Nuclear Instruments and Methods in Physics Research B9 (1985) 673-678

North-Holland, Amsterdam

673

CHARGE EXCHANGE SPECTROSCOPY FOR PLASMA DIAGNOSTICS *

R.C. ISLER

Fusion Energy Division, Oak Ridge National Laboramy, Oak Ridge, TN 37831. USA.

The use of charge-exchange spectroscopy as a diagnostic tool in fusion plasmas is discussed. This technique has been employed to measure the concentrations of fully stripped ions, to perform in situ calibrations of grazing incidence spectrometers, and to determine

ion temperatures and rotation velocities from the Doppler effect. The influence of collisions and external fields on the effective emission cross sections of charge-exchange excited spectral lines is assessed.

1. Introduction

The study of impurity production, transport, and control in plasma fusion devices is of paramount inter- est. It is recognized that small amounts of conta~n~ts can prevent the self-sustained burning of D-T fuel [I]. The major method of studying impurities is by passive spectroscopy of electron-excited spectral lines. This method is insufficient for completely assessing the role of low-Z impurities. Although they radiate strongly in the periphery of discharges where the electron tempera- ture is only a few tens of eV, they are completely ionized over most of the plasma voIume in present machines and, therefore, do not emit distinctive spectral lines. Fig. la shows calculated distributions of the ioni- zation stages of oxygen in a tokamak plasma having a central temperature of 700 eV, an edge temperature of 20 eV, and an average transport velocity near lo3 cm/s. Stages below 0 VII are not shown; they exist in narrow shells between 25 and 27 cm. The bare nuclei can be detected through charge-exchange excitation (CXE) if a neutral hydrogen heating or diagnostic beam exists on the device. A spectrometer instalied so that its field of view encompasses such a beam can detect reactions that produce excited hydrogenic ions [2,3]; e.g.,

Ho + OS++ H+f(O’+)*. (1)

Charge-exchange reactions have also been observed with incompletely ionized impurities, in particular as a result of transfer from thermal hydrogen at the edge of plas- mas where its ~n~ntration is high 1451, but in the present paper the emphasis is placed on the fully ionized stages and collisions involving energetic neutral beams.

The experimental arrangement of the spectrometers on the ISX-B tokamak is shown in fig. 2. Two of the

* Work Sponsored by the Office of Fusion Energy, US De- partment of Energy, under Contract No. DE-AC05-

84OR21400 with Martin Marietta Energy Systems Inc.

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

instruments are installed with views across the south and the west beamlines. The neutral beams attenuate rapidly as they pass through the plasma, and the grazing incidence instrument primarily detects CXE signals from the south beamline whereas the normal incidence instru-

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= 0.6 z

; 0.4

5 0.2

0.0

I / I I I I

1 (0)

3

0

-20 -10 0 10 20

RADIUS (cm)

Fig. 1. (a) Calculated distribution of the higher ionization

stages of oxygen in a discharge with central and edge tempera- tures of 700 eV and 20 eV respectively. (b) Calculated radial profiles of CXE lines. Solid - normal incidence spectrometer

with the west beamline operating; dash - grazing incidence spectrometer with the south beamline operating.

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R. C. Isler / Charge exchange spectroscopy for plasma diagnostics 675

ment detects signals from the west beamline. Calculated radial ~st~butions of the CXE signals are shown in fig. I b for the two vacuum spectrometers. They are evaluated

from the overlap of the neutral particle density with the distribution of O*+. The signal from the normal inci- dence instrument and the west bearnline is localized near the center of plasmas because its optic axis is almost tangential to the magnetic axis. The signal from the south beamline and the grazing incidence spec- trometer is not so well localized. In the PDX [6] and T-10 [7] tokamaks steerable diagnostic beams have been used to actually trace the spatial distributions of 08+ and C6+.

The distinctive signature of lines produced by CXE as compared with lines from low ionization stages in the edge is shown in fig. 3. Here, the temporal behaviors of the 633 A line of 0 VIII and the resonance line of 0 V

1000 [ 1 / I I (a)

800 - i

80 1 / 1 I I

1000 I I I

(c) I1

5 E - 600 -

< GAIN = 0.057

z Fi

TIME (ms)

Fig. 3. (a) Emission from the 5-4 transition of 0 VIII excited by charge transfer with neutral-beam atoms, (b) neutral-beam current as a function of time, and (c) emission from the resonance transition of 0 V which is excited by electron collisions.

are compared. The latter is seen to be much the brighter from comparison of the relative gains specified in the figure. The 0 V line results entirely from electron collisions. It exhibits a typical burnout peak near the beginning of the discharge when the plasma tempera- ture is rising rapidly. Thereafter, the radiation increases slowly until 180 ms when a large puff of deuterium is introduced. The signal follows the waveform of the puff as additional electrons pass through the periphery where the 0 V ions are located. The behavior of the 0 VIII CXE line is very different. It does not appear until the beam current, fig. 2b, is initiated. Its intensity drops, rather than rises, following the sharp increase of elec- tron density at 180 ms as a result of greater attenuation of the beam before it reaches the field of view of the spectrometer.

Although, the n = 5 to n = 4 transition can, in prin- ciple, be produced by electron collisions with O’+, which has a peak density at a radius several cm outside the center (fig. la), as well as from charge transfer with Os*, the electron contribution is negligible, because of the small cross sections for exciting such high levels. The same is not generally true of the 3-2 and the 2-l transitions, however, because of the larger electron colli- sion cross sections for populating the lower levels. But the large beam current and the very favorable viewing positions shown in fig. 2 do allow CXE to dominate the 3-2 radiation in many circumst~~es.

2. Effective emission cross sections

Analysis of signals such as those shown in fig. 3a requires that the effective emission cross sections be determined from the partial n, 1 charge-transfer cross sections and from cascading. The most suitable cross sections for the particle velocities considered here are those based on a modified perturbed stationary state formulation [8,9]. These calculations extend to energies as high as 30-35 keV, the range of the full-energy component of the beams on ISX-B. They also apply to the important one-half and one-third energy compo- nents and to the so-called halo of neutral hydrogen that exists in the vicinity of a beam as a result of charge transfer to thermal protons. The currents carried by each component are approximately the same, and the halo particle current is about 20% of the total beam current.

Table 1 lists the charge transfer cross sections com- puted by Shipsey et al. [9] for 35.2 keV hydrogen atoms incident on 08+. Only the n = 5 and 6 states are signifi- cantly populated. More detailed calculations show that the higher angular momentum states have the greatest probabihty of being populated. Effective emission cross for the CXE transitions of O’+ are illustrated in fig. 4. The An = 1 transitions are predicted to be the most

V. HIGHLY IONISED ATOM SPECTROSCOPY

676 R. C. Isler / Charge exchange spectroscopy for plasma diagnostics

Charge transfer cross sections into the excited levels of 07+

(lo-l6 cm’) from ref. [9]

Table 1 Table 2 Comparison of fine structure splittings (0 VIII, n = 6) With

Stark and Zeeman shifts for A j = 1 states (cm- ’ )

Hydrogen n-level

energy (keV) 4 5 6 I F.S. 52 18 9 6 4

A %(max) 0.021 0.064 0.128 0.213 0.320

A &(max) 0.155 0.559 0.998 1.450 1.906 35.2 2.88 21.54 22.86 2.66

17.5 4.80 31.55 20.30 2.23

12.15 5.31 36.12 20.20 1.34

1.35 0.26 47.24 14.98 0.19

strongly excited, and this result is confirmed by experi- ment. All the An = 1 and An = 2 transitions shown in fig. 4 have been observed except those that terminate on the ground state. Similar transitions from carbon and nitrogen have also been measured.

These calculations of effective cross sections have implicitly assumed that I can be treated as a good quantum number. This is not completely valid in a plasma en~ronment where electric and magnetic fields as well as collisions can potentially couple the nearly degenerate states of hydrogenic ions. Sampson gives the following criterion for complete statistical mixing [lo],

27.5

N 2 -1.18 X 10” cme3. e n8.5 (2)

The densities required are greater than the typical val- ues of 8 X lOi cme3 attained in ISX-B, by a factor of 10 for the n = 5 level in carbon and by a factor of 20 for the n = 6 level in oxygen. This criterion is based on considering states of the same j within a given n-level to be locked together by collisions, since they are sep- areted only by very small radiative interactions, and

0.09 0.12 0.07

1

Fig. 4. Effective emission cross sections for charge-exchange excitation of 0 VIII. Excitation cross sections are taken from

ref. [9], and it is assumed that I is a good quantum number for computing cascades.

assuming equilibrium to be established when collision rates between the j = i and j = + levels exceed their radiative transition probabilities. Equilibrium between higher j-states is established at lower densities. Ratios of collision cross sections calculated from the expres- sions in ref. [lo] show that the transfer between the Gal,

and Hii, states is 3-4 times less than the transfer between the !+ and P3,2 states. So although the sum of the 6h transition probabilities is a factor of 15 smaller than the sum of the 6p transition probabilities, it still appears that the electron densities of the present experiments are a factor of 3-5 below the levels where collisional transfer would influence the analysis.

Coupling of the A j = 0 states can also occur via the Stark effect as well as by collisions. The electric fields, E = V x B, experienced by the ions in their rest frames are appro~mately lo3 V/cm for 1 keV oxygen ions in a 10 kG magnetic field. The microfields from the proton working gas are of the same order. Even for these modest parameters, which are characteristic of ISX-B, coupling occurs in oxygen for levels with n 2 3. The n = 2 level is not influenced significantly, although it might be in machines with greater magnetic fields and ion temperatures.

Finally, the possibility of coupling states with A j = 1 by external fields must be investigated. The extent of mixing is more severe in higher energy levels so it is appropriate as an example to investigate the n = 6 level of 0 VIII. Table 2 shows the fine structure intervals with the ma~mum Zeeman and first order Stark shifts for a mean oxygen ion energy of 1 keV in a 10 kG field. The Stark shifts (and couplings) are seen to be negligi- ble. The Zeeman shift is about one-half the fine struc- ture interval for the H9,2-H11,2 pair and, therefore, is expected to induce some mixing; it is not so important for the lower j-j’ pairs. In a machine with a larger magnetic field, such as JET, A j = 1 Stark coupling will still be negligible unless the temperature is very high, but the Fs,2-F7,2 and the G7,2-G9,2 pairs ought also to mixed by the Zeeman effect.

For the present experiments in ISX-B it appears that the best model for the CXE cascades is to assume that states of the same j within a given n-level are com- pletely mixed. Calculations based on this assumption differ little from those assuming that I can be treated as

R. C. Isler / Charge exchange spectroscopy for plasma diagnostics 677

a good quantum number; the effective emission cross sections differ by only 4%. Under conditions where A j = 1 mixing is important the effective emission cross sections could differ greatly from the ones we have used because strong selection rules no longer exist.

3. Applications of CXE for diagnostics

The most obvious application of CXE spectroscopy is to measure the concentrations of fully stripped ions, but is has also been used for other applications - the relative calibration of grazing incidence spectrometers [ll], ion temperature measurements [12,13], and de- termining toroidal plasma rotation [14]. Fig. 5 shows signals recorded with the normal incidence spectrometer on ISX-B when the west beamline is operating. All the low-Z impurities are detected within the range of this instrument. The signals are relatively strong because the tokamak has not been gettered (1 gigarayleigh = 1015 photons/cm2 s), and the central densities are calculated to be 1.0, 0.7, 0.4 in units of lOi cm-’ for C, 0, and N respectively.

It is interesting to note that although different com- putational approaches [8,9,15-171 give absolute cross sections for the charge transfer that can differ by factors of two, the relative values of the effective emission cross sections for the An = 1 transitions generally differ by no

0 100 200 300 400

TIME (mr)

Fig. 5. CXE signals oxygen, carbon, and nitrogen from the

normal incidence spectrometer when the west beamline is oper-

ating with 800 kW of injection power.

more than 15% for any single calculation. These consid- erations have allowed the CXE lines to be exploited for an in situ method of obtaining the response of a grazing incidence spectrometer [ll]. The five spectral lines noted in fig. 4 have been used to obtain the relative calibration from 102 A to 1164 ,&, and additional points have been determined by fitting similar transitions from carbon and nitrogen to the curve. The absolute sensitivity is established directly from the tokamak discharges by comparing a calibrated H, monitor to the grazing inci- dence signal for the Lyp line.

One of the most promising uses of charge-exchange spectroscopy is to measure Doppler shifts and widths in order to obtain ion temperatures and plasma rotation velocities. It is one of the few ways of observing wave- lengths long enough for doing such measurements from the interior of high temperature discharges. Investiga- tions have been done in both the D III [12] and the PDX [13] tokamaks by seeding helium into the dis- charge. In ISX-B the central rotation has been measured from oxygen signals using the normal incidence spec- trometer [14]. Since the CXE lines do not appear until the beam is actually operating and the plasma has begun to rotate somewhat, it is necessary to use a nearby line from a non-rotating, or at least a slowly rotating, ion near the periphery as a fiducial. Profiles of both the 0 V line at 629.7 A and the 0 VIII line at 632.7 a measured by using a scanning mirror in the normal incidence spectrometer are shown in fig. 6. The 0 V line serves as a wavelength standard for the CXE line. It is known from measurements in the visible region where the plasma can be viewed both radially and tangentially that this ion does not rotate faster than 1 x lo6 cm/s. Central velocities obtained by using this pair are illustrated in fig. 7 for a sequence in which two staggered 900 kW beams are employed. The west beam turns on at 100 ms and induces rotation velocity of the

I38 140 142 144 146 148

TIME (mr)

Fig. 6. Profiles of spectral lines taken with the scanning mirror on the normal incidence spectrometer (fig. 2). The full sweep is

accomplished in a 10 ms period. The 0 V line from the plasma

periphery, which rotates very slowly, is used as fiducial to

evaluate the central rotation from the 0 VIII CXE line.

V. HIGHLY IONISED ATOM SPECTROSCOPY

678 R. C. I&r / Charge exchange spectroscopy for plasma diagnostics

I

L 10 l m l . .ra

0

O 5 0

;= .@

> 0

5 0

00 . . . . . . . . . . . . . . . . ..__..___..........................

l 0.9

2 -5

5 -10

0 100 200 300 400

TIME (ms)

Fig. 7. Rotation measured form the 633.7 A CXE line of 0 VII.

A co-directed, 900 kW beam is turned on at 100 ms; at 200 ms counter-directed injection that reduces the rotation to zero is initiated.

order of 10’ cm/s. The counter directed injection from the south beam is initiated at 200 ms, and the rotation is stopped within 30-35 ms. The systematic uncertainty incurred by using a separate wavelength standard for the Doppler shift is estimated to be 1.5 x lo6 cm/s.

In summary, charge-exchange spectroscopy is now being employed for variety of diagnostic measurements in high temperature plasma devices, and it is expected to be used even more widely on those machines that either employ neutral-beam heating or that have lower power beams solely for diagnostic purposes.

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