19].In preparation for D-T operations, a program of improvement of the JET ICRH system was implemented over several years.The goal was ICRH power maximisation and improving the equipment reliability, availability, and repairability while taking into account tritium safety constraints and radiological safety constraints.
To prevent tritium permeating through the vacuum windows from accumulating into the transmission lines (this would be a potential threat for the generator-hall occupants) a constant air flow is maintained between the 3 bars dry air pressurised RF transmission lines and the JET active gas handling system (AGHS), this is illustrated in figures 1 and 3. Prior the D-T campaign, the following activities were performed: systematic identification and repair of transmission line air leaks; check of the bleeding flow from the ICRH transmission lines to the JET AGHS; check of the satisfactory operation of the emergency isolation valves that can isolate the main transmission lines from the antenna pressurised transmission lines in case of a DCF failure; improvement of the air distribution system and modification of the venting procedure (venting through the JET Active Gas Handling System now).Also, any breach to the transmission lines in the JET torus hall or the generator hall is now subject to a health-physics survey.
For radiological safety reasons, the generator's area could not be accessed when JET was running in the D-T campaign; hence, a number of projects were conducted to improve the reliability of the plant and enhance capabilities to control and monitor the ICRH plant remotely.The focus for remote control activities were for the most common actions that previously required physical access to the plant (implementation of remote RESETs instead of local RESETs, implantation of remote human machine interface (HMI) instead of local HMI, etc), and the implementation of a new data acquisition system to monitor the operation of the RF generators in more detail.220 new signals were added to have a better insight on the operation of all amplification stages.

Operations and maximization of launched ICRH power
ICRH was used in ∼80% of JET-DTE2 campaign pulses.Figure 4 shows the launched power vs frequency in DTE2 ICRH pulses.Almost the whole available frequency range for the ICRH system on JET (23-57 MHz) was used.Also noticeable, the maximum launched power achieved is lower for lower frequencies; this is caused by the reduced coupling  To maximize the outcomes of the T and D-T campaigns while not exceeding the allocated T and neutron budgets, the procedure to prepare the system before a discharge was also improved.The target was to get the maximum coupled ICRH power on the first pulse in each plasma scenario sequence.At the beginning of campaign days, test load pulses were performed to verify the state of the generators and identify limits if any (limits can be defined in the RF Local manager, see next paragraph), so that appropriate actions can be taken before an actual JET pulse is performed.The same procedure was applied after each frequency change.Reference pulses (existing pulses with plasma conditions expected to be similar to the pulse being prepared) were systematically used to set the position of the transmission line matching elements.Figure 5 shows the ICRH power delivered during one of the campaign days.The 1st part of the experimental program required a frequency of 32.5 MHz; 4 MW was delivered from the 1st pulse  Prior to D-T operations, the program that handles real time control of the generators was improved as recommended in [20].The control scheme aims at delivering the required launched power (or maximum power given antenna coupling and generator limits) while minimising the risk of failures due to arcing or generator performance issues.By default, the software equalises the transmission line voltages for an antenna unless an individually preset voltage limit is met on one or more lines (exceeding those limits would result in increased risk of arcing) in which case the power is automatically redistributed between the other lines.If relevant, power limitations of individual generators are also considered: options to account for preset and self-diagnosed limits emerging during a pulse are implemented.This algorithm ensures maximum power delivery to the plasma given the antenna coupling (plasma conditions dependent) and limitations (potentially due to faults) of the RF system.An example of the new control algorithm operation in an ELMy H-mode plasma is shown in figure 6.In this example, antenna C&D coupling resistance is higher; RF-voltages on C&D have not quite reached the 30 kV limit; this is partly because the Tritium injection module TIM15 (shown in figure 1) located between antennas C and D is used, hence P ICRH,C&D > P ICRH,A&B .Note that in this pulse, the power modulation from 10.5 s was programmed to allow studies of the ICRH wave absorption in the plasma [10].

Plasma operations with the presence of the ω = 2ω C,T ICRH resonance on the inner wall
JET operating instructions (JOIs) are the first line of defence for machine protection.As a second line of defence, real time protection will act directly on systems during a pulse if a dangerous situation is detected.For the protection of the JET plasma facing component, a network of real time cameras [21] is used to detect hot spots; if hot spots are detected, depending on their temperature and location then a pre-programmed response is initiated by the real time first wall protection [22].Main chamber hot spots trigger a reduction or removal of auxiliary heating (NBI and ICRH) power.
To prevent damaging and melting of plasma facing components, one of the JOIs prohibits in principle plasma operations with ICRH resonances layers for H, D, T and 3 He located close to the inner or outer walls.The hydrogen minority ICRH scheme [23] with ω = ω c,H close to the plasma centre which provides dominant collisional electron heating was the main ICRH scheme in most of the JET tritium and D-T plasmas.In this case, the 2nd harmonic tritium resonance (ω = 2ω c,T ) is also located in the plasma close to the inner wall (see figure 7).The operating instructions related to location of ICRH resonances was relaxed to allow hydrogen minority ICRH during D-T plasma operations.ICRH simulations using TOMCAT [24] were performed; with a few percent of a hydrogen minority in T or D-T plasmas ICRH waves are very efficiently absorbed at ω = ω c,H in the plasma centre, no RF power travels to the inner wall to be absorbed at ω = 2ω c,T there (see figure 8(b)).Parasitic inner wall absorption can only be conceived in the hypothetical case where no hydrogen at all is present in the plasma.In this case most of the RF power is still absorbed at the plasma centre by the ω = 3ω c,T ICRH mechanism (see figure 8(a)).Overall, no sign of abnormal heat loads have been observed experimentally when using ω = ω c,H ICRH, even in pure tritium plasmas with T-NBI power up to  25 MW.This is illustrated in figure 9 where we compare JET in-vessel camera images in two identical deuterium-tritium pulses, one using the hydrogen minority ICRH scheme, and the other using the central ω = 2ω c,T ICRH scheme which does not have parasitic wall resonances.Configurations with an ICRH resonance near the outer wall, and in particular inside the antenna box where hot spots or arcs would be difficult to detect, are still not allowed in JET.
Similar observations were reported in Alcator C-mod; ICRH could be operated with fundamental H harmonic near the central column in D( 3 He) plasmas without excessive impurities or melting of the molybdenum tiles covering the central column, while impurity events were often observed if H cyclotron resonance intersected first wall components on the outboard side [25].

Lessons learnt from water leak in the JET ITER-like antenna
In September 2020, one of the ITER Like Antenna (ILA) [26] capacitors failed (C2 from ILA upper row).Figure 10 represents a sketch of a capacitor cooling circuit.The capacitor filled with water after a micro-leak developed in the bellows between the water-cooling circuit and the capacitor.In autumn 2020 a differential pumping system was installed to evacuate the upper row capacitors water cooling circuit.Operations with the ILA lower row resumed in January 2021, but in August 2021 a second fault developed on day-2 of the JET-DTE2 campaign.Despite the pumping, some water was still being retained in C2; during a plasma pulse (JPN 99361) when using the lower row of the ILA, a crack developed either in the C2 capacitor ceramic or in the brazing joint, and the water was released into the JET vacuum vessel.During this pulse, RF voltage was induced on C2 (6 kV) by the lower row-upper   row RF cross talk, but the exact mechanism leading to the 2nd catastrophic failure of C2 is so far unknown.It took three weeks to identify the origin of the leak into the JET torus, to fully evacuate the water and recondition the machine before JET operations could resume.As a precaution, the ILA was not run during the DTE2 campaign, only the A2 antennas were used.
The ability to drain (and it is important that in this draining process no water can remain captive in dead ends), fill, inject marker gases and isolate/pump the different ILA water cooling circuits separately was crucial to localise the faulty cooling circuit.The differential pumping system installed in the autumn 2020 presently mitigates the effect of this double fault in C2 and allows JET to run.More generally, fusion reactors should avoid integrating complex assemblies in-vessel for example involving cooling of movable parts and/or hydraulic actuation.This in a way validates the ITER ICRH antenna design [27] which does not use in-vessel matching capacitors.
As a final note, in May 2023, another ILA capacitor, C4 from the lower row, failed with a suspected water ingress from the cooling circuit into the capacitor private vacuum.This did not result in a leak to the torus.Before the JET DTE3 campaign an additional differential pumping circuit was installed after draining this cooling circuit.
Overall three of the ILA in-vessel capacitor failed (C7 failed in 2009 and was refurbished in 2010-2012), each failure leading to a loss of power capability, and one failure leading to a water leak.To compare with another critical component, none of the vacuum windows (double conical windows) for the JET A2 or ILA antennas failed during the whole life of these JET antenna systems.

ICWC operations
ICWC was part of the tritium recovery strategy after JET D-T operations [12], with a total of ∼915 s ICWC dischargetime.JET ICWC plasma discharges are described in [28].For tritium removal, ICWC discharges (typically 10 17 -10 18 m −3 ) were produced in JET using the ICRH system in the presence of the JET toroidal magnetic field (1.9 T).RF antennas are operated in monopole phasing to facilitate the ICWC discharge breakdown and maximise coupling to the low density plasmas.18 s long D2 gas ICWC discharges with 250-300 kW of coupled RF power at 29 MHz were generated, with on-axis resonance layer for the 2nd harmonic ion cyclotron resonance frequency for D+ ions (note however that ion-resonance absorption is not the main process involved in ICWC, see [29]).A 'barrel-shape' poloidal field of typically 15 mT on-axis was applied to maximise the wetted areas by ICWC plasmas [12].Figure 11 shows RF voltages applied on the transmission line feedings the antennas at the start of a typical ICWC discharge.
To run safely (from the ICRH system point of view) ICWC discharges, the following technical points must be considered.Firstly, only low frequency (f < 33 MHz) ICWC operations are allowed to prevent occurrence of voltage nodes in the VTL and eliminate any possibility of multipactor-triggered low-voltage arcing which is notoriously difficult to detect.Three A2 antennas were used, A&B fed by the 3 dB hybrid coupler network and D fed independently (non ECT mode for antenna D because of the fairly complex arc detection scheme when using C&D in ECT mode, see [14]).We implement more stringent arc protection for ICWC; the detection trip thresholds based on high voltage standing wave ratio (VSWR) are reduced because of the highest risk of generating arcs, making operations with high reflection particularly difficult.The maximum number of RF power reapplication attempts after VSWR trips is reduced (10 instead of 25 during normal operations).The RF voltage in the transmission lines feeding the antennas is limited to 20 kV; this is implemented via electronics control and real-time software.The over-pressure trip threshold in the antenna vacuum transmission lines is reduced from 10 −4 mbar to 5 × 10 −5 mbar during ICWC.The minimum permissible plasma current required to operate the RF system, normally 500 kA, is reduced to 0 kA.Finally, it is important to note that to limit the risk of arcing in the antenna boxes and vacuum transmission lines, RF power is applied before gas is introduced in the JET vessel (see figure 11).This implies that the transmission line system must be matched to antennas operating in vacuum.Usually, before ICWC operations, a few days of vacuum operations at ∼20 KV per line are required for the RF system where the vacuum matching point for the transmission lines are checked and tuned with the ICWC-related protection measures implemented.

Conclusions
Key physics and technology information of direct relevance to prepare ITER operations and that also feed into the design of next fusion reactors were obtained during the JET DTE2 and tritium campaign in 2020/21.Prior to the campaign, a program of enhancements was implemented to adapt the ICRH system and procedures to specific constraints for D-T operations (mainly tritium and radiological constraints), ensure good availability and reliability of the system and maximise the launched power.Unfortunately, a fault developed in an invessel matching capacitor of the ILA on day-2 of the DTE2 campaign, leading to a water leak into the vessel.The leak was isolated and tokamak operations could resume after a few weeks, but the ILA was not used for the rest of the campaign.ICRH was used in 80% of the DTE2 campaign pulses; in most of the cases the standard hydrogen minority ICRH scheme was used to provide central plasma electron heating and impurities (especially W) chase-out, contributing to discharge stability.Using this ICRH scheme, we have verified that no measurable RF power is dissipated on the JET wall despite the presence of the ω = 2ω c,T resonance close to the inner wall.Finally, the ICRH system was also used during the tritium recovery phase to generate ICWC plasmas; this was possible thanks to the flexibility of the system that allows operation without a plasma load, and after a reconfiguration of some of the ICRH protection system elements.

Figure 1 .
Figure 1.Overview of the JET ICRH system.

Figure 2 .
Figure 2. Top view of JET showing the NBI and ICRH auxiliary heating systems.Also shown are some Outer Mid-Plane gas injection modules (TIMs for tritium, GIMs for deuterium) used to improve wave coupling to the plasma.Reproduced with permission from [11].© 2023 Authors.Published by AIP Publishing.

Figure 3 .
Figure 3. Schematic view of the transmission lines pressurized air system.

Figure 4 .
Figure 4. Overview of ICRH usage vs frequency during the DTE2 experimental campaign (mostly D-T plasmas).Reproduced with permission from [11].© 2023 Authors.Published by AIP Publishing.

Figure 5 .
Figure 5. Averaged ICRH power launched during a DTE2 session.Pulse numbers without data either did not require ICRH or stopped prior to the heating phase.

Figure 6 .
Figure 6.Example showing real time optimisation of RF voltages in the transmission lines feeding the A2 antennas to maximise the ICRH launched power.Pulse 99633 with I P = 2.3 MA, B T = 3.45 T, 3 He minority ICRH with f RF = 32.5 MHz.(a) RF-voltages in the 8 transmission lines feeding antennas A&B (fed via 3 dB-hybrid couplers).RF amplifiers power is adjusted so RF-voltages reach the maximum permissible, 30 kV.(b) RF-Voltages in the 8 transmission lines feeding antennas C&D (fed via ECTs).(c) Power launched by antennas A&B, and C&D.(d) Total ICRH power.Reproduced with permission from [11].© 2023 Authors.Published by AIP Publishing.

Figure 7 .
Figure 7. Cross section of a plasma used in scenario development [4], with B T = 3.45 T and f ICRH = 51 MHz.The location of the ω = ω c,H and ω = 2ω c,T resonances is shown.Reproduced with permission from [11].© 2023 Authors.Published by AIP Publishing.

Figure 8 .
Figure 8. TOMCAT simulations of a pure tritium and tritium NBI plasma, with B T = 3.4 T and f ICRH = 51 MHz.In case (a) we assume no hydrogen in the plasma, most of the RF power is absorbed by the ω = 3ω c,T ICRH mechanism, some RF power is also absorbed at R ∼ 2 m by the ω = 2ω c,T ICRH mechanism.In case (b) with 1% H in the plasma no RF power flows to the inner wall.Indicated in the figures is the fraction of power absorbed on different species.

Figure 9 .
Figure 9. JET in-vessel camera images at t ∼ 10.95 s for two identical JET DT pulses heated with ∼13.0 MW of D-NBI and ∼13.0 MW of T-NBI and with (a) pulse 99886 with ω = 2ω c,T ICRH in the centre (f ICRH = 32.5 MHz) no resonance on the inner wall; (b) pulse 99596 with ω = ω c,H ICRH (f ICRH = 51 MHz) P ICRH = 4.5 MW with ω = 2ω c,T resonance on the inner wall.

Figure 10 .
Figure 10.Simplified diagram of ILA capacitors cooling circuit and vacuum arrangement showing the suspected location of the faults leading to a water leak into the JET vessel.Reproduced with permission from [11].© 2023 Authors.Published by AIP Publishing.

Figure 11 .
Figure 11.Example of an ICWC discharge, pulse 100293.(a) Transmission line voltages for antenna A; (b) transmission line voltages for antenna B; (c) transmission line voltages for antenna D; (d) D2 gas flux; (e) Dα signal.