KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Density Regime of Complete Detachment and Operational Density Limit in LHD J. Miyazawa1), R. Sakamoto1), S. Masuzaki1), B.J. Peterson1), N. Tamura1), M. Goto1), M. Shoji1), M. Kobayashi1), H. Arimoto2), K. Kondo2), S. Murakami3), H. Funaba1), I. Yamada1), K. Narihara1), S. Sakakibara1), K. Tanaka1), M. Osakabe1), S. Morita1), H. Yamada1), N. Ohyabu1), A. Komori1), O. Motojima1), and the LHD Experimental Group 1) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan 2) Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japan 3) Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan Today, I will talk about “Density Regime of Complete Detachment and Operational Density Limit in LHD”. 1/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Introduction High-density operation in fusion reactor Future fusion rector will operate in a density range of order 1020 m-3. Higher density is more favorable, since the fusion reaction rate increases with density squared. Reduction of divertor heat load by detachment is expected at high-density. High-density experiments in existing devices High-density plasmas of order 1020 m-3 have been studied in medium devices. Alcator C-Mod tokamak (C-Mod): R = 0.68 m, a = 0.22 m, B  8 T. Frascati Tokamak Upgrade (FTU): R = 0.935 m, a = 0.31 m, B  8 T. Wendelstein 7-AS stellarator (W7-AS): R = 2 m, a  0.16 m, B  2.5 T. Ex) LHD: R = 3.6 m, a = 0.64 m, B  2.75 T (inward-shifted configuration). Power density in LHD (0.5 MW/m3), is much smaller than in W7-AS ( 4 MW/m3) where volume-averaged density of 4  1020 m-3 was attained with detachment. Future fusion reactor will be operated in a density range of 10 to 20th per cubic meter. Higher density is more favorable, since the fusion reaction rate increases with density squared. Furthermore, reduction of divertor heat load by detachment is expected at high-density. High-density plasmas of order 10 to 20th per cubic meter have been studied mainly in small devices, such as C-Mod, FTU, and W7-AS. Compared with these small devices, the heating power density in LHD is much smaller. 2/14

Density limit prediction KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Density limit prediction Density limit of net current free helical plasmas Sudo density limit scaling (derived from H-E, H-DR, W7A, and L2): ncSudo = 2.5 (Ptot B / (a2 R) )0.5 (units: 1019 m-3, MW, T, and m). e.g. Greenwald Limit : ncGW (1020 m-3) = Ip/(a2) = (5B)/(qaR), … Since the qa scarcely changes in net-current-free plasmas, ncGW is roughly a constant at a given set of B and R (ncGW ~ 1.8  1020 m3, for B = 2.71 T, R = 3.65 m, and qa ~ 0.7). It has been considered that the power dependence in the Sudo scaling is resulted from the power balance between the heating power and the radiation loss that is proportional to ne2, however, - Radiative collapse is often triggered at a small radiation loss fraction of 30 %. At complete detachment, the radiation loss fraction ranges from 30 – 100 % without radiative collapse. Strongly peaked density profile is not within the scope of the Sudo scaling. So-called Sudo scaling has been often used to discuss the density limit in helical plasmas. This scaling is based on the power balance between the heating power and the radiation loss that is proportional to density squared. However, radiative collapse is often triggered at a small radiation fraction of 30 %. At complete detachment, on the other hand, the radiation loss fraction ranges from 30 to 100 % without radiative collapse. These suggest that such a simple power balance is not sufficient to describe the density limit physics. Furthermore, it should be noted that strongly peaked density profile generated by pellet injection is not in the scope of the Sudo scaling. 3/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Detachment in LHD Radiative Collapse 100eV < 0.8 Complete Detachment Plasma column shrinks and Wpdia decreases. Isat decreases at all the measured divertor tiles. 100eV < 1 Marfe Toroidally axisymmetric radiation belt. Sustainable in W7-AS. Serpens Mode Sustainable complete detachment. A helical radiation belt is formed inside of the LCFS: serpent The serpent rotates in the EB direction. 100eV ~ 0.9 Transient Partial Detachment Localized in the gas puff port. Without high recycling. Wpdia slightly decreases. Hot plasma boundary: 100eV Radial position where Te = 100±50 eV. Line radiations from right impurities increase at Te< 100 eV. Density 100eV > 1 4/14

Complete detachment in gas-fueled plasmas KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Complete detachment in gas-fueled plasmas (Transient partial detachment) Isat decreases only in the gas puff port. (Complete detachment) The hot plasma boundary shrinks below the LCFS (r100eV < 1) and Isat decreases at all the measured divertor tiles. The density ramp up rate increases even though the gas puff rate is unchanged.  Fueling efficiency is improved. (Serpens mode) r100eV is sustained at ~0.9 The serpent appears. In the case of gas-fueled plasmas, detachment takes place at high-density. In this discharge, density is increased by strong gas puffing. As the density increases, the plasma column shrinks and the hot plasma boundary decreases. Here, we define the hot plasma boundary by the normalized minor radius where the electron temperature is 100 electron volt and we call this rho- hundred-eV. Complete detachment takes place when the electron temperature at the last closed flux surface decreases to about 100 eV, or in other words, rho-hundred- eV decreases to 1. Then, the ion saturation current decreases at all the measured divertor tiles. The density ramp up rate increases even though the gas puff rate is unchanged. This means that the fueling efficiency is improved at complete detachment. Transition to self-sustained detachment, named the Serpens mode, takes place when rho-hundred-eV decreases to about 0.9. Rotating radiation belt appears in the Serpens mode phase and we call this a serpent. Serpent Marfe Hydrogen volume recombination Observed Radial position On/Inside LCFS Shape Helical Axisymmetric Rotation E  B Toroidal (W7-AS) LCFS Serpent 5/14

Density regime of complete detachment KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Density regime of complete detachment Pellet-fueled: Attach Detach Collapse regime Complete detachment regime Gas-fueled: Threshold for complete detachment Threshold for the Serpens mode During the Serpens mode Attachment regime This figure summarizes operational density regimes in LHD. Volume-averaged density is plotted against the total heating power. If we increase the density at a fixed heating power, complete detachment occurs when the density reaches these blue symbols. The Serpens mode appears when the density reaches these red open symbols. During the Serpens mode, the volume-averaged density increases further since the density profile changes from hollow to flat. The upper bound of the Serpens plasmas reaches 2.2 times as high as the Sudo scaling. In gas-fueled plasmas, complete detachment takes place in the highlighted density region. Crosses are the pellet-fueled data, where white and black denote attached and detached data, respectively. In pellet-fueled plasmas, the maximum density reaches 3.5 times as high as the Sudo scaling. From this figure, it seems like that there are a large difference between gas-fueled and pellet-fueled plasmas. Density regime of complete detachment is surrounded by the threshold density for complete detachment ( ) and the Serpens mode data ( ). Radiative collapse takes place above the complete detachment regime. High-densities in the collapse regime are achieved by applying pellet injection. 6/14

Maximum density in pellet-fueled plasmas KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Maximum density in pellet-fueled plasmas <ne> reaches 3  1020 m-3, in spite of small absorbed power density in LHD. The record ne0 in helical plasmas of 5  1020 m-3 has been achieved in LHD. A superdense-core (SDC) is formed inside of the internal diffusion barrier (IDB) and the central plasma pressure reaches 1 atm.  EX/8-1 N. Ohyabu (on Friday) These have been achieved in pellet-fueled plasmas with strongly peaked density profiles. The highest central density in helical plasmas of 5 times 10 to 20th per cubic meter has been also achieved in LHD. This was obtained at an outward-shifted configuration. After the maximum central density is reached, the density decreases and the temperature increases, as is known as reheat. What is interesting in this discharge is that the density decay rate is faster in the edge region than in the core region and as a result, superdense core, SDC is formed. The central pressure reaches one atmosphere during the SDC phase. This is the world record in helical plasmas, so far. 7/14

Edge densities are similar! KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Edge densities are similar! (Attached data) Even in the pellet-fueled plasma with a strongly peaked density profile, ne100eV is similar to that of the gas-fueled plasma at the threshold for complete detachment. (Detached data) ne100eV stays unchanged at various core density.  Local densities, ne100eV, at r100eV, are similar for each of attached and detached datasets. Here, shown are various radial profiles of electron temperature and electron density at similar heating power. Strongly peaked density profile is obtained in pellet-fueled case as shown by white crosses. Even in this case, the edge density defined by the electron density at rho- hundred-eV is similar to that of gas-fueled plasmas at the threshold for complete detachment, which is shown by black squares. Colored circles are detached data with various density profile of hollow, flat and peaked. Also in these detached cases, the edge density stays unchanged at various core density. We can say that local densities of ne-hundred-eV are similar for each of attached and detached datasets. 8/14

<ne> linearly increases with the peaking factor KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) <ne> linearly increases with the peaking factor (Attached data) In both of gas-fueled and pellet-fueled plasmas, ne100eV are well approximated by 0.8 ncSudo. Large <ne> in pellet-fueled data is due to the strongly peaked density profile. Attached data (100eV ~ 1): Gas-fueled Pellet-fueled 9/14

Critical edge density increase with P 0.5 KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Critical edge density increase with P 0.5 Collapse threshold Pellet-fueled: Attach Detach Gas-fueled: Threshold for complete detachment Threshold for the Serpens mode During the Serpens mode Detachment threshold Critical edge densities for complete detachment and radiative collapse increase with the square root of heating power. This is also expressed in the Sudo scaling: ncSudo = 2.5 (Ptot B / (a2 R) )0.5. 10/14

Parameter dependence of the edge temperature KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Parameter dependence of the edge temperature Attachment regime Critical edge temperature Complete detachment regime Te at the LCFS is well fitted by (Ptot0.5/ne)2/3, as long as Te > 100 eV. The critical LCFS density that results in the critical LCFS temperature of 100 eV increases with Ptot0.5. 11/14

Evolution of the edge density KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Evolution of the edge density Edge density at a fixed , ne(), increases as the hot plasma column shrinks and 100eV decreases, as long as  < 100eV. Outside 100eV ( > 100eV), ne() decreases with 100eV. ne100eV is a good representative of the maximum of ne() at each . 100eV is the radial position inside which one can increase the density by fueling. 12/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Maximum edge density ne100eV approximates the maximum local density and increases with Ptot0.5 in the edge region. A plot of ne100eV / Ptot0.5 versus 100eV corresponds to the radial profile of maximum density in the edge region. ne100eV / Ptot0.5 in attached plasmas reach the maximum (~ 0.8 ncSudo) at 100eV ~ 1. ne100eV / Ptot0.5 increases as 100eV decreases and saturates to ~ 1.5 ncSudo. 13/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Summary The highest central density in helical plasmas of 5  1020 m-3 has been achieved in LHD. In pellet-fueled plasmas with strongly peaked density profile. The volume-averaged density reaches 3  1020 m-3, in spite of small heating power density of < 0.5 MW/m3 and the magnetic field of < 3 T. Even in these high-density pellet-fueled plasmas, edge densities are similar to those in gas-fueled plasmas with flat or hollow density profiles. Complete detachment takes place when the edge temperature at LCFS decreases to a critical value of ~100eV (100eV = 1). In the edge region, the electron temperature is a function of the square root of heating power divided by the electron density. The critical LCFS density for complete detachment is ~ 0.8 ncSudo. High edge density of ~ 1.5 ncSudo is sustainable in the Serpens mode plasmas, where the volume-averaged density reaches ~ 2.2 ncSudo . 14/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) The End 15/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Radiation loss At the Serpens mode, Prad and the impurity irradiation such as CIII increase. However, these do not necessarily trigger the transition to the Serpens mode, as seen in the unstable detachment discharge (blue lines in the right figure). i.e. the unstable detachment discharge does not enter the Serpens mode even though Prad and the CIII intensity exceed the values in the Serpens mode discharge (shown by red lines). In the unstable detachment discharge, the electron density is lower than the Serpens mode discharge. Electron density is more important than the total radiation loss. 16/14

KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Neutral Pressure The neutral pressure, p0, increases with the edge density in attached plasmas. At complete detachment, p0 decreases even though gas puffing is continued and the edge density increases. In the Serpens mode after gas puff turned off, p0 decreases to ~1/3 of that during gas puffing. Under a low recycling condition, p0 decreases further and reattachment takes place. Fueling and recycling control is a key to achieve the Serpens mode. 17/14

Maximum <ne> in LHD KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Maximum <ne> in LHD The volume averaged electron density (<ne>) exceeds 3  1020 m-3, in spite of small absorbed power density in LHD (< 0.5 MW/m3) compared with W7-AS ( 4 MW/m3) where <ne> = 4  1020 m-3 was attained with detachment. At the inward shifted configuration (R = 3.65 m). Attached plasma. Hollow temperature profile (transient). First of all, let me show you the maximum density achieved so far in LHD. In spite of small power density in LHD, of less than 0.5 MW per cubic meter, which is much less than that in W7-AS, the volume-averaged density of 3 times 10 to 20th per cubic meter has been achieved. This was obtained in pellet-fueled plasma at an inward-shifted configuration. 18/14

Complete detachment and the Serpens mode KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Complete detachment and the Serpens mode LCFS Serpent At complete detachment, the hot plasma boundary shrinks inside the LCFS. After the transition to the Serpens mode, complete detachment is sustained with a rotating helical radiation belt, named the serpent. 19/14

Hydrogen recombination KTM & CWGM 2006 Kyoto University, Kyoto (19 - 22 Sep., 2006) 21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006) Hydrogen recombination During the Serpens mode, the ratio of Hg / Ha increases to 3 – 5 times of that in the attached phase.  Similar ratio is observed in the detached divertor region and the Marfe radiation belt in W7-AS. The Hg signal is fluctuating as the Ha signal. Each of the peaks in Ha and Hg fluctuations appears as the serpent passes by the measurements. Hydrogen volume recombination in the serpent is suggested. In this respect, the serpent in LHD and the Marfe in W7-AS resemble each other. 20/14