Petty, C.C.

Smatlak, D.L.

Smith, D.K.

DSpace@MIT

PFC/JA-87-38Time-of-flight analyzer for ionendloss of a mirror plasmaPetty, C.C.; Smith, D.K.; Smatlak, D.L.October 1987Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02139 USASubmitted for publication in: Review of Scientific InstrumentsSupported by the U.S. Department of Energy Contract No.DE-AC02-78ET51013AbstractA time-of-flight analyzer has successfully been used to measure the ex-tracted charge-state distribution and impurity level of an ECRH mirrorplasma. Endloss temperatures for each ion species have been determinedfrom the rise time of the analyzer output pulses. Ions entering the line-of-sight device are first accelerated by a two aperture lens and are then focusedwith an einzel lens. Electrostatic deflection plates chop the ion beam intoshort pulses which travel down a 1.20m tube to an electron multiplier. Ionspectra are summed at a rate of 1kHz using a hardware signal averager.1IntroductionIn a mirror plasma, a plasma potential (V,) develops which confines elec-trons and expels ions. An ion leaving the plasma as endloss therefore hasa total energy equal to the sum of its kinetic energy (E;) plus its potentialenergy (qiV,). If the ion is then accelerated through an externally appliedpotential (V), the time required for it to transverse a distance L ist = L()t 2=q(VL+Tbj E;LFor the condition Ej < qi(V, + Vb), this becomest = L 1- .(22q;(4, + 4) 2qi(V, + b)Thus the time-of-flight (TOF) of an ion is proportional to the square rootof its mass-to-charge ratio. The endloss temperature of each ion species canbe determined from the rise time of its pulse. From Eqn. 2, the difference inarrival times (At) between an ion with initial kinetic energy E; and an ionwith no initial kinetic energy isAt = Lm 2  (3)[2qi(V + Vb)] 3 /2(If the ion endloss current is assumed to have a one-dimensional Maxwelliandistribution of energies, I oc exp(-E;/T), then substituting for E; fromEqn. 3 givesx exp {[2qi(V, + Vb)] 3 /2At (4)Lm! /2T2Thus by fitting an exponential to the rising edge of the analyzer output pulse,the endloss temperature of that ion species can be determined.A TOF analyzer is presently being used to study ion endloss from theConstance B quadrupole-mirror experiment. The main function of the an-alyzer is to measure the extracted charge-state distribution and ion endlosstemperature of non-hydrogen plasmas. A secondary function is to identifyand monitor impurity levels in the plasma. Knowing the extracted charge-state distribution allows the total ion particle current to be found from theelectrical current collected by a faraday cup. For hydrogen plasmas, the TOFanalyzer can find the H+ to H+ ratio in order to determine the correct numberof ionizations per H, photon for source measurements. The Constance exper-iment is described in detail elsewhere.1' 2 Briefly it uses several kilowatts ofECRH to create a hot electron plasma (3 ~ 20%) with neh = 2 X 10"cm- 3and Tth = 400keV. The hot electrons are responsible for stripping heavyatoms to high charge states. There is also a cold population of electronswith nec = 1 x 10"cm-3 and Tec = 50eV. The ion endloss current-density istypically 5piA/cm 2. Pulse lengths for Constance are 1-2 seconds.TOF analyzers have previously been used as mass spectrometers in chem-ical ionization sources3 and other apparatus. 45 As mass spectrometers, theyare designed to achieve high mass resolution by minimizing the thermalspread of the ions. More recently, TOF analyzers have been used to de-termine the charge state distribution of ion sources.' The TOF analyzer3described here differs from these because it combines mi/qi resolution andsensitivity with the ability to measure ion temperatures. It also operates atlow ion current densities and on fast time scales.I. Analyzer designThere are four sections to the time-of-flight analyzer: 1) the accelerat-ing/focusing lens, 2) the deflection plates, 3) the time-of-flight length and4) the ion detector. A drawing of the first two sections is shown in Fig. 1.For simplicity the analyzer was designed to be a line-of-sight device. If poten-tial is measured relative to the plasma, then the entrance aperture is negativeby the plasma potential (V,). The second aperture is biased at a high neg-ative voltage (V) to accelerate the ions to 2-3kV. It is important that thisbias voltage be much greater than the ion temperature, otherwise the TOFwill depend more on temperature then on m1/qi. These two acceleratingapertures make up a convergent lens with a focal length approximately equalto three times the distance between them.7 Thus after several centimetersthe ion beam becomes strongly unfocused and the ion current density startsto decline. An einzel lens is therefore placed after the accelerating lens inorder to refocus the beam and thus increase the amount of current reachingthe ion detector. The einzel lens does not change the ion's energy becausethe entrance and exit apertures are at the same potential. The middle aper-ture is biased with a second power supply at a less negative potential (V).4The ions exit the accelerating/focusing lens with paths parallel to the axiswhen the sum of the mid-focal lengths of the two lenses equals the distancebetween their reference planes. The current at the ion detector is typicallyincreased by a factor of fifteen when the einzel lens is used. The optimalfocusing voltage for the einzel lens depends on both the bias voltage and theplasma potential, and thus can change by several hundred volts as plasmaconditions change.After leaving the accelerating/focusing lens, the ion beam is collimatedand passes between two electrostatic deflection plates. Normally one plate isbiased relative to the other, creating an electric field which deflects the ions.The angle of deflection (0) is given bytanG = Vd(5)2d(V + V)'where I is the deflection plate length, d is the separation between plates,and V is the potential difference between the plates. Note that the angleof deflection is independent of ion mass or charge. Our parameters are I =12.7mm, d = 6.4mm and V = 100V. There is an exit aperture located63.5mm past the deflection plates which deflected ions cannot pass. In orderto send a burst of ions through the exit aperture to the ion detector, thedeflection plate voltage is pulsed to zero by the circuit shown in Fig. 2.Typical pulse lengths are 100-300ns, with 7ns rise and fall times. This fastrise time is necessary in order to determine the ion temperature from theanalyzer output pulse.5After passing through the exit aperture, the ions travel down a long sec-tion of tubing. This serves as the time-of-flight length. In general, the longerthe tube, the better the mi/q; resolution and the more accurate the ion tem-perature measurement. In our case the maximum length was set by spacelimitations to be L = 1.20m. This length gives typical TOF's of severalmicroseconds.At the end of the flight an electron multiplier serves as the ion detector. AVacumetrics AEM-1000 multiplier is used which has a rise time of iOns and issmall enough to fit inside a standard 1 -inch diameter tube. The multipliergain is 5 x 10' at 2kV. The electron multiplier supply voltage is the samebias voltage used to accelerate the ions. The advantage of this is that onlyone high voltage power supply is needed. The disadvantage is that changingthe multiplier gain means changing the accelerating voltage for the ions, andthus the TOF and the focusing properties also change. Because large peakcurrents are drawn from the multiplier (typically 100piA), a 300pF vacuumcompatible, glass capacitor is placed across the resistor from the last dynodeto ground. The multiplier output is amplified by a PAR 115 wideband pre-amp with a gain of ten. All signal cables are terminated in 501.The magnetic field lines from the Constance baseball magnet are parallelto the axis of the TOF analyzer, with a typical field strength of 90G atthe entrance. The entire length of the analyzer is magnetically shielded toinsure that there are no ion gyroradius effects and to prevent gain loss of6the electron multiplier. Ion gyroradius effects would result in a limit to theperpendicular temperature that an ion could have and still reach the electronmultiplier. The magnetic shielding doubles as a high voltage shield since theentire analyzer floats at the bias voltage.II. Data acquisition and averagingThe TOF analyzer can produce an ion spectrum every 20ptsec. Since theusual time scale of interest is several hundred milliseconds, data averagingis performed. Besides the signal-to-noise ratio improvement, data averag-ing smooths out the statistical fluctuations in particle counts which comesfrom low ion currents. On Constance, the major source of noise is fromsoft X-rays. Because the TOF analyzer is a line-of-sight instrument, X-rayscan pass through the apertures and strike the electron multiplier. Summingmany spectra averages the X-rays out to an offset current. (It is possible,however, to altogether block X-rays from entering the TOF analyzer by tak-ing advantage of the curved magnetic field lines.) A second source of noise isfrom hot electron endloss. Electrons with kinetic energy greater than about40keV will enter the analyzer and pass by the deflection plates unaffected,thus contributing to the noise signal.The primary method of signal averaging is to digitize the electron multi-plier output at 100MHz, and then sum the spectra using a hardware signalaverager. This setup consists of a Transiac 2001s transient digitizer linked7to a Transiac 4001 signal averaging memory. Spectra can be summed with8 bits of accuracy at a rate of 1kHz. Thus over a period of one second asignal-to-noise ratio improvement of 32 can be obtained.A second method of data averaging is with a gated integrator and boxcaraverager. The gate of the boxcar is scanned over the ion spectrum in orderto perform the data averaging, with a scan lasting from .1-1 second. Thesignal-to-noise ratio improvement with the boxcar is twelve times less thanwith the hardware signal averager; this can be improved on somewhat bynarrowing the portion of the spectrum being averaged over by the boxcar.The boxcar has an advantage, however, in that the gate can be set to lookat one particular ion species. It can then follow the amplitude of that ionspecies on a very fast (20kHz) time scale.III. Experimental resultsA typical TOF spectrum is shown in Fig. 3 for an argon plasma. Thisdata is the sum of 1000 spectra taken over 1 second. The TOF parametersare V = 115V and V1b 2.50kV. A deflection plate pulse length of 150nswas used in order to resolve the highest charge states. This amount of timewas too short, however, to allow the slowest Ar+ and Ar2 + ions to passby the deflection plates; thus their heights are too low by 90% and 40%respectfully. Because of fringe electric fields, the minimum required pulselength is actually twice as long as it takes an ion to travel the length of the8deflection plates. The highest charge state of argon extracted was Arl+.In addition many impurities are evident, the dominant ones being atomicand molecular hydrogen (hydrogen is the normal working gas of Constance).Also seen are nitrogen, oxygen and carbon, along with various molecularcombinations. All ion species are peaked on the lowest charge state.The ion particle current can be found by dividing the electron multiplieroutput current by the multiplier gain and the electron yield for ion impact (y).An absolute calibration of y for various ion species is difficult, mainly because-y is strongly affected by the surface condition of the dynode material.8 '9Many experiments have shown that -y decreases with ion mass and increaseswith charge state, 8-1 0 but the reported amounts vary considerably. Since therelative increase in y with charge state falls as the ion energy increases, thiseffect is deemphasized for the TOF analyzer. We calibrated -y by analyzingthe height of individual counts from the electron multiplier corresponding todifferent ion species. We found no correlation between -y and ion mass orcharge to within the 20% experimental uncertainty for elements up to argon.A blowup of the first two argon charge states is shown in Fig. 4 for adeflection plate pulse length of 300ns. This pulse length allows all of theargon ions to pass through the deflection plates. The plasma conditions arethe same as for Fig. 3. The exponential rise of the pulses gives an ion end-loss temperature of 33eV for Ar+ and 60eV for Ar 2+. These temperaturesare too high to be accounted for by electron drag alone; it is believed that9the ions are being heated by the strong electron microinstability. These iontemperatures, and in particular the increase in ion temperature with chargestate, were independently confirmed from doppler broadening of argon spec-tral lines. Similar results were obtained in oxygen plasmas. This confirmationis important because large fluctuations in the plasma potential could havedistorted the analyzer output pulses, leading to TOF ion temperatures whichwere too high. The temperature increase with q. is consistent with mirrorconfinement models.11ConclusionsA TOF analyzer has successfully been used to measure the extractedcharge-state distribution and ion endloss temperature of an ECRH mirrorplasma. Because an electron multiplier is used as the ion detector, the mainsource of error in determining the ion particle currents is in knowing theelectron yield for ion impact of the dynode material; however, -y appears tobe relatively independent of ion mass or charge for elements up to at leastargon. The rise time of the TOF analyzer pulses can be used to find the ionendloss temperature for each ion species. As with all such measurements,care must be taken that a hot ion tail is not mistaken for the temperatureof the bulk ions. Large fluctuations in the plasma potential are anotherpossible source of error, but do not appear to be a concern because the TOFtemperatures agree with temperatures determined from doppler broadeningof visible spectral lines. It should be remembered that the ion temperatures10measured by the TOF analyzer are endloss temperatures, and may not bethe same as the confined temperatures.AcknowledgmentsThe authors would like to thank Ken Rettman for fabricating many ofthe parts for the TOF analyzer. This work was performed under D.O.E.contract no. DE-AC02-78ET51013. C. Petty was financially supported bythe Magnetic Fusion Energy Technology Fellowship administered by OakRidge Associated Universities.11References'R.S. Post, et. al., Plasma Physics and Controlled Nuclear Fusion 1984:Tenth Conference Proceedings, London, 12-19 September 1984 (IAEA,Vienna, 1984), Vol. 2.2D.L. Smatlak, X. Chen, B.G. Lane, S.A. Hokin, R.S. Post, Phys. Rev. Lett.58, 1853 (1987).'J. David Pinkston, Martin Rabb, J. Throck Watson and John Allison, Rev.Sci. Instr. 57, 583 (1986).4D. Rathmann, N. Exeler and B. Willerding, J. Phys. E: Sci. Instrum. 18,17 (1985).'J.A. Browder, R.L. Miller, W.A. Thomas and G. Sanzone, Int. J. MassSpectrosc. & Ion Phys. 37, 99 (1981).'I.G. Brown, J.E. Galvin, R.A. MacGill and R.T. Wright, Rev. Sci. Instr.58, 1589 (1987).7E. Harting and F.H. Read, Electrostatic Lenses (Elsevier Scientific Pub-lishing Company, New York, 1976).'S.C. Brown, Basic Data of Plasma Physics (M.I.T. Press, Cambridge,1966).'S. Fliigge, Handbuch der Physik (Springer, Berlin, 1955).12'0C.F. Barnett, G.E. Evans and P.M. Stier, Rev. Sci. Instr. 25, 1112 (1954)."David E. Baldwin, Model for ion confinement in a hot-electron tandemmirror anchor, Lawrence Livermore Lab. Rep. UCID-18802 (1980).13-6cSCc c10---=.0 U)CC01.-4 -0-I- y- --+-:44-+U')14i(00c'-1+ + E0)0 -0040b3rU0(DV3U)0-100n(mylar) (ceramic) -ToIRFD12 deflectionplates-L HV IsolationtransformerFIG. 2. Fast switching circuit for deflection plates.Pulsein.iF .0pF vII+z+ 0zA+ ++.++4+ + ++-.- 0 0 0 ,"+b.. b6bQ (0.ID0(A) IndIno jaildiiiflw uO4P9I3+ I-I+I00Q(00*I I I I00000000-0000(0000D000I-01.40~0+ '0+r-.40 -4. U-- 0I I Ii I IC3a>00.-5 -10150 10250 10350 10450 10550 10650Time (nsec)(a):3-2-0 Tj 60 eV--4-0-72000 730 740700 7Time (nsec)(b)FIG. 4. Logarithmic plot of the electron multiplier output pulse to calculater2+io -nls eprtrs o )A+ )A

Time-of-flight analyzer for ion endloss of a mirror plasma

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Time-of-flight analyzer for ion endloss of a mirror plasma

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