Resistive Infernal Modes in Tokamak Plasmas

• Resistive infernal modes are pressure-driven, long-wavelength MHD instabilities that occur in low or vanishing magnetic shear regions, localizing near rational surfaces.
• They exhibit broad infernal-like eigenfunctions with enhanced toroidal coupling and reduced field-line bending stabilization, driven by pressure gradients.
• Applications include advanced tokamak scenarios such as CFETR, where edge-localized modes and rotation stabilization critically influence operational stability.

Searching arXiv for the cited paper IDs and topic context. Using arXiv search to confirm the primary sources and related context. Resistive infernal modes are pressure-driven, long-wavelength MHD instabilities that become prominent when a tokamak equilibrium contains extended regions of very low or vanishing magnetic shear near a rational surface, so that field-line bending stabilization is weakened and toroidal sideband coupling is enhanced. In the most general usage represented here, the term covers both internal resistive infernal spectra in advanced tokamak cores and the edge-localized infernal components that can dominate a global $n=1$ resistive wall mode (RWM). In both settings, the defining feature is the emergence of infernal-like eigenfunctions localized in flat-$q$ or reversed-shear regions, with resistivity permitting either a discrete spectrum of unstable modes or strong coupling to the resistive wall boundary condition (Han et al., 2021, Coste-Sarguet et al., 3 Sep 2025).

1. Definition and physical character

Infernal modes and infernal components arise in low-$n$ ideal MHD when the safety-factor profile contains a region of very low or zero magnetic shear, $dq/dr \approx 0$, typically in a $q$ plateau associated with reversed shear in the core or a pedestal transport barrier at the edge. When the local $q$ value in such a flat region is near a low-order rational $m/n$, the eigenfunction becomes strongly localized there and is dominated by one poloidal harmonic; this is the classic infernal-mode structure. More generally, when the flat-$q$ region is not exactly at a rational surface, the local pressure drive and vanishing shear still generate “infernal components” of a global low-$n$ mode: individual poloidal harmonics that peak in the plateau rather than at rational surfaces and contribute strong pressure-driven drive to the instability (Han et al., 2021).

A resistive infernal mode is distinguished from an ideal infernal mode by the role of finite resistivity. In one realization, the infernal eigenfunctions develop fine-scale structure in the inner layer and assemble into a discrete spectrum of fast-growing modes for a single $(m,n)$, with subdominant modes increasingly oscillatory in radius. In another, an infernal component couples to and dominates an RWM, so that the wall diffusion time and resistive-wall boundary conditions become essential to the global dynamics. In both cases, low shear reduces field-line bending, the pressure-gradient drive becomes comparatively more important, and toroidal coupling to neighboring harmonics broadens or strengthens the mode structure (Coste-Sarguet et al., 3 Sep 2025).

A common misconception is to identify resistive infernal modes with ordinary tearing modes. The distinction stated in the source material is that tearing modes are current-driven, localize near rational surfaces, and rely on resistive inner layers, whereas infernal modes are pressure-driven, long-wavelength, toroidally coupled internal modes that exploit vanishing shear. In shaped low-shear plasmas the two branches can nevertheless become closely connected: the 2025 study states that double-tearing branches can morph continuously into resistive infernal branches as pressure increases in reversed shear (Coste-Sarguet et al., 3 Sep 2025).

2. Low-shear geometry, pressure drive, and infernal localization

The equilibrium ingredients are the safety factor $q$0 and the magnetic shear

$q$1

Advanced, hybrid, and reversed-shear scenarios are especially susceptible because they contain wide regions where $q$2 or changes sign. In that regime, proximity to a rational surface $q$3 weakens field-line bending stabilization and amplifies toroidal sideband coupling, allowing broad infernal-like eigenfunctions to develop across the low-shear region even for finite $q$4 and $q$5 (Coste-Sarguet et al., 3 Sep 2025).

The pressure-curvature drive enters through the Mercier term. In the large-aspect-ratio, low-$q$6, weak-shear ordering cited for shaped plasmas,

$q$7

where $q$8, $q$9 at the resonant radius $n$0, and

$n$1

This expression makes the shaping dependence explicit through elongation $n$2 and triangularity $n$3. The source emphasizes that this leading-order toroidal/shaping correction makes $n$4 nonzero even at $n$5, so shaping can render average curvature unfavorable and make interchange-like drive finite at $n$6 (Coste-Sarguet et al., 3 Sep 2025).

The energy-principle interpretation is central. Ideal MHD stability is governed by the potential energy functional $n$7, and the infernal character can be understood from the reduced effectiveness of field-line bending in flat-$n$8 regions. In symbolic form,

$n$9

The magnetic and compressional terms are stabilizing, whereas the pressure-gradient term provides drive. In a flat-$dq/dr \approx 0$0 region the field-line bending penalty is reduced, so the pressure-gradient drive can dominate for harmonics near $dq/dr \approx 0$1, yielding a localized infernal component that strongly reduces $dq/dr \approx 0$2 (Han et al., 2021).

This suggests a unified interpretation of core and edge infernal behavior: whether the mode appears as an internal resistive spectrum or as an infernalized RWM depends primarily on where the low-shear region lies and how strongly that region couples to either the resistive layer or the external vacuum/wall problem.

3. Coupling to resistive wall modes and the CFETR edge-dominant case

The clearest reactor-relevant example in the source material is the CFETR 1 GW steady-state operating scenario, in which the $dq/dr \approx 0$3 RWM was analyzed with the AEGIS code. The equilibrium has a large bootstrap-current fraction, producing a deeply reversed magnetic shear in the central core region and a locally flattened $dq/dr \approx 0$4 profile within the edge pedestal. Infernal components therefore develop in both locations, but the edge infernal component dominates the $dq/dr \approx 0$5 RWM structure and determines the destabilization (Han et al., 2021).

The cited equilibrium parameters are $dq/dr \approx 0$6, $dq/dr \approx 0$7, $dq/dr \approx 0$8, $dq/dr \approx 0$9, $q$0, $q$1, and $q$2. The $q$3 profile contains a flat edge region spanning $q$4–$q$5, with $q$6 at $q$7. Current density peaks locally at $q$8 in the core and $q$9 at the edge. In the reference equilibrium without the edge flat-$q$0 region, the $q$1 harmonic peaks in the core flat-$q$2 region and the $q$3 harmonic spans between two $q$4 surfaces. In the full steady-state scenario with the edge plateau present, the $q$5 component peaks inside the edge flat-$q$6 region and dominates the $q$7 eigenfunction, even though the edge pressure gradient is locally weaker than in the core (Han et al., 2021).

The reason given is that the RWM is an external mode. Edge-localized current and pressure gradients therefore have greater leverage on the global $q$8 behavior than analogous structures in the core. In the presence of a resistive wall, external kink-like eigenfunctions couple to wall currents, and infernal components at the edge pedestal amplify the external drive and can render the wall ineffective. The edge-localized $q$9 harmonic thus sets the $m/n$0 stability threshold (Han et al., 2021).

This edge dominance is reflected in the reported stability limits. With a resistive wall at $m/n$1 and $m/n$2, the reference equilibrium has

$m/n$3

whereas the steady-state equilibrium with edge flat-$m/n$4 has

$m/n$5

The design target $m/n$6 therefore lies in the unstable window for the CFETR steady-state case when neither rotation nor feedback or kinetic damping is included. Moreover, varying $m/n$7 from $m/n$8 to $m/n$9 shows that both $q$0 limits drop as $q$1 approaches approximately $q$2, and near $q$3 the no-wall and ideal-wall limits become nearly identical, indicating complete loss of wall stabilization from a single dominant infernal harmonic, $q$4 for $q$5 (Han et al., 2021).

4. Mathematical frameworks and dispersion structure

In the CFETR RWM study, the governing model is linearized ideal MHD with rotation in toroidal geometry:

$q$6

with

$q$7

Rotation enters through the Doppler shift

$q$8

under the stated subsonic assumption with centrifugal and Coriolis effects neglected, and compressibility enters through the apparent-mass Greene–Johnson approach (Han et al., 2021).

The generalized analytic framework for advanced tokamak resistive infernal spectra is broader. For monotonic non-resonant $q$9 profiles with shaping, the 2025 study gives a generalized dispersion relation in terms of Bessel functions,

$n$0

with associated eigenfunctions

$n$1

These expressions encode the hallmark infernal structure: broad, infernal-like radial displacement across the low-shear zone. The corresponding sideband-coupling coefficient,

$n$2

makes the toroidal coupling to $n$3 explicit (Coste-Sarguet et al., 3 Sep 2025).

For reversed-shear profiles, the same work derives a compact relation linking $n$4 to $n$5 and then a final generalized dispersion relation with shear and shaping. The paper states that this relation recovers the most unstable roots and, crucially, the entire discrete spectrum in reversed-shear cases, and that numerically it tracks full-code growth rates much better than the shearless dispersion when the $n$6 profile has finite core shear (Coste-Sarguet et al., 3 Sep 2025).

The resistive formulation uses inner–outer matching. The outer-region perturbed-flux equation is

$n$7

while the inner-layer asymptotics are written as

$n$8

with

$n$9

Matching yields the slope jump $(m,n)$0, and the GGJ resistive dispersion relation is given as

$(m,n)$1

The source explicitly states that pressure and shaping must be retained in the inner-layer quantity $(m,n)$2 to recover the correct $(m,n)$3 trend of tearing and infernal growth rates, whereas approximating $(m,n)$4 from a pressure-free outer ODE is numerically robust except at very high $(m,n)$5 (Coste-Sarguet et al., 3 Sep 2025).

5. Discrete spectra, shaping effects, and numerical observability

A defining result of the generalized theory is that resistive infernal modes can form a discrete spectrum for a single $(m,n)$6. The largest growth rate corresponds to the broadest infernal eigenfunction spanning the low-shear region, while lower-growth members are increasingly oscillatory and more sharply localized. The source states that numerical eigenvalue tracking and analytic dispersion roots agree closely for the first several modes, and that this reproduces the behavior predicted analytically for low-shear cores while retaining infernal corrections (Coste-Sarguet et al., 3 Sep 2025).

Shaping qualitatively modifies this spectrum through the Mercier term. The summary given is specific: positive triangularity with elongation can suppress the spectrum entirely, leaving only the dominant mode, whereas elongation without positive $(m,n)$7, or with negative triangularity, can reintroduce a wide spectrum of ideal or resistive infernal modes, even at $(m,n)$8. In resonant cores with $(m,n)$9, negative triangularity removes the usual pressure stabilization of tearing and, with finite resistivity, launches a resistive internal-kink spectrum characterized by broad displacements across the low-shear region and fine-scale features inside the resistive layer. The radial magnetic-flux perturbation remains finite at $q$00 for all members, indicating reconnection potential (Coste-Sarguet et al., 3 Sep 2025).

The numerical methodology also matters. The solver described in the 2025 work treats the main harmonic $q$01 and the nearest sidebands $q$02 in a generalized eigenvalue problem, with finite differences on an adaptive radial grid refined near rational surfaces, regularity at the axis, and decay at the edge. Two modeling cautions are especially emphasized. First, pressure and shaping can be omitted from the outer $q$03 calculation as a robust approximation, but they must be retained in the inner-layer dispersion. Second, “partially electromagnetic” simplifications that set $q$04, and hence $q$05, modify the Mercier term and grossly suppress interchange and infernal growth, often eliminating these modes entirely except at extremely low $q$06 and shear. Combined with a constant toroidal field $q$07, the suppression becomes even stronger, especially for negative-triangularity and low-$q$08 tearing or infernal cases (Coste-Sarguet et al., 3 Sep 2025).

A plausible implication is that some null results in reduced-model calculations may reflect model exclusions rather than genuine stability, particularly in equilibria deliberately designed with low core shear.

6. Rotation stabilization and wall-coupling control

The CFETR study quantifies rotational stabilization in detail. The core Alfvén frequency used for normalization is

$q$09

and the distance above the no-wall limit is expressed as

$q$10

with $q$11. The reported result is that edge rotation is the most critical ingredient for stabilizing the $q$12 RWM dominated by the edge infernal component. At $q$13, a uniform rotation magnitude

$q$14

fully suppresses the mode over the achievable $q$15 range (Han et al., 2021).

Wall-position scans at fixed $q$16 show that, without rotation, the growth rate parameter $q$17 tends to infinity as $q$18 approaches the critical wall position $q$19. With rotation, a stable window opens between $q$20 and $q$21, and $q$22 moves inward as $q$23 increases, so the window widens. The physical interpretation given is that the Doppler shift $q$24 moves the mode frequency relative to the shear-Alfvén continuum, avoids resonances that would amplify the response, enables continuum damping, and reduces resonant amplification of the wall-coupled external mode (Han et al., 2021).

The radial localization of rotation is decisive. Two radially descending profiles, labeled “type 1” and “type 2,” are ineffective in the steady-state CFETR case: the $q$25 boundary barely moves and the stable window does not open appreciably. By contrast, Gaussian profiles

$q$26

peaked at the edge, with $q$27, are far superior. The lower boundary $q$28 shifts inward most when the rotation maximum aligns with the $q$29 infernal peak. With $q$30 and sufficiently broad $q$31, specifically the type A profile with $q$32, full stabilization is achieved (Han et al., 2021).

This makes clear that stabilization is not only a question of total rotation magnitude. In the presence of an edge-dominant infernal component, the magnitude and radial localization of edge rotation are the decisive control parameters.

7. Operational implications and broader significance

For CFETR, the operational conclusion is explicit: at the designed $q$33, the $q$34 RWM is unstable in the steady-state equilibrium with edge flat-$q$35 when passive or active damping is not included. The strong edge infernal component lowers the $q$36 limits and erodes wall stabilization near $q$37. The identified mitigation strategies are to maintain sufficient edge rotation, to tailor the $q$38 profile so as to reduce edge flattening near integer $q$39, to adjust pedestal pressure gradients so as to increase $q$40 and suppress infernal localization, to move the wall inward where possible, and to supplement fluid-MHD stabilization with active RWM feedback and drift-kinetic damping mechanisms that were excluded from the reported calculations (Han et al., 2021).

In the broader advanced-tokamak context, resistive infernal modes imply that low-shear optimization for confinement can create a discrete family of pressure-driven instabilities with broad radial structure and reconnection potential. The practical guidance stated in the 2025 study is to avoid extended low-shear plateaus near rational surfaces, keep $q$41 finite through the core especially if $q$42, limit the width and depth of the reversed-shear $q$43 well near rational surfaces, and use shaping strategically: positive triangularity combined with elongation can suppress infernal spectra, whereas negative triangularity increases unfavorable curvature and facilitates resistive infernal spectra and loss of tearing pressure stabilization (Coste-Sarguet et al., 3 Sep 2025).

The two lines of work together establish a coherent picture. In core-dominated low-shear regimes, resistive infernal modes appear as internal discrete spectra governed by weakened field-line bending, toroidal coupling, and resistive inner-layer physics. In edge-dominated pedestal plateaus, infernal components can instead dominate a global RWM and collapse the distinction between no-wall and ideal-wall stability when a single near-resonant harmonic becomes sufficiently localized. This suggests that “resistive infernal mode” is best understood not as one narrowly defined instability, but as a class of resistive, pressure-driven low-shear MHD phenomena whose observable manifestation depends on where the infernal localization occurs and how that region couples to the surrounding ideal, resistive, and vacuum dynamics (Han et al., 2021, Coste-Sarguet et al., 3 Sep 2025).