Boivin, R.L.

Koltonyuk, M.

Mayo, R.M.

Munson, C.P.

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PFC/JA-96-14Time-of-Flight Neutral ParticleAnalyzer for Alcator C-ModR.L. Boivin, M. Koltonyuk, C.P. Munson', R.M. Mayo2May 1996'Los Alamos National Laboratory, Los Alamos, NM 87545.2North Carolina State University, Raleigh, NC 27695-7909.Submitted to Review of Scientific Instruments.This work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.A neutral particle analyzer, based on the Time-Of-Flight (TOF) tech-nique is scheduled to be installed on the Alcator C-Mod tokamak. Theinstrument was originally designed and used in the ZT-40 experiment, andlater, briefly used on the CTX experiment at Los Alamos National Labora-tory. The design consists of a chopper wheel mounted on a turbomolecularpump, a -2m long flight tube, a CuBe secondary electron emitting surfaceand an electron multiplier, which together are used as the neutral particledetector. Among the changes introduced in the system from the original de-sign, of particular interest is the location of the instrument. The instrumentis mounted behind a magnetic-type neutral particle analyzer and shares thesame line of sight. Both of them can be scanned, poloidally down to the X-point and tangentially to R/R0 =0.7. This will enable us to compare neutralflux through 2 different techniques with an overlap in energies from -0.5 - 4keV, with the instrument capable of detecting neutral particles with energiesas low as -20 eV. This should aid in characterizing neutral and ion behaviorat the edge of the tokamak plasma, especially near the X-point. We will alsodescribe changes made to the controls for improved versatility and ease ofoperation.21I. IntroductionAlcator C-Mod is a compact, diverted tokamak, with high magnetic field,high density and high power density (ohmic and ICRF). One of the key as-pects of our program resides in the study of divertor dynamics, including thestudy of the edge near or at the separatrix. It also has a natural applicabilityto the study of H modes, which are essentially an edge phenomenon. Themeasurement of escaping neutral population and its energy dependence isparticularly important and can lead to a better understanding of the dynam-ics of neutrals and ions near the edge. However, the energy range of interestfor edge populations (-250 eV and below) cannot be seen by a standard neu-tral particle analyzer based on a stripping cell technique. At those energies,the stripping efficiency of deuterium or hydrogen on helium is too low toreceive appreciable signal for good statistics. Consequently, we are revertingto the well-known technique of time-of-flight spectroscopy1 - to extend ourneutral particle spectroscopy to lower energies.The time-of-flight .(TOF) technique consists in measuring the particlesenergy distribution based on their drift time along a known distance. A fastrotating chopper (with an open duty factor of less than 1%) allows neutralparticles to pass only during a very short time period. The particles arethen allowed to drift along a flight tube and are detected at the other end,usually by an electron multiplier, micro-channel plate or similar instrument.For neutral particles of low energy, the detection efficiency can be quite lowand it is usually necessary to go through an intermediary such as formingsecondary electrons from a Cu-Be surface. It is also important to restrict the2neutral particles to only one species as this technique can not discriminatebetween different mass particles such as hydrogen and deuterium.The instrument described here has been developed and used on ZT-40M5and CTX,6 both located at LANL. However, some key features have beenmodified for its installation on Alcator C-Mod and will be described below.II. Diagnostic DescriptionA. VacuumThis neutral particle analyzer consists mainly of 3 components, a particlechopper, a flight tube and the detector assembly. The chopper is a flat tita-nium alloy (Ti-6Al-4V) disk, 26.67 cm in diameter and 0.0254 cm thick. Thewheel is mounted on a Leybold NT-150 turbomolecular pump, which nom-inally rotates at approximately 360 rev/sec. The pump controller has beenslightly modified to allow for control of the pump speed. The bearings of thepump were also retrofitted with ceramic components, eliminating the needfor oil changing, reducing stress on the internal components and allowing thepump to run in any orientation. In principle, the instrument can be mountedat any angle (from horizontal to vertical) but care must be taken that thewheel itself is appropriately supported. Since the maximum inclination ofthe instrument will be less than 13* from a vertical position (the chopper be-ing nominally vertical), we do not believe that any additional support wouldbe needed in our application. The chopper wheel consists of 16 radial slits(2 cm x 0.0254 cm), located 0.85 cm away from the outer edge. The wheel3is placed behind a single stationary slit (2 cm x 0.0254 cm), which acts asa collimator and a vacuum break between the instrument and, in this case,the other diagnostic (or the tokamak vacuum vessel, in normal applications).With the stationary and chopper slits of same sizes, the temporal acceptanceis mainly triangular (with very small corrections due to the circular natureof the chopper) with a half opening time of ~ 0.9 psec. Opposite to theneutrals path, 180' away from the stationary slit, a HeNe laser beam is pro-jected through the chopper slits onto a photodiode (fiber optic receiver HPmodel HFBR-2402), allowing monitoring of the chopper speed. Originally,this was intended to be used also as the time fiducial, but because of smalljitter and precision in alignment, the use of the UV pulse from the plasmathrough the slits was found to be more reliable (see Section C).Once the neutral particles passed through the stationary slit and thechopper, they drift down the flight tube (- 2 m long), which is 3.5 cm indiameter (inner). Background pressures are kept low ( Z 10 -6 torr) withthe use of a turbomolecular pump (Balzers 510), and with the chopper pumpitself, which keeps most of its pumping efficiency, even with the presence ofthe chopping wheel. The low pressure is needed for good neutral transmissionand for arc prevention in the detector. The pressure is monitored near thedetector with a Granville Philipps nude gauge, interlocked with the HighVoltage circuitry to protect the detector against arcing.Since neutrals with energies as low as 20 eV are difficult to detect, weuse a Cu-Be plate (3.81 cm diameter) to generate secondary electrons whichare then accelerated towards a 20 stage electron multiplier (Johnston Labs4MM-1). The copper plate is tilted approximately 100 from the flight tubeaxis (see Fig. 1), and is mounted directly above the electron multiplier whoseaxis is perpendicular to the flight tube.B. Mechanical supportOne of the key features of this diagnostic will be its mounting in "paral-lel" with a magnetic-type (EIIB) neutral particle analyzer (see Fig. 2). Theexisting high-energy neutral particle analyzer7 is mounted on a cradle whichallows the line of sight to be scanned between shots poloidally from themidplane down to near the X-point, and toroidally from perpendicular to atangential view of R/R,=0.7. This analyzer has a built-in port in the backwhich views through the beamline and the stripping cell, which is sometimesused for measuring unstripped particles. In our case we are planning to at-tach the time-of-flight on that open port. In principle, we should be ableto measure the neutral particle spectrum, from 20 eV, to up to 300 keV, onone line of sight. Since their energy range overlaps between 0.5 to 4 keV, itshould also be possible to cross-calibrate the analyzers, obtaining a betterhandle of the escaping neutral particle flux and power. However, it may benecessary to decrease the stripping cell pressure in order to limit attenuationof the low energy particle. Since the pressure is remotely controlled, it wouldbe a relatively easy operation to switch from one instrument to the other.5C. ElectronicsThe TOF detector is nominally configured for current mode, where theanode is at ground and the first stage typically biased between -1.7 kV and -2.5 kV, although designed to go as "high" as -3.5 kV. A 5 kV negative powersupply (Bertan model 502C rated at 2 mA) is used for both Cu-Be plateand multiplier. The multiplier signal is processed through a pre-amplifier(fast current to voltage converter at 6 MHz) and directed to a fast CAMACtransient digitizer (TRANSIAC 2008 with 256 kB or LeCroy 8818 with 2 MBof memory). Since the detector assembly has a direct line of sight with theplasma, care must be taken that the electron multiplier does not saturatedue to the brief pulse of UV light during slit alignment. Since the multiplierhas a relatively long saturation recovery time (~ 100 psec), the Cu-Be biasis reversed from the normal negative -300V (with respect to the multiplierfirst stage) to positive 100V during the expected UV pulse. The switching isdone, using high speed, high voltage transistor (model STI-70) with a timeconstant of ~ 0.2 psec. Despite care taken to gate the detector and avoidsaturation on the UV pulse, some UV light signal is still evident and can beutilized for several purposes. Firstly, it serves as the time fiducial (t = 0)for the particle drift times, and provides the triggering for the high-voltagecircuitry based on the rising edge of the pulse. Finally, it can be used (atlow gain) to verify the acceptance function of the slit assembly by looking atthe time dependence of the UV signal.Since we are mostly interested in the lower part of the energy spectrum(~ 250 eV or less), it may be necessary to increase the gain (i.e. HV) on6the multiplier. However, the multiplier may now saturate on the neutralsignal as well (see Fig. 3) originating from the "bunched" high energy end ofthe spectrum. Consequently, we modified the HV circuitry to allow for anexternal output determining the delay in the HV switching from the nominalt=2-3 psec, to as high as t=100 psec. Naturally, the maximum time allowedshould still be lower than the maximum particle drift period, which is around175 Psec, as determined by the chopper wheel rotation speed and the numberof slits. The lower limit in time delays also defines the maximum detectableenergy, which is approximately 4 keV for deuterium in the nominal setting.III. Expected signal levelsThe calculations leading to the signal levels for the analyzer involve manyparameters, of which some are not very well known, such as the neutral den-sity profile inside the plasma. Coupled to uncertainties in ion density andtemperature profiles, the uncertainties in escaping neutral flux, both in mag-nitude and in the spectral shapes are rather high, potentially up to a feworders of magnitudes. However, we dispose of a few measurements, one withthe TOF instrument on CTX, and with the high energy analyzer on AlcatorC-Mod. If necessary, we can increase the signal levels by increasing the slitdimensions, reducing the chopper speed, plasma-chopper distance, but usu-ally at the expense of resolution, mainly in particle energy.8 However, themost powerful knob resides in the electron multiplier gain, adjusted by theHV bias, which has a range from unity up to 107. For example, with a biasof ~ -2kV, the multiplier has a nominal gain of -10'. Shown in Fig. 3 is a7simulated signal as seen by the electron multiplier for the following plasmaconditions: edge neutral density no,=1x10 6 m-3 , central electron densityneo=1x102 0 m-3, and central ion temperature Tio= 2 keV. Included in thecalculations are the collection fraction of secondary electrons from the CuBeplate (~ 0.5-0.8), the secondary electron emission coefficient 3 q(E) (taken tobe -9.7 x 10- (E(eV) - 14.6) ) and G, the gain of the electron multiplier setnominally at 10'. Since the majority of Alcator C-Mod plasmas have a hy-drogen content of less than 1%, no significant "contamination" is expected inthe deuterium spectrum. Care must be taken that the multiplier current doesnot significantly exceed 1-2 pA. Consequently, the simulations indicate thatreasonable signal levels are expected, although large uncertainties still existin the neutral density profile and that adequate alignment can be performed.IV. AcknowledgmentsWe would like to acknowledge the help of H. Alvestad, K. Schoenberg,both at LANL, C. Fiore, W. Burke, W. Parkin at MIT, and the support ofC. W. Barnes and G. Wurden (LANL). Supported by US DOE Contract No.DE-AC02-78ET51013 and No. W-7405-ENG-36.8References'D. E. Voss, Low Energy Neutral Atom Emission and Plasma Recyclingin the PLT Tokamak, PhD thesis, Princeton University, Princeton, NJ,1980.2D. E. Voss and S. A. Cohen, Rev. Sci. Instrum., 53, 1696 (1982).3H. Verbeek, J. Phys. E, 19, 964 (1986).4Y. Miura and F. Okano, Rev. Sci. Instrum., 61, 3581 (1990).-G. A. Wurden et al., Ion Heating Studies in the ZT-40M Reversed FieldPinch, in Controlled Fusion and Plasma Physics, (Proc. of the 15 th Eu-ropean Conf., Dubrovnik,1988), Vol 12B, pages 533-536, Geneva, 1988,European Physical Society.6R. M. Mayo et al., Nucl. Fusion, 31, 2087 (1991).'C. Kurz and C. L. Fiore, Rev. Sci. Instrum., 61, 3119 (1990).8W. van Toledo, R. van Buuren, A. J. H. Donne, and H. de Kluiver, Rev.Sci. Instrum., 63, 2223 (1992).9Side view1-NeutralParticesJohnston'Electron-MultiplierC A-o o -Copper-Beryllium Plate HolderBCuto VConI VCAA2-5LeCy n erIon Gauge Ion Gauge _Controllerack view HighPressureTripAnode(Groundpotential)rrentoltage UV pulseverter triger High Voltageolt/gA Switch -MA and Dividers~MACDC Bertan HVMHz Power Supplyroy 8818 5 kVFig. 1: Details of the detector assembly and the associatedelectronics components.100Ga CZ CuCux004-) 1Q4)44Cr-I0-IjtJHH44~44 Qr4i01 HDfo-r-4 -r0 0-4-OQri4-r)iEa 4.411DeuteriTio=2nio= 1.noa 1- - I . ...- ..-.- umkeV1 x 1020 M-3x 1016 M-31.21.00.80.60.40.2010110-110-2------------ ----------LOG Scale-U---------------------------------------30 40110000Fig. 3: Expectedfunctionparticle400 100Energy (eV)50count rate, in linear and log scale, as aof arrival time, and equivalently, ofenergy.12I - - -10-3' ) 10I20Time (sec)I25I| I

Time-of-flight neutral particle analyzer for Alcator C-Mod

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Time-of-flight neutral particle analyzer for Alcator C-Mod

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