Spherical tokamak generation and merging on UTST using ex-vessel poloidal field coils only

Ryota Imazawa, Shuji Kamio, Ryuma Hihara, Keita Abe, Morio Sakamura, Qinghong Cao, Takuma Yamada, Michiaki Inomoto, Yuichi Takase, Yasushi Ono

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)


We successfully generated two spherical tokamaks (ST) at two null points by using ex-vessel poloidal field (PF) coils only and succeeded in merging them. This scheme is called a double null merging (DNM) scheme. First, two pairs of ex-vessel PF coils generate two null points where the poloidal field is zero at the upper and lower regions inside the vacuum vessel. Then a poloidal flux swing generates two STs at two null points, because the distance to the wall along the magnetic field is long at the null points. Finally, the coil currents push two STs toward the mid-plane and merge them into a single ST. Since a magnetic reconnection during merging transforms magnetic energy into thermal energy, this merged ST plasma is expected to have a high beta. It must be noted that the DNM scheme generates an ST without a center solenoid coil. The DNM scheme was demonstrated on the TS-3/4 (Japan) and MAST (UKAEA). However, these devices have all PF coils inside the vacuum vessels, and the initial plasmas were generated around the PF coils, not the null points. Since internal coils are not feasible in a fusion reactor due to high neutron flux, it is important to demonstrate the DNM scheme by using ex-vessel PF coils.

Original languageEnglish
Pages (from-to)18-24
Number of pages7
JournalElectrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi)
Issue number4
Publication statusPublished - Jun 2012
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Energy Engineering and Power Technology
  • Electrical and Electronic Engineering


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