The RFX-mod2 experiment

Summary

This presentation introduces the RFX-mod2 magnetic confinement fusion experiment, detailing the machine’s evolution from RFX and RFX-mod. It highlights key upgrades including the removal of the Inconel vacuum vessel to improve passive shell proximity, active MHD and resistive wall mode control capabilities, operation modes as a Reversed Field Pinch (RFP) up to 2 MA and ohmic/shaped tokamak, and broader applications including fusion-fission hybrid reactors.

Title Slide

The RFX-mod2 experiment CUP: B53C22003070006 L. Marrelli and the RFX-mod2-team NEFERTARI Final Workshop Padova, 22-25/3/2026 Finanziato dall’Unione europea NextGenerationEU | Ministero dell’Università e della Ricerca | Italiadomani | Consiglio Nazionale delle Ricerche | Consorzio RFX | CNR-ISTP

RFX-mod2 is a Reversed Field Pinch

RFX-mod2 is a Reversed Field Pinch • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines)

RFX-mod2: Toroidal field Coils

RFX-mod2: Toroidal field Coils • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines) • Toroidal Field Coils: Low Engineering Demand • Reduced External Field: Plasma-generated bulk field. • Moderate Mechanical Stress: Low Lorentz forces on windings

RFX-mod2: Poloidal Field Coils

RFX-mod2: Poloidal Field Coils • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines) • Toroidal Field Coils: Low Engineering Demand • Reduced External Field: Plasma-generated bulk field. • Moderate Mechanical Stress: Low Lorentz forces on windings • Poloidal field coils: Air core high flux swing (15 Vs) • Pure Ohmic Heating: No auxiliary heating required for RFP

RFX-mod2: Power supplies

RFX-mod2: Power supplies • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines) • Toroidal Field Coils: Low Engineering Demand • Reduced External Field: Plasma-generated bulk field. • Moderate Mechanical Stress: Low Lorentz forces on windings • Poloidal field coils: Air core high flux swing (15 Vs) • Pure Ohmic Heating: No auxiliary heating required for RFP • Versatile MW-class power plant featuring modular ac/dc units

RFX-mod2: high power test facility

RFX-mod2: high power test facility • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines) • Toroidal Field Coils: Low Engineering Demand • Reduced External Field: Plasma-generated bulk field. • Moderate Mechanical Stress: Low Lorentz forces on windings • Poloidal field coils: Air core high flux swing (15 Vs) • Pure Ohmic Heating: No auxiliary heating required for RFP • Versatile MW-class power plant featuring modular ac/dc units • Qualification tests for ITER Fast Discharge Units FDU (70kA, 24kV), JT-60SA Quench Protection Unit QPC (25 kA, 4kV) and development of next step reactors Magnetic Energy Storage and Transfer system (MEST)

RFX-mod2: active control

RFX-mod2: active control • RFX-mod2 is the second modification of the Reversed Field eXperiment which is a Reversed Field Pinch (RFP) (R=2m, a=0.49) • Paramagnetism: Bulk B-field self-generated by plasma. • Field Reversal: Inverted edge pitch (Cyan vs. Red lines) • Toroidal Field Coils: Low Engineering Demand • Reduced External Field: Plasma-generated bulk field. • Moderate Mechanical Stress: Low Lorentz forces on windings • Poloidal field coils: Air core high flux swing (15 Vs) • Pure Ohmic Heating: No auxiliary heating required for RFP • Versatile MW-class power plant featuring modular ac/dc units • Active control of MHD instabilities; (J.D. Lawson 1977 proposal) • 3mm thick copper shell • 48x4 independent coils

References: J.D. Lawson, (1977) Reversed Field Pinch Reactor Studies. Part I., Physical Principles, CLM-R171 C.M. Bishop, (1989) Plas. Phys. Contr. Fus, 31, 1179

RFX-mod results: multiple Resistive Wall Modes stabilization

RFX-mod results: multiple Resistive Wall Modes stabilization • RFX-mod proved that multiple RWMs can be simultaneously suppressed [1,2]

Without Control
Controlled after 150ms
Fully controlled

References: [1] S. Ortolani, Active MHD control experiments in RFX-mod. Plas Phys Contr Fus. 2006;48(12B):B371 [2] Bolzonella T, et al. Resistive-Wall-Mode Active Rotation in the RFX-Mod Device. Phys Rev Lett 2008; 101: 165003

RFX-mod-results: highest plasma current for an RFP

RFX-mod-results: highest plasma current for an RFP • Reversed-Field pinch 2 MA plasma current target level was achieved in RFX-mod thanks to • Advanced mode control strategies [2] • Power supplies configuration flexibility [1] (15.8 Vs flux swing)

No MHD control
Simple Virtual Shell [3]
Advanced ModeControl [2]
Power supply reconfiguration [1]

References: [1] Novello, et al (2011). “Enhancement of the power supply systems in RFX-mod towards 2 MA plasma current”. Fusion Engineering and Design 86: 1393-1397 [2] Zanca, Marrelli, Manduchi, Marchiori (2007). “Beyond the intelligent shell concept: the clean-mode-control”. Nuclear Fusion 47: 1425-1436 [3] Ortolani and RFX-mod team, (2006) Plasma Physics and Controlled Fusion 48, B371

RFX-mod results: From Multiple Helicity (Part 1)

RFX-mod results: From Multiple Helicity • At low plasma currents many tearing modes are present, spooling performances and generating localized bulging of the plasma column. RFX (and RFX-mod with non optimal control)

RFX-mod results: From Multiple Helicity (Part 2)

RFX-mod results: From Multiple Helicity • At low plasma currents many tearing modes are present, spooling performances and generating localized bulging of the plasma column. • RFX-mod was able to partially mitigate wall locking by slowly rotating the bulging RFX (and RFX-mod with non optimal control) | RFX-mod with Advanced Mode Control

RFX-mod results: From Multiple Helicity to Quasi single Helicity

RFX-mod results: From Multiple Helicity to Quasi single Helicity • At high plasma currents (Ip > 1MA) one tearing mode dominates and the core plasma becomes helical with internal transport barriers

Reference: Lorenzini, (2009). “Self-organized helical equilibria as a new paradigm for ohmically heated fusion plasmas”. Nature Physics 5: 570-574.

RFX-mod as a ohmic tokamak, Very Low q and Ultra Low q

RFX-mod as a ohmic tokamak, Very Low q and Ultra Low q • Thanks to TF coils design (0.5T for 1 s) and active feedback • RFX-mod also operated below the q(a)<2 limit [1] • Performed disruption prevention studies [2] • Performed runaway mitigation with RMP [3]

References: [1] Zanca P, et al.. Plasma Phys Control Fusion. 2012 Sep 1;54(9):094004 [2] Zanca P., et al. (2015). “An active feedback recovery technique from disruption events induced by m = 2, n = 1…”, Nuclear Fusion 55: 043020. [3] Gobbin, et al (2024). “Runaway electron mitigation by 3D fields application in ASDEX upgrade, COMPASS, and RFX-mod”. Frontiers in Physics 12

RFX-mod: shaped tokamak

RFX-mod: shaped tokamak • Reconfigurable connections to field shaping coils allow shaping • Positive and Negative Triangularity • Single (upper and lower) Null, Double Null, Snow Flake

Reference: D. Abate, Plasma Phys. Control. Fusion 62, 085001 (2020)

From RFX-mod

From RFX-mod • RFX-mod performances were limited by the highly resistive Inconel Vacuum Vessel, causing wall locking of multiple Tearing Modes and subsequent localized Plasma Wall Interaction Components shown:

Mechanical support structure
Passive stabilizing Cu shell
Vacuum vessel
First wall (graphite)

From RFX-mod to RFX-mod2

From RFX-mod to RFX-mod2 • RFX-mod performances were limited by the highly resistive Inconel Vacuum Vessel, causing wall locking of multiple Tearing Modes and subsequent localized Plasma Wall Interaction • Thanks to POR FESR regional funding «MIAIVO», RFX-mod have been modified to remove the Inconel Vessel Comparison shown between RFX-mod and RFX-mod2.

RFX-mod2 expectations (Part 1)

RFX-mod2 expectations • With the new boundary the localized edge deformation decreases by a factor 3

Reference: Zuin, et al (2017). Nuclear Fusion 57: 102012.

RFX-mod2 expectations (Part 2)

RFX-mod2 expectations • With the new boundary the edge deformation decreases by a factor 3 • Wall locking is simulated to occur at 500-800 kA (instead of 100-150kA) RFX-mod Inconel vs RFX-mod2 Copper

References: Zuin, et al (2017). Nuclear Fusion 57: 102012. Marrelli L, et al., Nuclear Fusion. 2019 Jul 1;59(7):076027

RFX-mod2 expectations (Part 3)

RFX-mod2 expectations • With the new boundary the edge deformation decreases by a factor 3 • Wall locking is simulated to occur at 500-800 kA (instead of 100-150kA) • 25% reduction of tearing mode energy

References: Zuin, et al (2017). Nuclear Fusion 57: 102012. Marrelli L, et al., Nuclear Fusion. 2019 Jul 1;59(7):076027

RFX-mod2: status of vessel complex reassembly

RFX-mod2: status of vessel complex reassembly • RFX-mod2 Vessel Complex components in the assembly hall

The NEFERTARI opportunity: plants revamping and key diagnostics

The NEFERTARI opportunity: plants revamping and key diagnostics • Revamped key auxiliary plants • Vacuum system (WP1) • Toroidal field power Supplies (WP1) • Wall conditioning systems: Glow Discharge (WP1) Cleaning and Pulse Discharge Cleaning (WP1) • Remote Manipulation (WP1) • Electromagnetic measurements and model based control (WP2) • Revamped and enhanced existing diagnostics; allows applications of codes for experiment interpretations • Edge diagnostics (WP3) • Core diagnostics (WP4 & WP5) • Foster the integration of expertise and pave the way for future common research • High Voltage Insulation (WP6) • Plasma Wall Interaction (WP7) • Neutron and SXR imaging diagnostics (WP8) • Spectroscopic diagnostics & advanced modelling (WP9)

RFX-mod2 research themes

RFX-mod2 research themes • Complete exploration of 2MA Reversed Field Pinch Exploit real-time control capabilities both for RFP and tokamak • Model based (machine learning) control algorithms • FEM, BEM of passive structures and plasma response • Non magnetic real-time plasma position control with reflectometry • Explore 3D physics in high current Reversed Field Pinch plasmas • Plasma Wall Interaction • MHD dynamics with controlled 3D boundary • Explore 3D effects in ohmic tokamak • Error fields • Experimental characterization of disruptions • electromagnetic induced forces direct measurements • Fundamental Plasma physics studies in multiple configurations • Edge turbulence and microinstabilities • Magnetic reconnection and particles accelerations • Transport barriers characterization

Spare Slides

Extreme configurations at low q

Extreme configurations at low q • Tokamak operation with qedge < 2 (circular and shaped) accessible thanks to advanced real-time magnetic control and adjustable plasma-stabilizing shell proximity • Ultra-low q regime (q < 1) • Validation of modelling extrapolation of stress forces due to kink instability • Validation of MHD and gyrokinetic modelling • Anomalous ion heating (Ti > Te) • Influence of magnetic topology on energy confinement • Fundamental mechanisms behind density limit physics in multiconfiguration regimes

Magnetic topology at various q Reference: M. Zuin, Nucl. Fusion 62, 066029 (2022)

RFX-mod2 the only device exploring the 2 MA Reversed-Field Pinch

RFX-mod2 the only device exploring the 2 MA Reversed-Field Pinch • RFX-mod2 will assess the Reversed-Field Pinch as an alternative magnetic configuration for fusion plasma confinement featuring low cost and construction simplicity, thanks to the confining magnetic field mainly produced by the plasma current. The required external applied toroidal field is very low • RFP’s are exploited in medium size experiments also in Stockholm (Sweden), Hefei (China) and Kyoto (Japan). • RFX-mod2 will exploit a new optimized magnetic front-end with reduced passive stabilizing shell-to-plasma distance, aimed to minimize residual edge magnetic perturbations and to expand and improve the positive confinement properties scaling with plasma current • RFX-mod2 will guide the feasibility study for an efficient low-cost volume neutron source based on a Reversed-Field Pinch as fusion core suitable for Breeding Blanket test RFX-mod2 will guide the feasibility study for a fusion-fission hybrid reactor for efficient Tritium production based on a Reversed-Field Pinch

Reference: L. Marrelli, Nucl. Fusion 61, 023001 (2021)

RFX-mod2 will give crucial indications for a Pilot Fusion-Fission Hybrid Reversed-Field Pinch based reactor for Tritium production

RFX-mod2 will give crucial indications for a Pilot Fusion-Fission Hybrid Reversed-Field Pinch based reactor for Tritium production Hybrid reactors cover: efficient Tritium production for fusion reactors, nuclear waste transmutation, nuclear fuel fertilization and energy production High density production of thermal neutrons with efficient Tritium production Simplicity, efficiency, ease-of-maintenance and safety of fusion reactor with reduced/null Tritium production needs

Advantages of a Reversed-Field Pinch as fusion core in a hybrid reactor (Pfus/Pinput ≈1): • Extremely low applied magnetic field (< 0.1 T) generated by toroidal field coils -> copper coils at room temperature • Vacuum vessel, toroidal field coils and blanket outside the cryostat • No plasma current limit: @Ip=10 MA, B_φ (on axis) ≈ 4.7 T, almost fully self-generated -> fusion conditions with ohmic heating only; no additional heating, no aspect ratio constraints • Simple, robust and with high accessibility machine -> low cost

Reference: R. Piovan, IEEE Trans. Plasma Science, 48, 1708 (2020)

RFX @ Consorzio RFX

RFX @ Consorzio RFX • RFX was a toroidal device for magnetic confinement in Reversed Field Pinch (RFP) configurations • (R=2m, a=0.457) • Operated in 1991-1999 • RFX was modified and became RFX-mod • Operated in 2004-2015