A new plasma operation scenario, Super I mode, was discovered and demonstrated on the Experiment Advanced Superconducting Tokamak (EAST). The new high-confinement and self-organizing mechanism represents the reliability and advancement of the machine itself, while also offers insights into how to better maintain the plasma operating stably and for long duration.
The exciting discovery was made during campaign of EAST last year, in which a stationary plasma with a world-record pulse length of 1,056 seconds was realized.
The researchers were surprised to find that the new mode even dramatically improved the energy confinement compared with the I mode originally observed on other fusion devices, hence it was called Super I mode.
This newly discovered mode has caught the attention of scientists for its exciting features. It exhibits a much higher energy confinement than I-mode and is comparable to H-mode.There are more. During long term high plasma performance, the thermal load of the element exposed to high temperature plasma is moderate because there is no edge-localized-modes.
After analyzing the data, the researchers figured out why the energy confinement had increased significantly.
"We have maintained the internal electron transport barrier at the plasma center," said Dr ZHANG Bin, a young operator in EAST team, "which works together with I-mode at the edge, then the energy is greatly confined."
In addition to improved energy confinement, the Super I mode has other advantages, such as no metallic impurity central accumulation at the core, favorable particle fluxes on the divertor to remain extremely stable, and sustained quiet stationary plasma-wall interactions. Overall, it can serve as the basic operation scenario of the International Thermonuclear Experimental Reactor (ITER), the world's largest tokamak joined by 35 nations.
An advanced and open experimental platform
Fusion is the process of two atoms of hydrogen fusing together to form an atom of helium while releasing a large amount of energy, through which the sun and stars are powered. For ambitious fusion scientists who are trying to replicate sun power on Earth, the hope is that the fusion reaction can take place under a well-controlled way, so that whole society can be powered in a totally new but more efficient and green way.
Highly motivated by this shared goal for all humans, fusion scientists across the world have been for decades exploring in different ways. Dozens of fusion research machines have been designed and constructed and operated as result.
EAST is one of those experimental platforms built to address key technological and physical issues of long-pulse operation. It takes Tokamak as its technological route and applies magnetic configuration like ITER, one of the most extensive international scientific collaboration involving 35 countries and spanning 35 years.
Since its first discharge in 2006, EAST has two rounds of campaign each year. Thousands of experiments have been conducted independently by its in-house teams or collectively with the international fusion community.
In May last year, the machine reached a plasma temperature of 120 million degrees Celsius for 101 seconds. Soon after, it had its 100,000th shot, and at the end of the same year, the high temperature Tokamak plasma set another record of 1,056 seconds.
As people cheered the record-breaking plasma exercise, scientists immediately buried themselves into data analysis.They wanted more from the "story behind the record," said Prof. QIAN Jinping, deputy head of Tokamak Experiment Division, "we were all excited about the record. That was for sure a milestone of our machine. But I think what the experiments bring us more inspiration to look forward to."
As an advanced experimental platform, the machine plays an essential role in fusion physics, technology and engineering. Earlier this year, the EAST team and their international collaborators proved experimentally, for the first time, a current driven by turbulence, using observations from recent experiments and simulations.
"It enables possibility that we testify ideas, observe physical phenomenon, and analyze the data to know why," said QIAN, elaborating on how an advanced experimental platform facilitates research "every step forward".
An experimental platform can also be the ground for international scientific collaboration. EAST opens applications for scientific proposals to the world every year. International participation is seen in the experiment conducting and data sharing.
Waveform of the thousand-second discharge #106915. (Image by EAST Team)
Diagram of fusion triple-product [ni(0)Ti(0)τE] versus plasma duration t(s) for mega-ampere and megawatt class tokamaks. (Image by EAST Team)
Identification of turbulent current. (Image by EAST Team)
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