Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear

https://iopscience.iop.org/article/10.1088/0029-5515/32/1/I04/pdf

Shear Reversal and Mhd Activity During Pellet Enhanced Performance Pulses in Jet

The q‐profile plays a key role in determining plasma equilibrium and stability in tokamaks. With the development of the motional Stark effect (MSE) diagnostic, accurate q(R,t) profiles have been measured and utilized for equilibrium and stability analysis. A multichannel MSE polarimeter system on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)] has obtained data for several plasma conditions including L‐mode, supershot, current ramps, and high βp. For sawtoothing discharges on TFTR, it is found that q(0)∼0.7, well below one, and remains below one throughout the entire evolution of the sawtooth cycle with an increase in q(0) of ≤0.1 after a sawtooth crash. During high βp operation or coinjection of neutral beams on TFTR a significant broadening of the current profile and an increase of q(0) is observed.

q‐profile measurements in the Tokamak Fusion Test Reactor*

The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 ? 10l6 s?1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 ? 1019 m?3. The corresponding fusion product nD(0)?ETi(0) is between 7.0 and 8.6 ? 1020 m?3?s?keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

https://iopscience.iop.org/article/10.1088/0029-5515/31/5/003/pdf

H-mode confinement in JET with enhanced performance by pellet peaked density profiles

The physical properties of the JET lower hybrid antenna have been investigated numerically with a two-dimensional computer code based on the linear coupling theory. The antenna is made from an array of multijunction units which divide the incident power along the toroidal direction. The main properties of this new coupler are investigated and compared with those of the conventional grills generally used in previous lower hybrid experiments. In the light of this study, the general design of the multijunction antenna is presented; the importance of the geometrical parameters of the multijunction unit (e.g. choice of the location of the E-plane junctions, septum width between waveguides) is stressed. These parameters are optimized and their values are taken into account in launcher manufacturing. With such an optimization, the electric field enhancement in each secondary waveguide is minimized for an electron density at the launcher of about 1018 m−3, the power reflection coefficient is below 1.5% and the 'n1-weighted antenna directivity' is expected to lie between 60% and 70% in a large range of plasma densities around the optimum density. By using two shorted passive waveguides on each side of the antenna, the edge effects are reduced. The effect of the accessibility limit on coupling is also investigated.

https://iopscience.iop.org/article/10.1088/0029-5515/30/3/009/pdf

Coupling of slow waves near the lower hybrid frequency in JET

This paper describes the upgraded JT-60. The objectives of this upgrade are to improve plasma performance with minimum modification and to obtain physical and engineering database for the design of...

JT-60 upgrade device for confinement and steady state studies

Emphases in JT-60 experiments are placed on (1) lower-hybrid (LH) current drive characteristics with a multi-junction type launcher, and (2) the confinement study with combination of neutral beam injection LH current drive and pellet injection The new multi-junction LH launcher provides a sharp N/sub /// spectrum with high directivity for N/sub ///=1-34 The current drive efficiency and the radial distribution of high energy electron production show clear correlation with injected N/sub ///: the current drive efficiency has the maximum at low N/sub ///( approximately 13) while flattening of plasma current is more effective in large N/sub /// A broad radial distribution of high energy electron current and approximately 30% reduction in sawtooth inversion radius were obtained by high N/sub /// ( approximately 25) LH injection To fully suppress the sawtooth activity, low N/sub /// ( approximately 13) injection was found to be more effective Improved energy confinement has been obtained with hydrogen pellet injection Energy confinement time was enhanced up to 40% relative to usual gas fuelled discharges The discharge has a strongly peaked electron density profile with ne(0)/(ne) approximately 5 and ne(0) approximately 20*1020 m-3 The improved discharges are characterized by a strongly peaked pressure profile within the q=1 magnetic surface, and degrades when a large sawtooth recovers or the pressure gradient may reach a critical value When large (3 mm, 4 mm) and fast (22 km/s) pellets were injected, 30% energy confinement improvement was obtained even during the NB heating of 14 MW Further investigations of IDC characteristics have been made The oxygen impurity lines from the main plasma and the main radiative loss drop first Then the plasma stored energy starts to rise The particle recycling is reduced around the main plasma, and is localized in the neighborhood of the X-point with a time lag of approximately 02 sec Eventually the discharge shows a significant remote radiative cooling power at the divertor region

I Aoki

Papers

Recent results in JT-60 experiments