Spontaneous Formation of Spherical Tokamak by ECH on LATE Graduate School of Energy Science, Kyoto University *Graduate School of Science, Kyoto University **Japan Atomic Energy Research ***Tsukuba University STW /09/ /01, Kyoto University H.Tanaka, Y.Abe, K.Hayashi, J.Yamada, T.Matsumoto, T.Yoshinaga, M.Uchida, S.Yamaguchi*, T.Maekawa, S.Maebara**, T.Imai***

Contents * Introduction * Spontaneous formation of initial closed flux surfaces by 2.45 GHz microwave (Pinj =< 20 kW) * Spontaneous formation of initial closed flux surfaces by 5 GHz microwave (Pinj =< 130 kW) * Summary

Study on Formation of Initial Closed Flux Surfaces Formation of Spherical Tokamak by ECH only without Ohmic Heating Vertical field is essential to generate plasma current! Change of topology: Open field configuration ==> Formation of initial closed flux surfaces * How does it occur? * What is the difference with the Ohmic case. Experiments under steady vertical field

Current Jump and Spontaneous Formation of Closed Flux Surfaces under Steady Vertical Field Vertical Field in Vacuum Poloidal Flux after Current Jump

Dependence on the Vertical Field Strength Before Current Jump After Current Jump  C0 is inverted after current jump  urrent jump occurs 11 G =< Bv =< 28 G at Pinj = 5 kW Plasma current after the jump is nearly proportional to Bv. Plasma current before the jump is not inversely proportional to Bv, suggesting that the plasma pressure is changed as Bv is changed.

Change of ne, Vs, Te Profile after Current Jump Before Current JumpAfter Current Jump n e (R,z) V s (R,z) T e (R,z) The contor lines of oloidal flux and n e profile almost coincide each other. Space potential decreased, suggesting that the confinement of electrons is improved by the formatioi of the closed flux surfaces. Te is eV and the profile is broad and do not have any apparent structures corresponding to the closed flux surfaces.

Magnetic Probes and Flux Loops 12 ch magnetic probes are inserted into plasma. Together with 5 ch flux loops wound around the center stack, we can measure the change of poloidal flux with negligeble eddy current effect.

Current Profile Model We adopt the following model to express the current profile and obtain 7 values from the measured poloidal flux by least-squares-error fitting method: The plasma currrent flows in the area composed of 4 one-quarter-ellipses with parabolic profiles. 7 fitting parameters: -positioin of the maximum of current density : Rc, Zc -the value of the maximum current density : j0 - length of the each axis of ellipse : a1, a2, a3, a4

Model Calculation of Currrent Distribution t = ms Ip = 0.44 kA t = ms Ip = 0.61 kA t = ms Ip = 0.71 kA t = ms Ip = 0.76 kA t = ms Ip = 0.81 kA t = ms Ip = 1.09 kA Pinj = 5 kW Bv = 18 G Current Jump

Maximum Ip Increases with Bv and Pinj f = 2.45 GHz I T = 51 kAT Bv = 34 G Bv = 32 G Pinj = 20 kW

5 GHz ECH System Setup Oblique Injection : * ~15 degree from normal to B T * left-hand circularly polarized

5 GHz ECH System (Photograph)

Magnetic Field Configuration I T = 90 kAT f = 5GHz R(  =  e) = 10.1 cm R(  = 2  e) = 20.2 cm R(  = 3  e) = 30.2 cm R(  = 4  e) = 40.3 cm  =  e  = 2  e  = 3  e  = 4  e

Wavefore of Discharge (5GHz, 130 kW, 60 ms) Bv (G) P H2 (x Pa) Pinj(5GHz) (kW) Pref(5GHz) (kW)  c0 (mWb) Ip (kA) Rp (m) neL(27cm) (x10 13 cm -2 ) neL(17cm) (x10 13 cm -2 )  c0 > 0 : Field Reversal          I T = 90 kAT Bv = 85 G = 27.4cm) n e L = 3 x cm -2 P inj ~ 130 kW, 60 ms

Maximum Ip Increases with Bv (5GHz) 5 GHz, steady Bv 2.45 GHz, steady Bv After Current Jump

Maximum Ip Increases with Bv (5GHz & 2.45 GHz) 5 GHz, steady Bv 2.45 GHz, steady Bv 2.45 GHz, Bv ramp

Maximum Ip Increases with Pinj (5GHz) 5 GHz, steady Bv 2.45 GHz, steady Bv

Pinj Dependence after Current Jump I T = 90 kAT Bv = 60 G

Model Calculation of Currrent Distribution t = 89.5 ms Ip = 2.14 kA t = 91.7 ms Ip = 4.11 kA t = 94.7 ms Ip = 7.36 kA t = ms Ip = 6.75 kA t = 60 ms Vacuum

A Speculation of Mechanism of Spontaneous Formation To start the process of current jump, some amount of plasma current should be made flowing in the open field lines. Such current may enhance the local magnetic mirror and the number of trapped electrons increases. Perpendicular heating by ECH may assist trapping effectively and the plasma pressure will increases, resulting in the increase of pressure-driven current. Small closed flux surfaces appear if the enough current are driven. Then the auto-selected current could flow because of the different direction of shift of drift surfaces of passing electrons around the closed flux surfaces. The shift of the drift surface of electrons carrying counter-current is inward and they may escape beyond the separatrix to the wall or hit the inner wall (center stack), while that of electrons carrying co-current is outward and may be effected nothing. Such positive feedback mechanism increases plasma current till the initial closed flux surface is formed and balanced by the MHD equilibrium condition * Contribution of bootstrap current and/or EC driven current is an open question.

Summary By injecting microwave power under steady vertical field at low gas pressure, plasma current suddenly increases in the course of slow rise ("current jump"). By this process, initial closed flux surfaces are spontaneously formed. The steady value of plasma current after the formation of the closed flux surfaces is determined by the vertical field strength so as to maintain the MHD equilibrium. Increasing both vertical field strength and injected microwave power, more plasma current can be generated. So far, plasma current up to 6.5 kA is obtained when 5 GHz, 130 kW microwave power is injected at Bv = 85 G. By the magnetic measurement and model calculation of current distribution, it is suggested that elongated current distribution in the open field lines enhances the local magnetic mirror, and during the current jump, small closed flux surfaces are formed and become large as the current profile spreads toward outboard side.

LATE : Side View Vacuum Vessel :  100 cm x 100 cm Center Stack : O.D. =  11.4 cm 60 kAT (Steady State) 875 R = 13.7 cm 714 R = 16.8 cm 180 kAT (0.1sec flat top) 1786 R = 20.2 cm Vertical Coils (Bv-I) : 16.4 kAT (Steady State) (Bv-O) : 8 kAT (Steady State) (Bv-R) : 12 kAT (Steady State) RF Source 2.45 GHz, 5 kW x 2, CW (Magnetron) 2.45 GHz, 20 kW, 2.5 sec (Magnetron) 5.0 GHz, ~ 200 kW, 0.1 sec (Klystron) 2.0 GHz, 350 kW, 0.1 sec (Klystron) Vertical Field Coils ~ 2.4 m ~ 3 m Flux Loops Return Limbs Center Stack TMP  =  e  = 2  e

LATE : Top View (NaI Scintillator) (20 ch x 4) Oblique Injection : * ~15 degree from normal to B T * Linearly polarized E-field on the equatorial plane

Threshold Pressure Increases with Pinj f = 2.45 GHz I T = 60 kAT Bv = 22 G cm) The gas pressure at the beginning of the current jump increases as Pinj increases.

Summary : 2.45 GHz Experiment By injecting a 2.45 GHz microwave pulse up to 10 kW for 4 seconds, a plasma current has been initiated and ramped up over Ip = 4 kA by a slow ramp of the external vertical field to keep the higher plasma current in equilibrium. Magnetic measurements show that a Spherical Tokamak equilibrium, having the last closed flux surface with an aspect ratio of R 0 /a = 22.3 cm/16.6 cm ~ 1.35 and an elongation of  ~ 1.34 has been produced and maintained. The soft X-ray CT image of the plasma cross section and the visible light plasma image coincide with the magnetic measurements. A unidirectional high energy electron tail is produced during the discharge. The line-averaged electron density is 1.3 x cm -3 which is almost twice the plasma cutoff density, suggesting that electron cyclotron heating by mode-converted electron Bernstein waves may take place. When the decay index of Bv is small, plasma elongation and cross section of the last closed flux surface become large and plasma current increases.

5 GHz Klystron 5 GHz Klystron (Toshiba E3720SPM) Original Specification (JAERI) Now testing... Beam Voltage Beam Current Frequency Output Power Pulse Width Repetition Rate 77 kV 15 A 5.00 GHz 800 kW 15  s 700 pps Beam Voltage Beam Current Frequency Output Power Pulse Width Repetition Rate 60 kV 10 A 5.00 GHz >200 kW 0.1 s 1 pulse per 3 min

Advantages in Using Higher Frequency Microwaves Shorter Wavelength : Wavelength << Plasma Size ( cm) Larger L n / 0 Higher Cutoff Density Stronger Magnetic Field Wavelength ( 0 ) L n / 0 L n = 3cm) Cutoff Density (n c ) Resonant Magnetic Field 2 GHz15.0 cm x cm G 2.45 GHz12.2 cm x cm G 5 GHz 6.0 cm x cm G Launching wave is nearly a plane wave Better conversion rate to EBW Higher plasma density Better confinement L n : Scale length of density gradient

Mode Conversion Rate f = 5 GHz f = 2.45 GHz Left-hand Circularly Polarized  e /  = 0.4 N // = N //opt = 0.535

Polarization Converter (Photograph)

Polarization Converter : Cold Test Confirmation of Conversion From Rectangular TE 10 mode to Left-hand Circularly Polarized Wave Polar plot of detected signal

Change of SX Camera Signal