Abstract. A set of Ohmic density ramp experiments addressing the role of parallel connection length in modifying Scrape Off Layer (SOL) properties has been performed on the TCV tokamak. The parallel connection length has been modified by varying the poloidal flux expansion f x . It will be shown that this modification does not influence neither the detachment density threshold, nor the development of a flat Scrape Off Layer (SOL) density profile which instead depends strongly on the increase of the core line average density. The modification of the SOL upstream profile, with the appearance of what is generally called a density shoulder, has been related to the properties of filamentary blobs. Blob size increases with density, without any dependence pn the parallel connection length both in the near and far SOL. The increase of the density decay length, corresponding to a profile flattening, has been related to the variation of the divertor normalized collisionality Λ div [1,2], showing that in TCV the increase of Λ div is not sufficient to guarantee the SOL upstream profile flattening.

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Electron energy distribution functions (EEDF) in argon RF plasmas have been measured by using an improved driven probe. The presence of RF potential fluctuations between probe and plasma distorts a Langmuir probe characteristic. Therefore it introduces large errors into plasma parameters. The improved method, which removes the effect of the RF fluctuation on probe characteristics, consists of superimposing the fluctuation of the space potential measured by an emissive probe to the dc voltage applied to the probe. It offered an efficient improvement of probe characteristics. This technique was applied to the measurements of EEDFs in argon plasmas at 13.56MHz. The measured EEDFs in the RF plasmas were non-Maxwellian. It was found that the high energy electrons present in the vicinity of the boundary between the powered electrode sheath and plasma. The electrons will play an important role to maintain the RF discharges. ƒL •[ ƒ• •[ ƒh:•Ã "d ƒv ƒ• •[ ƒu,ƒG ƒ~ ƒb ƒV ƒu ƒv •[ ƒu,RFƒO ƒ• •[ •ú "d,‹ó ŠÔ "d ˆÊ,"d Žq ƒG ƒl ƒ‹ ƒ ŠÖ •" 1. ‚Ü ‚¦ ‚ª ‚« RFƒO ƒ••[ •ú "d ƒv ƒ‰ ƒY ƒ} ‚Í ƒv ƒ‰ ƒY ƒ} ƒv ƒ•ƒZ ƒV ƒ" ƒO ‚É •L ‚-p ‚¢ ‚ç‚ê ‚Ä ‚¢ ‚é ‚ª,‚» ‚Ì ƒf ƒ| ƒW ƒVƒ ‡ ƒ"‚ ‚é ‚¢ ‚̓G ƒbƒ`ƒbƒ`ƒ" ƒO ‚É•d-v ‚ÈŠˆ•‚ÈŠˆ•« Ží,ƒC ƒI ƒ"‚È ‚Ç‚Ì• ¶ •¬ ‚Í,'á ƒK ƒX‰· "x ‚Ì ‚½ ‚ß ƒv ƒ‰ ƒY ƒ} ' † ‚Ì"d Žq ‚Ì ƒG ƒl ƒ‹ ƒM •[ ‚É‹-‚-ˆË ' ¶ ‚· ‚é•B •] ‚Á ‚Ä,RFƒv ƒ‰ƒY ƒ} ‚Ì "d Žq ƒG ƒl ƒ‹ ƒM •[ •ª •z ŠÖ •" ‚ð'ª 'è ‚· ‚é ‚± ‚AE‚Í,ƒv ƒ‰ ƒY ƒ} ƒv ƒ• ƒZ ƒV ƒ"ƒO‚Ì •Å "K ‰» ‚Ì‚½ ‚ß ‚É•d-v ‚Å ‚ ‚è,RF•ú "d ‹@ •\ ‚̉ð-¾ ‚Ì ‚½ ‚ß ‚É ‚à'å •Ï ‹»-¡ •[ ‚¢ ‚± ‚AE‚Å ‚ ‚é•B ƒv ƒ• •[ ƒu-@ ‚ÍLangmuir(1)‚É ‚ae ‚Á ‚Ä ŠJ"-‚³‚ê OEà ‚-‚© ‚ç"d Žq ‰· "x,"d Žq-§ "x,"d Žq ƒG ƒl ƒ‹ ƒM•[ •ª •z ‚¨‚ae‚¨‚ae ‚Ñ‹ó ŠÔ(ƒv ƒ‰ ƒY ƒ})"d ˆÊ ‚È ‚Ç‚Ì,ƒv ƒ‰ ƒY ƒ}ƒp ƒ‰ ƒ• •[ ƒ^‚Ì 'ª 'è ‚É-p ‚¢ ‚ç‚ê ‚Ä ‚¢ ‚é•B ƒv ƒ••[ ƒu ‚AEƒv ƒ‰ ƒYƒ} ŠÔ ‚ÉRF"d ˆ³ •u •Ï "® ‚ª ' ¶ •Ý ‚· ‚é ‚AE,ƒv ƒ••[ ƒu ƒV•[ ƒX‚Ì "ñ •ü OE`"Á •« ‚Ì ‚½ ‚ß,Žž ŠÔ •½‹Ï ‚Ì ƒv ƒ••[ ƒu"d-¬-"d ˆ³ "Á •« ‚Í‚Ð ‚¸‚Ý‚¸‚Ý,‚» ‚Ì "Á •« ‚© ‚ç‹• ‚ß ‚ç‚ê ‚½ ƒv ƒ‰ ƒY ƒ}ƒp ƒ‰ ƒ••[ ƒ^ ‚É ‚Í'å ‚«‚È OEë •· ‚ð• ¶ ‚¸‚邸‚é•B •Ï "® ‚· ‚éƒv ƒ‰ ƒYƒ} ‚Ì Žž ŠÔ•½ ‹Ï ƒv ƒ• •[ ƒu"Á •« ‚Ö ‚̉e ‹¿ ‚Í-• ˜_ "I ‚ ‚é ‚¢ ‚ÍŽÀ OE± "I ‚ÉOE¤ ‹ † ‚³‚ê,-{ Ž¿ "I ‚É "¯-l ‚È OE‹ ‰Ê ‚ª "¾ ‚ç ‚ê ‚Ä ‚¢ ‚é(2)•`(4)•BBraithwaite ‚ç(5)‚Í,ƒv ƒ••[ ƒu ‚ÉRF"d OE¹ "d ˆ³ ‚ðˆó‰Á‚ðˆó‰Á ‚· ‚é ‚± ‚AE ‚É ‚ae ‚è,ƒvPˆê ƒu ‚AEƒv ƒ‰ƒY ƒ} ŠÔ ‚Ì"d ˆÊ •Ï "® ‚ð"\ "® "I ‚É•oe ‹Ž ‚· ‚é‹ì "® ƒv ƒ• •[ƒu(driven probe)-@ ‚ð ŠJ"-‚µ‚½•B ‚µ ‚© ‚µ‚È ‚ª ‚ç,ƒv ƒ‰ƒY ƒ} ‚Ì‹ó ŠÔ "d ˆÊ ‚Ì •Ï "® "gOE`gOE`‚ÌOEð-¬ •¬

Advanced Langmuir probe techniques for evaluating the plasma potential and electron-energy distribution function (EEDF) in magnetized plasma are reviewed. It is shown that when the magnetic field applied is very weak and the electrons reach the probe without collisions in the probe sheath the second-derivative Druyvesteyn formula can be used for EEDF evaluation. At low values of the magnetic field, an extended second-derivative Druyvesteyn formula yields reliable results, while at higher values of the magnetic field, the first-derivative probe technique is applicable for precise evaluation of the plasma potential and the EEDF. There is an interval of intermediate values of the magnetic field when both techniques-the extended second-derivative and the first-derivative one-can be used. Experimental results from probe measurements in different ranges of magnetic field are reviewed and discussed: low-pressure argon gas discharges in the presence of a magnetic field in the range from 0.01 to 0.08 T, probe measurements in circular hydrogen plasmas for high-temperature fusion (magnetic fields from 0.45 T to 1.3 T) in small ISTTOK and CASTOR tokamaks, D-shape COMPASS tokamak plasmas, as well as in the TJ-II stellarator. In the vicinity of the last closed flux surface (LCFS) in tokamaks and in the TJ-II stellarator, the EEDF obtained is found to be bi-Maxwellian, while close to the tokamak chamber wall it is Maxwellian. The mechanism of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. Comparison of the results from probe measurements with those obtained from calculations using the ASTRA and EIRENE codes shows that the main reason for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms.

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.