Plasma current ramp-up with 28 GHz second harmonic electron cyclotron wave

in the QUEST spherical tokamakT. ONCHI1, H. IDEI1, M. FUKUYAMA1, D. OGATA1, T. KARIYA2, A. EJIRI3, K. MATSUZAKI3, Y. OSAWA3, Y. PENG3, R. ASHIDA1, S. KOJIMA1,

K. KURODA1, M. HASEGAWA1, R. IKEZOE1, T. IDO1, K. HANADA1, A. HIGASHIJIMA1, T. NAGATA1, S. SHIMABUKURO1, I. NIIYA1,

K. NAKAMURA1, N. BERTELLI4, M. ONO4, Y. TAKASE3, A. FUKUYAMA5, S. MURAKAMI5

1Kyushu University, 2University of Tsukuba, 3The University of Tokyo, 4Princeton Plasma Physics Laboratory, 5Kyoto University

onchi@triam.kyushu-u.ac.jp

• Through oblique injection of the electron cyclotron wave (ECW), the observationpresents that the highest level of plasma current Ip as non-inductive EC ramp-up isattained with small loop voltage, Vloop < 0.1 V. The auxiliary ohmic heating (OH)increases Ip by 30-40 %. Energetic electrons have main roles to generate the currentand maintain the equilibrium pressure.

• As a result of quasi-normal beam injection and finely adjusted gas fuelling, bulkelectrons are heated efficiently, and hence the electron temperature reaches Te > 500eV at the density ne ∼ 1 x 1018 m−3 with the incident RF power of no more than 150 kW.

ABSTRACTECH CHARACTERISTICS with DIFFERENT N//

v Te and ne through the Thomson scattering measurement

v Data obtained in 2020 (44030 - 45119)v Data filtered by

Ø max Pe | ( t < 3.0 s) & (Te > 5 eV) & (|Ip| > 20 kA) & (σTe < 50 eV) & (ne > 1×1017 m-3)

• High Te with |Ip| < 40 kA + low N//• High Ip, but low Te with high N//• ne may weakly depend on Ip• Pe decreases logarithmically with Ip value• Energetic electrons own the equilibrium

pressure (Pe = 200-300 Pa) when Ip is high

AUXILIARY OHMIC HEATING and MODULATION of

LOOP VOLTAGE• Oblique ECW Injection:

• Loop voltage applied after EC current ramp-up

• Ip > 100 kA achieved• Ip increases by 30-40 % with the medium

Vloop (< 0.5 V)• Quasi-normal Injection:

• Modulation of the central solenoid current à Change of Vloop, thus #$à Te ≈ 830 eV

OUTCOME

• Plasma current ramp-up experiment by 28 GHz-ECH has been explored in the QUESTspherical tokamak*1,2. Multiple harmonic resonance layers (2nd – 4th) locate in theplasma confinement region.

• Oblique EC wave injection : With high refractive index parallel to magnetic field, N//,energetic electrons moving forward along the magnetic field resonate more effectivelythan those moving backward. Such symmetry breaking is consistent with the results ofthe current ramp-up experiment (Ip > 70 kA by ECH)*3.

BACKGROUND

STEERING ANTENNA*4

(1) Small convex mirror:HE11 beam à Gaussian beam

(expanded ). (2) Large concave mirror: expanded Gaussian beam à

focused beam ( 5-cm waist size at the ECR layer)

(3) Steering antenna to change the beam direction

BULK HEATING THROUGH

QUASI-NORMAL ECW

INJECTION• Plasma current is ramped up

to Ip ≈ 25 kA. Loop voltage isas low as Vloop ≈ 0.1 Vmeasured on the central post,R = 0.18 m

• Hard X-ray (HXR) pulse signalcounted by Cadmium Telluridedetector is also observedconcurrently with plasmaramp-up.

• The RF power is absorbed inboth bulk and energeticelectrons effectively, thereforemoderate current drive andbulk electron heating occursimultaneously.

RF INJECTION USING STEERING ANTENNA

The waveforms of discharge through quasi-normal ECW injection

• The ECH in QUEST, with multi-harmonic resonances in the core region, ischaracterized by parallel refractive index N// of ECW

• Bulk heating occurs with low N//. The highest Te is obtained with #%-modulation.• High Ip is realized with high N//, but pressure of bulk electrons decreases obviously.

The equilibrium pressure is maintained by the energetic electrons. Ip > 100 kA hasbeen achieved by auxiliary ohmic heatings.

CONCLUSION

Waveforms of the discharge 40527: (a)the 28 GHz-RF power monitor signal,(b)Ip, (c)Vloop, (d)line-integrated ne. (e) Two-dimensional map of magneticflux surfaces, obtained by an equilibrium reconstruction, with Ip = 100 kA.(f) #$,measured at R = 0.18 m, vs Ip.

ID: 844

The authors are most thankful to the QUEST team for technical support. This work was performed under the auspicesand support of the NIFS Collaboration Research Programs (NIFS19KUTR136/NIFS17KUTR128). This research waspartially supported by the Ministry of Education, Science, Sports and Culture, under Grant-in-Aid for Scientific Research(B) (No. 15H04231).*1 : H. Idei et al., Nucl. Fusion 57 (2017) 126045.*2 : H. Idei et al., Nucl. Fusion 60 (2020) 016030.*3 : T. Onchi et al., Phys. Plasmas 28 (2021) 022505.*4 : H. Idei et al., Fusion Eng. Des. 146 (2019) 1149.

ACKNOWLEDGEMENTS / REFERENCES

表1Te [eV] O-mode absorption X-mode

absorption50 6.37263302636915E-04 8.21735938533152E-03100 1.19299500754355E-03 1.52991841999937E-02200 2.40193661746246E-03 3.04609500126829E-02500 6.126412545710450000E-03 7.50960417850143E-021000 1.26614928781783E-02 1.466148445964E-01

0%

4%

8%

12%

16%

Ee [eV]50 100 200 500 1000

O-mode absorptionX-mode absorptionO-mode absorption X-mode absorption

1

(e)

(f)

A top view of QUEST. The second, third, and fourth harmonic resonance layers are shown by red, green, and blue half circles, respectively.

(3)

§ ne0 = 2.0e18 [m-3]§ N|| = 0.11

Rays of the ECW (N// = 0.11 at Rres2)

at R = 0.18 m

The waveforms of discharge 41194: (a)plasma current Ip, electric field Ef at R = 0.18 m. (b) Haline emission with gas injection timing. (c) Electron temperature and density measured at R = 0.40 m through Thomson scattering.

Typical discharge waveforms. Current isramped up through the 28 GHz-ECH. ECW isinjected obliquely as N// = 0.75.

Major radial dependence of ⁄'∥ ') on particle energy. Particles whose energy of 1 keV (black), 10 keV (blue), 30 keV (green), and 60 keV (red) satisfy resonance conditions at ⁄'∥ ')

Resonance ellipses and constant velocitycircles in velocity space with respect to thesecond resonance.

(f)