A new plasma operation scenario Super I Mode was discovered and demonstrated on Experiment Advanced Superconducting Tokamak (EAST). The new high-confinement and self-organizing mechanism represents the reliability and advancement of the machine itself but also offers insights into how to better maintain the plasma operating stably and for long duration.
The team from Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS) reported their finding in a recent published paper in Science Advances.
The exciting discovery was made during campaign of EAST last year in which a stationary plasma with a world-record pulse length of 1056 s was realized.
Scientists were surprised to find, compared with the I mode initially observed on other fusion device, this new mode had even dramatically improved energy confinement so it was called Super I mode.
The new-found mode attracted scientists' attention by its exciting features. It demonstrated energy confinement much higher than I-mode and comparable to H-mode. There's more. During high plasma performance in a long term, the heat load on components exposed to high temperature plasma was moderate due to the absence of large edge-localized-modes.
After data analysis, scientists got the reasons for the significantly enhanced energy confinement.
"We have maintained the electron internal transport barrier at the plasma center," explained Dr ZHANG Bin，a young operator in EAST team, "which worked together with I-mode at the edge, then energy was greatly confined."
In addition to improved energy confinement, the Super I mode also has advantages over other scenarios like no metallic impurity central accumulation at the core, facilitated the particle flux on the divertor remained extremely stable, and sustained quiet stationary plasma-wall interactions. Overall, it could be expected as the basic operation scenario of 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 fusion scientists who are trying ambitiously to replicate sun power on earth, they want the fusion reaction to take place under a well-controlled way. So that whole society could be powered in totally new but much 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 which was built with a target to address key technological and physics issues of long-pulse operation. It takes Tokamak as its technological route and applies magnetic configuration like ITER that is so far 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 were conducted independently by its in-house group or collectively with international fusion community.
Last year, the machine achieved a plasma temperature of 120 million degrees Celsius for 101 seconds in May. Shortly after it embraced its one hundred thousandth shot followed by a record long as 1,056 seconds for high temperature tokamak plasma at the end of the same year.
While when the crowd were cheered up by the record long plasma operation, scientists buried themselves immediately 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 experiment inspires us was more worth expectation,” added QIAN.
As an advanced experimental platform, the machine plays an essential role in exploration regarding fusion physics, technology and engineering. Earlier this year, the EAST and their international collaborators proved experimentally, for the first time, the current driven by turbulence, via observed data obtained in the latest experiment and simulations after.
"The machine enables possibility that we testify ideas, observe physical phenomenon then we analyze the data to know why," QIAN, elaborated how an advanced experimental platform facilitates research "every step forward".
An experimental platform can also be the ground for international scientific collaboration. Each year, EAST opens scientific proposal application to the world. 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)