Scrape-off layer ion temperature measurements at the divertor target in MAST by retarding field energy analyser

Elmore, S.; Allan, S.; Kirk, A.; Thornton, A.; Harrison, James R.; Tamain, P.; Kočan, M.; Bradley, James W.; Team, Mast

doi:10.1016/j.jnucmat.2013.01.268

Cited by 14 publications

(15 citation statements)

References 14 publications

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“…Uncertainty in α LP , given in equation 2, arrises from uncertainty in T i /T e . Previous measurements on MAST have shown that 1 ≤ T i /T e ≤ 2.5 is reasonable for L-mode conditions [26][27][28] .This then gives α LP = 2.7 ± 0.1. Since α BP P cannot be measured directly in the system described in this paper the empirical value of α BP P = 0.6 ± 0.3 6,10,12,14 found across a number of experiments is adopted.…”

Section: Electron Temperature Measurementsmentioning

confidence: 79%

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Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe

Walkden

Adámek

Allan

et al. 2015

Review of Scientific Instruments

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The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature, and radial electric field on the Mega Amp Spherical Tokamak. The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential, the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile, an estimate of the radial electric field is extracted which is shown to be of the order ∼1 kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from E × B motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

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Section: Electron Temperature Measurementsmentioning

confidence: 79%

“…Figure 6 shows the profile of α LP obtained with this technique in each shot. In these calculations T i = 2T e was assumed, which is motivated by previous measurements on MAST [26][27][28] . Figure 6 shows that the secondary electron emission can have a major impact on the calculation of α LP .…”

Section: Electron Temperature Measurementsmentioning

confidence: 99%

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe

Walkden

Adámek

Allan

et al. 2015

Review of Scientific Instruments

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“…Previous measurements made using the SST have shown for an average type III ELM that T i decreases from an initial peak, soon after the ELM occurs, to the previously measured inter-ELM value [20] as a function of time [9]. The measurements here are in an equivalent DN discharge to that in [9] which produces type III ELMs.…”

Section: Sst Measurements Of Type III Elmsmentioning

confidence: 78%

“…Nevertheless, ion temperatures of up to 20 eV are routinely found during ELM events and some measurements of T i ~ 30 eV have also been seen. The highest measured inter-ELM T i in an equivalent plasma was T i = 8 ± 2 eV at ΔR LCFS tgt = 10 cm [20]. By comparing measurements with the IR diagnostic it has been found that it is most likely that a series of filaments have impacted on the target RFEA following an ELM event, any variations of T i during the I sat signal measured at the time of the ELM are likely to be related to filament structures passing the RFEA probe radially due to the timescale of the variations.…”

Section: Discussionmentioning

confidence: 93%

Scrape-off layer ion temperature measurements at the divertor target during type III and type I ELMs in MAST measured by RFEA

Elmore¹,

Allan²,

Fishpool³

et al. 2016

Plasma Phys. Control. Fusion

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In future nuclear fusion reactors high heat load events, such as edge-localised modes (ELMs), can potentially damage divertor materials and release impurities into the main plasma, limiting plasma performance. The most difficult to handle are type I ELMs since they carry the largest fraction of energy from the plasma and therefore deposit the largest heat flux at the target and on first wall materials. Knowing the temperature of the ions released from ELM events is important since it determines the potential sputtering they would cause from plasma facing materials. To make measurements of T i by retarding field energy analyser (RFEA) during type I ELMs a new operational technique has been used to allow faster measurements to be made; this is called the fast swept technique (FST). The FST method allows measurements to be made within the time of the ELM event which has previously not been possible with T i measurements. This new technique has been validated by comparing it with a slower average measurement previously used to make ion temperature measurements of ELMs. Presented here are the first T i measurements during Type I ELMs made at a tokamak divertor. Temperatures as high as 20 eV are measured more than 15 cm from the peak heat flux of an ELM, in a region where no inter-ELM current is measured by the RFEA; showing that ELM events cause hot ions to reach the divertor target far into the scrape off layer. Fast camera imaging has been used to investigate the type of ELM filaments that have been measured by the divertor RFEA. It is postulated that most of the ion temperatures measured in type I ELMs are from secondary ELM filaments which have not been previously identified in MAST plasmas.2

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“…It was assumed in previous studies of the SOL width derived from LP data that the ion (T i ) and electron temperatures (T e ) were equal, and this is then used to derive the heat flux to the divertor. Measurements of T i at the divertor have shown that this is not the case [21], especially in the case of low collisionality discharges where there is little coupling between the ions and electrons through collisions. The poor coupling at low density leads to ion temperatures higher than electron temperatures.…”

Section: Regression Of Sol Width To Input Parametersmentioning

confidence: 99%

Scaling of the scrape-off layer width during inter-ELM H modes on MAST as measured by infrared thermography

Thornton,

Kirk,

Team

2014

Preprint

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The power load to the divertor surfaces is a key concern for future devices such as ITER, due to the thermal limits on the material surface. One factor that characterises the heat flux to the divertor is the fall off length in the SOL, which recent empirical scalings have shown could be as small as 1 mm. These predictions are based on a multi-machine scaling of the heat flux width fitted using an expression for the divertor heat flux profile which includes a term for the exponential decay in the scrape off layer (SOL) and diffusion about the last closed flux surface (LCFS) in the private flux region. This expression has been used to fit a database of inter-ELM H mode profiles at the upper divertor and extract the fall off length, λ q , for a range of different plasma parameters in double null plasmas. The MAST data shows good agreement with the formula, with the fitted fall off length spanning a range of 5 to 11 mm in the data base generated. Regression of this data has shown that the fall off length has the strongest dependence on the plasma current (or equivalently, the poloidal magnetic field at the outboard midplane) to the power -0.71. The scaling with the smallest χ 2 error utilises the poloidal magnetic field at the outboard midplane (B pol,omp ) and the power crossing the scrape off layer in the relation λ q [mm] = 1.84(±0.48)B −0.68(±0.14) pol,omp P 0.18(±0.07) SOL with χ 2 = 3.46 and R 2 = 0.56 as a goodness of fit. The equivalent scaling with plasma current is λ q [mm] = 4.57(±0.54)I −0.64(±0.15) p P 0.22(±0.08) SOL with χ 2 = 3.84 and R 2 = 0.55. The moderate goodness of fit suggests that additional plasma parameters are required to accurately reproduce the observed variation in λ q .

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Scrape-off layer ion temperature measurements at the divertor target in MAST by retarding field energy analyser

Cited by 14 publications

References 14 publications

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe

Scrape-off layer ion temperature measurements at the divertor target during type III and type I ELMs in MAST measured by RFEA

Scaling of the scrape-off layer width during inter-ELM H modes on MAST as measured by infrared thermography

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