South Korea’s KSTAR tokamak held high-confinement plasma continuously for 102 seconds during a campaign that ran from December 2023 through February 2024, while sustaining ion temperatures of 100 million degrees Celsius for 48 seconds within that window. Those two numbers, drawn from the Korea Institute of Fusion Energy’s own reporting, represent the longest and hottest stable plasma runs the device has achieved. The result matters because commercial fusion reactors will need to maintain such conditions for minutes at a time, and KSTAR just demonstrated that a combination of hardware upgrades and real-time plasma control can push performance well beyond previous limits.
Why the 102-second H-mode run changes the fusion timeline
The distinction between a brief flash of extreme heat and a sustained burn is the central engineering problem in fusion energy. Reaching 100 million degrees Celsius is necessary to force hydrogen isotopes close enough to fuse, but holding that state long enough to extract useful energy has proven far harder. KSTAR’s 102-second H-mode result, confirmed by the Korea Institute, is significant because H-mode is the high-confinement operating regime that future power plants will rely on. In simpler terms, H-mode means the plasma stays hot and dense near its core rather than leaking energy to the walls of the reactor.
Before this campaign, KSTAR had been steadily extending its pulse lengths, but two persistent problems kept runs short. First, the device’s older carbon-based divertor absorbed fuel particles and released impurities back into the plasma, degrading performance over time. Second, small asymmetries in the magnetic field, known as error fields, triggered instabilities that forced operators to shut down early. The latest results suggest both problems were addressed simultaneously, which is what makes the 102-second figure more than an incremental gain.
The practical consequence for anyone watching the fusion industry is timing. ITER, the international reactor under construction in France, is designed to sustain plasma for 400 seconds. KSTAR’s role is to test the control strategies and materials that ITER and its successors will need. Every extension of stable plasma time on KSTAR narrows the gap between laboratory demonstration and a working power source.
Tungsten divertor and error-field corrections working in tandem
Two specific upgrades drove the improvement. KSTAR installed a tungsten divertor before the December 2023 campaign began. Tungsten can withstand far higher heat loads than carbon, and it does not trap hydrogen fuel the way carbon tiles do. That swap alone was expected to extend pulse lengths by reducing the buildup of impurities that cool the plasma edge. But the divertor upgrade would have been insufficient on its own if magnetic instabilities continued to cut runs short.
The second upgrade involved real-time error-field correction. A peer-reviewed study in Nature Communications detailed how KSTAR researchers tailored the device’s error fields to suppress plasma instabilities and reduce energy transport losses. Error fields are tiny deviations in the magnetic cage that confines the plasma. Left uncorrected, they seed rotating islands of disrupted plasma that grow until confinement collapses. The KSTAR team showed that by actively adjusting correction coils in real time, they could keep these instabilities from forming during long pulses.
The same work is also accessible through a Nature portal, underscoring that the upgrade is not just an engineering tweak but a step-change in how error fields are measured and controlled. Instead of treating them as static defects to be minimized once at the start of a pulse, the KSTAR approach treats them as dynamic quantities that can be shaped throughout a discharge.
The hypothesis that these two upgrades produce gains greater than either one alone is consistent with the data, though a direct side-by-side comparison using matched heating power from pre-2023 and 2024 pulse databases has not been published. Earlier KSTAR campaigns showed that impurity accumulation and error-field-driven instabilities tended to appear together as pulses lengthened, suggesting that both wall conditions and magnetic symmetry had to be addressed. The 102-second result exceeded previous degradation thresholds, which points to the combined effect of cleaner walls and better magnetic control rather than a single fix.
Open questions about scaling KSTAR’s gains to reactor conditions
Several gaps in the public record prevent a definitive verdict on how far these results can scale. The 48-second figure for ion temperature at 100 million degrees Celsius comes from the institutional announcement, but the raw diagnostic data confirming exact temperature profiles over the full 102-second pulse has not been released in a peer-reviewed format. That matters because plasma temperature can vary across the cross-section of the device, and edge temperatures behave differently from core temperatures during long pulses.
Without full temperature and density profiles, it is hard to say how close KSTAR came to the conditions required for net energy gain in a power plant. High ion temperature is necessary, but so are sufficient density and good energy confinement. If the hottest region of the plasma shrank or shifted during the pulse, the effective performance for fusion reactions could be lower than the headline number suggests.
The tungsten divertor’s measured heat-flux limits before and after installation have also not appeared in a public technical report. Tungsten solves the impurity problem but introduces its own risk: if surface temperatures exceed a threshold, tungsten can melt or crack, sending heavy metal atoms into the plasma. How close the divertor came to those limits during the 102-second run will determine whether the same approach works at higher power levels and longer durations.
Another question involves how the new control strategies will behave under the harsher conditions of a reactor-scale device. KSTAR is smaller than ITER and operates at lower absolute power, which makes some instabilities easier to manage. As power increases, turbulence and edge-localized events can become more violent, potentially overwhelming correction coils that work well at current KSTAR settings. Demonstrating that the same control logic can handle stronger drives will require future campaigns with higher heating power and more aggressive operating points.
There is also the issue of repetition. A single record-setting pulse proves that the machine can reach a given state, but power reactors will need to operate reliably for thousands of cycles. The public summaries of the 2023–2024 campaign emphasize the headline 102-second discharge but say less about how often similar conditions were achieved. If the record pulse was an outlier that required unusually favorable conditions, the path to routine operation may be longer than the topline number implies.
KSTAR Research Center leadership has described the results as verification of a path toward continuous operations. That framing is encouraging but leaves open the question of what happens when heating power increases, auxiliary systems age, and components are subjected to repeated thermal cycling. The tungsten divertor, in particular, will need to demonstrate resilience over many campaigns, not just one.
Even with these caveats, the broader significance of KSTAR’s achievement is clear. By pairing materials upgrades with sophisticated magnetic control, the team has shown that long-pulse, high-confinement operation at reactor-relevant temperatures is not a distant aspiration but an active experimental regime. Each additional second of stable H-mode at 100 million degrees Celsius provides new data on how plasmas behave under conditions that future power plants must endure.
The next steps will likely focus on turning a single long pulse into a reproducible operating scenario, mapping out how performance changes with higher power, and publishing more detailed diagnostics so the global fusion community can test and refine its models. As that evidence base grows, the 102-second KSTAR run will either stand as a milestone on a smooth trajectory to commercial fusion or as a marker of the specific challenges that still need to be solved. For now, it is a concrete demonstration that the combination of advanced divertor materials and real-time error-field control can bend the fusion timeline toward longer, hotter, and more useful plasmas.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
