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-¬ •¬

The electron energy distribution function (EEDF) at different radial positions is derived from Langmuir probe measurements in the CASTOR tokamak edge plasma using the first derivative method. It is shown that the EEDFs are not Maxwellian but can be approximated as bi-Maxwellians with one dominant, low temperature electron population and one minority composed of hotter electrons. In the limiter shadow the measured EEDFs are Maxwellian. The values of the plasma potential and electron densities at different radial positions are also evaluated. The results presented in this paper demonstrate that the first derivative method allows one to acquire additional plasma parameters using the electron part of the current-voltage characteristics in strongly magnetized tokamak edge plasmas.

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.