Soft X-ray measurement in RF driven plasmas on QUEST Hiroki MIURA 1, Kazuaki HANADA 2, Hideki ZUSHI 2 ， Kazuo NAKAMURA 2, Akihide FUJISAWA 2, Hiroshi IDEI1 2, Yoshihiko NAGASHIMA 2, Makoto HASEGAWA 2, Hisatoshi NAKASHIMA 2, Shoji KAWASAKI 2, Aki HIGASHIJIMA 2, Osamu MITARAI 3, Takashi MAEKAWA 4, Atsushi FUKUYAMA 5, Yuichi TAKASE 6, Akira EJIRI 6, Naoyuki FUKUMOTO 7, Takashi YAMAGUCHI 6, Hiro TOGASHI 6 IGSES Kyushu Univ. 1, RIAM, Kyushu Univ. 2, Tokai Univ 3, Department of Nuclear Engineering Kyoto Univ. 4, Kyoto Univ 5, Graduate School of Frontier Science Univ. of Tokyo 6, Univ of Hyogo 7

QUEST is a medium size of spherical tokamak, which aims to realize non- inductive start-up and its maintenance. R ： major radius a ： minor radius A ： aspect ratio B ｔ ： toroidal magnetic field RF ： heating sourse cross-sectional view of QUEST Main purposes on QUEST are to research on non-inductive current start-up and current drive. R[m] ～ 0.64 a[m] ～ 0.36 A ～ 1.78 B t [T] ～ 0.25 RF 2.45GHz 8.2GHz 28GHz A purpose of newly installation of a 28GHz system plasma current

Typical waveform of plasma current with 28GHz RF and plasma current reached to more than 50kA. 28GHz RF An experiment with 28GHz RF is started since 2013 plasma current increase 28GHz Plasma current reached to more than 50kA. The 28GHz RF injected into the plasma from 1.8 to 3.46 sec.

Plasma current frequently degrades with additional 8.2GHz RF power. Plasma current frequently degrades with additional 8.2GHz RF power. The reason is still not understood There is something to be an interesting phenomenon between 28GHz and 8.2GHz plasma

Slow oscillation of plasma current sometimes happens in 28GHz and 8.2GHz plasmas. There is an interesting phenomenon at combination of 28GHz and 8.2GHz. Slow oscillation of plasma current A frequency of the oscillation is from 20Hz to 30Hz. Measurement of soft -ray (SXR) ( Measurement of SXR has high time and special resolutions ) A 2DSXR camera An AXUV detector array 2 types of SXR diagnostics were used to investigate the phenomenon.

An AXUV detector array was firstly installed to measure SXR with wide view. AXUV Detector array 2.79m 2.75m 1m1m 0.44m ch1 ch16 (b) (a) (b) An energy sensitivity of the array is from 1.12eV visible light to 100keV SXR. The array is covered with a SUS box The array measures SXR from plasma through a pin hole of 7mm in diameter. (a)

The array can detect SXR emitted from approximately half of plasma in top view. The array can detect SXR emitted from plasma with a wide view. The array is composed of 16 ch and can cover most of plasma region in the poloidal cross-sectional view. AXUV detector array CH1 CH16 cross-sectional view of dot-line AXUV detector array Top view of QUEST A A

A 2DSXR camera can detect SXR with high special resolution around core plasma region. A viewing area of the camera is a range of diameter 36 centi meter on cross-sectional view of QUEST R=26cm R=62cm An energy sensitivity of the camera is from 10eV to 10keV SXR A 2DSXR camera Al filter MCP phospher + + The operation of the camera needs to install an orifice and should take a distance between plasma and MCP. The camera may break up by arc if the camera operates in high pressure environment. MCP must be provided high voltage The camera is useful to measure SXR from core plasma in detail.

Typical waveforms of a slow oscillation from 20 to 30 Hz are illustrated. The oscillation happened at combination of 28GHzRF with 8.2GHzRF. 8.2GHz 28GHz It suggests the oscillation happens self-consistently. Both poloidal coil currents and RF power are kept constant. During the oscillation

CH1 CH16 The SXR oscillations are localized around the core plasma region. The SXR oscillations with high amplitude appear in the range of ch4 to ch9. The oscillation may be caused by the core plasma modification. An oscillation range expected by next graph AXUV Detector array CH9 CH4

The SXR oscillation around core plasma can be detected by the 2DSXR camera. Camera’s results are used to investigate core plasma behavior in detail Viewing area of the camera The pictures measured every 1 ms. The area with intense SXR signal repeats to expansion and shrink.

The intense SXR was emitted from 2 nd ECR layer of 28GHz and spread with the expansion of LCFS Z[cm] R Z R [cm] R Z 3 3 The intense SXR was emitted from 2 nd ECR layer of 28GHz and spread with the expansion of last-closed flux surface. The SXR oscillation can be partially explained by the modification of LCFS. -18 Two green lines show a location of a second harmonic resonance layer of 28GHz at R=32cm and a fundamental resonance layer of 8.2GHz at R=54cm Yellow lines illustrate a location of LCFS at the time.

SXR is reduced during the oscillation even in LCFS, and simultaneously increases outside. SXR signals are different between inside and outside of core plasma The reduction of SXR signal takes place even inside LCFS and takes a rise outside LCFS. This means particles and/or energy flow out from LCFS.

Particles of core plasma are exhausted from LCFS during the oscillation. The oscillation with the same frequency is also observed in line integrated density measured through the mid-plane. It suggests particles of core plasma are exhausted from LCFS during the oscillation.

Summary  The slow oscillation with 20-30Hz in plasma current was observed in 28GHz + 8.2GHz non-inductive plasmas.  The slow oscillation can be detected by SXR measurement and is localized around the core plasma.  The oscillation can be monitored in a 2D SXR camera. The intense SXR area spread with expansion of LCFS. The reduction of SXR was observed even inside LCFS during the oscillation and the increase simultaneously happened outside LCFS. This means particles and/or energy flow out from LCFS.  The oscillation with the same frequency was observed in line integrated density and this means at least particles flow out from LCFS during the oscillation.

Limiter An AXUV detector array AXUV Detector array Operating temperature limit of an AXUV detector array is 70 degrees. The array is covered with SUS box and cooled down by water-cooled tube. Baking temperature of QUEST is 100 degrees.

A structure of a 2DSXR camera High speed camera Visible light Pinhole +Be filter 25μm Al filter 5μm MCP +phospher Soft Xray An energy sensitivity of the camera is from 10eV visible light to 10keV SXR SXR goes through an Al filter. Next, MCP amplifies electron when MCP receives SXR through a pinhole. Finally, phosphor converts electron to visible light. A Be filter is used when it cuts off low energy SXR.

RF antenna system to inject of well- controlled microwave for EBWCD 19 CW 8WG Antenna EyEy ExEx n e0 = 2 x m -3, T e0 =100eV, total current : 20kA

Non inductive Current start-up (35kA) with 8.2GHz RF 20 ~35kA 35 kA was achieved at 100 kW. Although SSO was not established, Ip more than 20 kA was sustained for ~ 15 s. Wave absorption decreases with enhancing recycling. Plasma shape was oblate due to strongly curved Bz.

28 GHz Gyrotron is installed on QUEST (Collaboration with Tsukuba Univ.) Superconducting magnet MOU waveguide QUEST MOU 10m

Operation record 28GHz (Gyrotron) This gyrotron has oscillated 28 GHz/ 1.25 MW at the maximum output power of a power supply. QUEST experiments The maximum incident power into the QUEST plasma is 450 kW/ 600 ms. (unstable) Sometimes, an anomalous discharge is occurred by an output gas from a corrugation surface of the transmission line. The longest pulses length 2 second at Tsukuba Univ. (Output power: Gyrotron-600 kW/ MOU-532 kW) 140 ms at QUEST (Output power: Gyrotron-505 kW/ MOU-457 kW) Both, the pulses length were limited by a calorific capacity of a dummy load.

Main purpose of 28GHz: to get higher density for EBWCD EBWH/CD Exp. Scenario ： 1) 1 st : 8.2 GHz: production 2) 2 nd : 28 GHz : density ramp-up 3) 1 st : 8.2 GHz : EBWCD 2 nd :28GHz 1 st :8.2GHz 28GHz tube has been developed in Tsukuba Univ. and QUEST has a power supply for the 170GHz gyrotron

FULL-CD with 270kW gives more than 50 kA GHz 28GHz 2 nd harmonic resonance is located around inner edge of last closed flux surface.

Time evolutions of flux surface in 28GHz full non inductive plasma

8.2GHz RF has strongly the disturbing effect of plasma current