Integrated discharge scenario IAEA-FEC 2014 St. Petersburg, Russia PPC/2-1 Integrated discharge scenario for high-temperature helical plasma on LHD K. Nagaoka National Institute for Fusion Science H. Takahashi1, S. Murakami3, H. Nakano1, Y. Takeiri1,2, H. Tsuchiya1, M. Osakabe1, K. Ida1,2, M. Yokoyama1,2, M. Yoshinuma1, S. Morita1,2, M. Goto1,2, T. Oishi1,2, N. Pablant4, K. Fujii5 K. Tanaka1, N. Tamura1, Y. Nakamura1,2, D. Xiaodi2, T. Ido1, A. Shimizu1, S. Kubo1, H. Igami1, R. Seki1, C. Suzuki1, Y. Suzuki1,2, K. Tsumori1,2, K. Ikeda1, M. Kisaki1, Y. Yoshimura1, T. Shimozuma1, T. Seki1, K. Saito1, H. Kasahara1, S. Kamio1, T. Mutoh1,2, O. Kaneko1,2, H. Yamada1,2, A. Komori1,2, and LHD Experiment Group Thank you chairman It is my great pressure to present our recent study on “Integrated discharge scenario for high-temperature helical plasma on LHD” In the PPC category 1National Institute for Fusion Science, Toki, 509-5292, Japan 2Graduate School for Advanced Studies (SOKENDAI), Toki 509-5292, Japan 3Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan 4Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA 5Department of Mechanical Engineering and Science, Kyoto University, Kyoto 615-8540, Japan

This is outline of my talk After brief introduction, I would like to talk about two topics. The first one is “Discharge Scenario of ion ITB” The wall conditioning effects on ion ITB plasma production are discussed The second one is “Integration of ion ITB and electron ITB” The transport characteristics and the importance of profile control are discussed. Finally, I will summarize my talk.

Helical plasmas have good performance in high density regime and an significant advantage in steady state operation Therefore, the development of discharge scenario of high temperature helical plasmas is a crucial issue, And the characteristics of high temperature plasma should be studied. In the previous study, high-Ti plasma and high-Te plasma have been investigated separately, Because ion and electron heat transports don’t couple with each other, which is a different property from tokamak ITB plasmas. Therefore, the integration of ion ITB and electron ITB should be demonstrated. Here, you can see the typical high-Ti plasma in left-hand side And high-Te plasma in right-hand side. The high-Ti plasma is obtained with ion ITB formation. This plasma is characterized by Improved heat transport …… Reduction of …….. Were experimentally identified. Extremely hollow impurity profile, so called Impurity hole, is formed in the core. The high electron temperature plasma, so called Core electron root confinement (CERC), was obtained with electron ITB formation. And characterized by 1) Improved heat transport with positive Er in the core region 2) Er shear created by a bifurcation of Er suppress the transport. 3) Impurity hole formation is also observed in the core region.

Recently, the temperature regime of LHD plasmas has been significantly extended. This graph shows the high temperature regime in Ti and Te space. In the first part of this presentation, I will discuss about the wall conditioning effect on the ion ITB formation And the extension of the ion temperature regime. In the second part, I will talk about the integration of ion ITB and electron ITB. The temperature regime with comparable Ti and Te has been extended significantly in these two years. And, Improved factor of global confinement with respect to normal confinement is 1.5 for ion ITB and 1.7 for ion and electron ITBs are achieved based on international stellarator scaling.

Let me move to the first topic Discharge Scenario of ion ITB

Here I will show you the typical ion ITB plasma in LHD. The left-hand side graph show the discharge sequence of Heating power, Density Temperature. The Carbon pellet was injected to the NBI heated plasma The central ion temperature increases in the density decay phase. The peaked ion temperature profile with steep gradient (ion ITB) formed, You can see it in the central graph. And, no ITB was observed in the electron temperature and density profiles. Carbon impurity was expelled from the core in the high-Ti phase. And hollow profile of impurity, so called Impurity hole, is formed in the core region.

Here, let me discuss about wall conditioning effects. The achievable ion temperature is found to depend on the H alpha intensity, This indicates that the low wall recycling condition is preferable to extend Ti regime. The repetitive helium main discharges with ICRF is performed to reduce wall recycling. This graph shows the pressure in the vacuum vessel. The hydrogen pressure (blue line) is reduced during the ICRF conditioning discharges. And the low recycling condition is realized after conditioning discharges.

Here we compare the density profile before and after wall conditioning, in order to capture the wall conditioning effects on the core plasma. The edge density reduced significantly and the profile of the ion heating by NBI becomes a bit peaked. The peaking factor of ion heating power, estimated by this ratio, increases from 0.48 to 0.55 after the conditioning discharges. The charge exchange loss of energetic ions is also a potential candidate of wall conditioning effects. The profile of neutral hydrogen was experimentally measured with a high-dynamic range of Balmer-alpha spectroscopy. The neutral density was reduced even in the core region after wall conditioning discharges. Using the experimentally measured neutral density, the charge exchange loss of energetic ions are evaluated and reduced from 14% to 7% in the core region. These effects increase the ion heating power in the core region.

Increase of ion heating power due to the wall conditioning is not so much, which is the order of 10%. However, the ion heat transport improves further (further reduction of ci). This graph shows the flux gradient relation of ion heat transport. The ion temperature gradient strongly depends on the heat flux, because the heat transport of ion ITB plasma is an improved branch. The small amount of increment of heating power enhances the temperature gradient, which means the ion heat transport improves further. Therefore, the wall conditioning becomes more effective to increase Ti0 due to the further improvement of ion heat transport. The right-hand side graph shows the achieved ion temperature as a function of the density-normalized ion heating power. Until 2010, Ti increases with the upgrade of perpendicular NBI power. After that, Ti increases with the wall conditioning effects, and The central Ti of 8.1keV was achieved with intense conditioning discharges.

Contents Introduction Discharge Scenario of ion ITB Ion ITB plasma in LHD Wall conditioning effects Integration of ion ITB and electron ITB Strongly focused ECH Transport characteristics Summary Let me move on to the second topics. “Integration of ion ITB and electron ITB.”

Recently, the ECH line with horizontal injection was upgraded. This horizontal ECH can finely control the power deposition profile, and applied to ion ITB plasma in order to increase of core Te. Then plasmas with both Te and Ti of around 6keV were successfully produced, when ECH was strongly focused at the plasma center. This graph shows the temperature profile, and the wider ion ITB and narrower electron ITB are formed. “Impurity hole” was also intensively formed as you can see in this graph.

The heat transport of ion ITB plasma (left-hand side) and integrated ITB plasma (right-hand side) have been analyzed with TASK3D code. The ion heat transport is improved in the ion ITB core region., while electron transport keeps normal confinement level. On the other hand, Both ion and electron heat transport are improved in the core region of integrated plasma. Therefore, ion ITB and electron ITB are formed simultaneously.

The radial electric field was investigated with heavy ion beam probe (HIBP). And, the positive radial electric field (Er) was observed in the core region of integrated ITB plasma, while the weak negative Er is observed in the ion ITB. The neoclassical ambipolar Er calculated by GSRAKE code is consistent with the experimental observation. Important is that the ion heat transport also improved with positive Er

The characteristics of heat transport were investigated, and here you can see the different behavior between ion and electron temperature gradients. The scale length of the Ti gradient is observed to decrease with the temperature ratio, Te/Ti. While the achievable scale length of electron temperature gradient is unchanged with temperature ratio.

These observations indicate; The temperature ratio of Te/Ti < 1 should be kept for integration of ion and electron ITBs. The combination of wider ion ITB and narrower electron ITB is a possible solution to keep Te/Ti < 1. Therefore we would like to emphasize that the temperature profile control is a key for integration of ion and electron ITBs.

Thank you for your attention.

Characteristics of heat transport -density dependence- central Te central Ti The density dependence of achievable temperature is different in low density regime. The ion temperature decreases with the decrease of density, while electron temperature increases in low density. The scale length of ion temperature gradient also decreases with the decrease of density, while the maximum of scale length of electron temperature gradient is unchanged with the density. Scale length of grad Ti Scale length of grad Te

Effect of density profile change 119084 119128 The helium wall conditioning decreases the hydrogen recycling and changes the change of density profile more peaked. The ion heating power was calculated by FIT-3D code and became peaked profile after helium conditioning discharges. The peaking factor of ion heating power (Pi(reff/a99<0.5)/Pi_total) increases from 0.48 to 0.55 after helium conditioning discharges.

Effect of charge exchange loss of energetic ions Energy spectra of NBI ions were calculated with / without charge exchange loss. The CX loss becomes significant in low energy regime, indicating reduction of ion heating power. The neutral density profile was evaluated from a high-dynamic range of Balmer-a spectroscopy measurement. It was identified that the reduction of ion heating power from 14% to 7% in total ion heating power due to the reduction of CX loss of energetic ions. K. Fujii, RSI 2014, EXC/P6-31

-Extension of temperature regime- Introduction -Extension of temperature regime- Ion ITB Wall conditioning Ion and el. ITBs Recently, the temperature regime of helical plasma has been significantly extended in LHD. The central ion temperature of 8.1keV has been achieved in low Zeff plasma with intense helium wall conditioning. Te ~ Ti regime has been also extended by integration of ion ITB and electron ITB. Improved factor of global confinement with respect to normal confinement is 1.5 for ion ITB and 1.7 for ion and electron ITBs are achieved based on ISS04 scaling. In this talk, discharge scenario of high Ti plasma is presented and effects of wall conditioning is discussed. Then the integration of discharge scenarios of ion and electron ITBs are presented . The characteristics of heat transport in high temperature regime are discussed.

Wall conditioning effects on the core plasma Profile change Charge exchange loss no CX loss nH=1x1013 m-3 K. Fujii, RSI 2014, EXC/P6-31 1x1014 1x1015 The edge density become lower. The ion heating power calculated by FIT-3D code became peaked profile after helium conditioning discharges. The peaking factor of ion heating power (Pi(reff/a99<0.5)/Pi_total) increases from 0.48 to 0.55 after helium conditioning discharges. The charge exchange loss of energetic ions was calculated and neutral density profile was measured with a high-dynamic range of Balmer-a spectroscopy. The loss of energetic ions reduced from 14% to 7% in ion heating power is identified in this comparison. Here we compare the density profile between before and after wall conditioning, in order to capture the wall conditioning effects on the core plasma. The edge density reduced significantly and the profile of ion heating power coming from NBI becomes peaked. The peaking factor of ion heating power defined by ion heating power deposited inside of half radius divided by total power increases from 0.48 to 0.55 after the conditioning discharges. The charge exchange loss of fast ions is a potential candidate of wall conditioning effects. The energy spectra of NBI ions are calculated with the charge exchange loss effect, which is shown in the central graph.