January 2006
In this month, experiments related to three of major JT-60U objectives in 2005-2006, namely (i) sustainment of high βN with high confinement, (ii) sustainment of high bootstrap current fraction, and (iii) sustainment of high confinement under saturated wall conditions, have been started. Results including other experiments are shown below.
(1) Conditioning of heating systems
The pulse length of tangential positive ion NBs (P-NBs) has been extended to 29-29.5 s and that of #14 (one of perpendicular P-NBs) to 15 s. The negative-ion NB (N-NB) was injected for 5 s at 340 kV and 3 MW. The pulse length of ECRF was up to 3 s for 3 units (#1, 3, 4) and up to 1.4 s for 1 unit (#2). Injection of LHRF was started and the pulse length was up to 1 s in modulated operation with a duty of 50%.
(2) Sustainment of high βN with high confinement
The target of this experiment is to maintain a high βN of 2-2.5 with high confinement of HHy2 ~ 1 for a long time (>25 s), which is one of the major objectives in 2005 to 2006. Optimization of discharge scenario including the configuration scan was carried out. The plasma current and the toroidal field were mainly Ip = 1 MA and Bt = 1.76 T (q95 ~3.2). It was found that both a low particle inventory in the first wall including the divertor target and a low wall temperature before the discharge are important to maintain low particle recycling and high confinement. In first long-pulse discharges after the overnight GDC, βN =2.5 and HHy2 ~0.94 were maintained for 5 s and βN>1.8 and HHy2>0.7 were maintained for 26 s.
(3) Sustainment of high bootstrap current fraction
The target of this experiment is to maintain a discharge with a large fraction of bootstrap current fBS of 70-80% for a long time (~20 s), which is one of the major objectives in 2005 to 2006. The response and controllability of a plasma with a large fBS will also be studied. Optimization of discharge scenario was carried out to reproduce the shot E43046 in 2004, in which a fBS of 75% was maintained for 7.4 s, with a reduced NB power for long pulse operation (>10 s). The reference value of the plasma stored energy for feedback control and the toroidal momentum input from tangential NBs were adjusted in order to suppress mini collapses with a wide ITB radius maintained. Duration of high fBS up to 5 s was obtained.
(4) Particle control study and argon injection under saturated wall conditions
These are related to sustainment of high confinement under saturated wall conditions, one of major objectives in 2005 to 2006.
The particle control was studied under saturated wall conditions that were obtained by repeating long pulse ELMy H-mode plasmas (1.2 MA, 2.3 T, PNB ~ 4-8 MW, ne-bar ~0.6nGW). The saturation of particle inventory retained in the wall was observed in the 3rd discharge for 4 MW heating, while it was observed in the 2nd discharge for 8 MW heating. This indicates that the particle balance is affected by the rise in the wall temperature and resultant particle release from the wall. The steady sustainment of the plasma density under saturated wall conditions was demonstrated by using divertor pumping in an optimized configuration with a smaller distance between the divertor hit points and the pumping slot (2-3 cm). The H factor was similar to that in a discharge with wall pumping.
Argon injection was attempted under saturated wall conditions for the first time in JT-60U, in order to achieve high confinement with high radiation under saturated wall conditions. High radiation fraction (Prad/PNB ~0.8) with reduced ELM activities was obtained and sustained for 4 s during argon injection. The H factor was higher (H89PL ~ 1.55 at ne/nGW ~0.68) than that in a discharge without argon injection (H89PL ~1.35, Prad/PNB ~0.5 at ne/nGW ~0.65).
(5) Slow Ip start-up by ECRF without a central solenoid
Ip start-up by ECRF was attempted by adjusting the vertical field slowly, without applying a one-turn voltage from any coils. While the ratio of the 'VR' coil current to the 'VT' coil current was set 1:(-2) to obtain a broad region with a positive n-index, the vertical field was ramped down from 50 G to 30 G. A plasma current was generated when the vertical field (Bv) became 40 G, and Ip of 8 kA was maintained for 1.5 s for Bv = 30 G.
(6) Dynamic transport analysis
The ECRF power and the NB power were modulated in time to study relation between the heat flux divided by the electron density and the temperature gradient dynamically. In both a positive shear plasma and a reversed shear plasma, the electron temperature (Te) gradient inside the ITB decreased during formation of a Te ITB. This indicates that formation of a strong ITB with locally reduced radial transport is accompanied by enhancement of the radial transport outside the ITB layer.
(7) Impact of electron heating on ITBs
The impact of electron heating on an ion temperature (Ti) ITB was studied by injecting an ECRF wave into a high βp H-mode plasma. A higher plasma current at Ip = 1.2 MA was attempted to compare the previous data at Ip = 1 MA. The degradation of a Ti ITB was observed for 3 units ECRF injection, but the change in the Ti profile was smaller than that observed for 1 unit ECRF injection at Ip = 1 MA. This suggests Ip dependence of the ECRF effect on the ITB degradation.
(8) Bootstrap current overdrive
The OH coil ('F coil') current was fixed during the flat-top of a reversed shear discharge at Ip = 0.6 MA, Bt = 3.7 T without co-tangential NB injection, in order to demonstrate the bootstrap current overdrive. The plasma current was maintained for 4 s without a one-turn voltage from the OH coil, though it gradually decreased to 0.53 MA with repetitive mini collapses at intervals of about 1 s.
(9) Fast ion Confinement with ferritic steel inserts
The decay time of a neutron emission rate after a short pulse NB injection (beam blip) was measured and compared to the calculation with the Orbit Following Monte Carlo (OFMC) code, in order to evaluate the confinement of fast ions. A large-volume plasma configuration was used to enhance the effect of ripple reduction due to ferritic steel inserts. ECRF heating was employed to extend the beam slowing down time. The observed decay time was closer to the calculation with the magnetic field generated by the ferritic steel than to the calculation without it, though the difference between the latter two was small.
(10) Excited waves near the ion cyclotron range of frequency
ICRF antennas were used as pick up coils for detecting electrostatic and/or electromagnetic fluctuations. Fluctuations whose frequencies corresponded to the ion cyclotron frequency (and its harmonics) of helium-3 ions, triton and deuteron were observed for the injection of tangential P-NBs, perpendicular P-NBs and tangential N-NBs, respectively. The toroidal wave number was evaluated from the phase difference between signals of two antenna straps arrayed in the toroidal direction. The fluctuations due to deuteron have zero or small wave numbers, while those due to fusion products (triton and helium-3 ions) have finite wave numbers.
(11) Fluctuation profile measurement using MSE
A new data acquisition system with 16 channels has been prepared to record the MSE data at 500 kHz for 50 s, in order to measure density fluctuations and magnetic fluctuations. In a discharge aimed at sustainment of a large bootstrap current fraction (0.8 MA, 3.4 T, reversed shear), 5-20 kHz fluctuation was observed in the signal of only one MSE channel near the plasma edge. Similar fluctuation was observed in the interferometer signals, while no magnetic fluctuations were observed in pick up coil signals. The observed fluctuation seems to be an electrostatic density fluctuation localized near the plasma edge.
(12) Absorption of 3rd harmonic resonance ECRF
Absorption properties of an ECRF wave for the 3rd harmonic resonance condition, which will appear in JT-60 modification with superconducting coils, were studied for the first time in JT-60U. The target plasma parameters were Ip = 0.8 MA, Bt = 1.4 T and ne-bar = (1.2-1.8)x1019 m-3. As a reference, absorption for the 2nd harmonic resonance condition was also investigated at Bt = 1.9 T. The absorption fraction was estimated by the time derivative of the plasma stored energy, dW/dt, at the start of ECRF injection. Though dW/dt for the 3rd harmonic resonance condition was about 20% of that for the 2nd harmonic resonance condition in OH plasmas, it was enhanced up to 80% when NB of 7-8 MW was injected. This tendency does not seem to contradict with the prediction of a Fokker-Planck code, though precise profile measurements are required for quantitative evaluation.