Nuclear instruments and Methods in Physics Research B10/11(1985) 487-491 Norm-Homed. Amsterdam

487

ACCELEXATOR-BASED DIAGNOSTICS IN TANDEM-MIRROR FUSION EXPERIMENTS *

R BASTASZ, W.L. HSU, A.E. PONTAU and K.L. WILSON

Sandia National laboratories, Liuermore, Calgomia 94550

Several accelerator-based analysis techniques have proven useful in diagnosing plasma-edge and first-wall conditions in two tandem-mirror fusion experiments. This Paper describes the various techniques and the use of solid-state probes in the tandem-mir- ror devices TMX and TMX-U in conjunction with subsequent analysis by Rutherford backscattering spectrometry (RBS), nttclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS). RBS was used to measure the concentration of deposited impurities on solid-state probe samples. NRA, utilizing the D(3He, n)H reaction, gave quantitative data about deuterium retention levels in carbon samples exposed to the plasma. SIMS analysis produced near-surface depth profiles of implanted deuterium in exposed silicon samples. Taken together, these techniques provided detailed information about impurity levels, the flux and energy of particles striking the first-wall, and plasma-edge temperatures. This information has contributep towards an understanding of the ~nf~ern~t properties of tandem mirrors.

1. Iwtrodwetion

Solid-state particle collector probes have become widely used in large-scale fusion experiments to study the plasma boundary layer [l-lo]. probes inserted near the plasma edge are used to trap particles escaping from the plasma. Their main attraction is that it is relatively simple to place a removable probe in a fusion experi- ment and then subsequently analyze exposed probe samples in the laboratory. With instrumentation sep- arate from the fusion experiment, it is possible to utilize a variety of analysis methods. Several accelerator-based analysis methods have been used to provide information about deposition of plasma impurities and retention of hydrogen isotopes in collector probes. This information enables one to estimate plasma impurity levels, the flux and energy of particles striking a probe sample, and plasma-edge temperatures.

In this paper, we will first survey accelerator-based methods that can be used for analysis of solid-state probe samples. Following this, we will describe how accelerator-based analysis methods have been used in conjunction with solid-state probes for studying the plasma boundary layer in the tandem mirror experiment (TMX) and tandem mirror experiment-upgrade (TMX- U) fusion devices.

2. Accelerator-based methods used for probe analysis

The following methods will be discussed: low-energy ion scattering (LEIS), secondary ion mass spectrometry

l This work was supported by the U.S. Department of Energy.

0168-583X/85/$03.30 6 Elsevier Science Publishers B.V. (North-Holland physics Publishing Division)

(SIMS), Rutherford backscattering spectrometry (RBS), elastic recoil detection (ERD), and nuclear reaction analysis (NRA). The primary data sought from these methods are the amount and depth distribution of par- ticles implanted in probe samples. In table 1, the meth- ods are grouped according to the type of analyzed particle and energy range of the required accelerator. LEIS and RBS are similar in that the energy loss of reflected ions is analyzed while the other methods detect emitted target atoms or reaction products. LEIS and SIMS use low-energy accelerators to enhance surface

Table 1 Accelerator-based methods used for probe analysis

Particle Ana1.yzed

reflected target reaction ion atom product

I‘EIS ; sn4s: ___

EZXWJY i Roneer

high P.BS i EI(D

:....“...4..“““i

NRA :

(>lOO keV) I i hydrogen analysis i . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Em : Elastic recoil detection LESS: Low-energy ion scattering NRA : tiuc1ear reaction analysis RBS : Rutherford backscattering spectroscopy SIRS: seconda?Cy ion mass spect?ametry

IV. CONTROLLED THERMONUCLEAR REACTIONS

488 R. Bastasz et al. / Acceieraior - based diugnosiics

sensitivity or to obtain maximum sample erosion rates. The other methods require high-energy accelerators to provide low ion neutralization rates, large ion penetra- tion depths, or to reach nuclear reaction thresholds. A reasonable boundary between the low and high energy regions is the Bohr electron velocity (e2/A = 2.2 X lo6 m/s), above which the probability for ion neutralization becomes small [ll]. For hydrogen and helium projectiles this occurs between 25-100 keV.

2.1. Low-energy ion scattering

Surface compositional analysis can be obtained by measuring the scattered ion energy loss due to elastic collisions with surface atoms [12]. Surface atoms must have greater mass than the incident ion mass in order to be detectable, except in special cases where the ion beam strikes the surface at a grazing angle. At low energies, nearly complete neutralization of inert-gas ions that scatter from subsurface target atoms occurs, giving this method extreme surface sensitivity. Consequently, probe studies using this method must be done carefully, avoiding air transfer of probe samples after their ex- posure, otherwise scattering by adsorbed atmospheric gases will dominate the measured energy spectra.

2.2. Secondary ion mass spectrometty

SIMS provides surface compositional analysis of all elements including hydrogen [13]. Sputtered target atoms that are ionized can be analyzed directly in a mass spectrometer. Controlled erosion of the target during analysis yields a depth profile of up to several hundred nm with a resolution of a few mn over the entire depth. One complicating factor, however, is that SIMS meas- urements are affected by local surface conditions, which can alter the probability for secondary ion emission. This makes quantitation of SIMS data difficult for poorly characterized samples containing many impuri- ties. Best results are obtained when the method is used to analyze small amounts of an impurity in an otherwise homogeneous target. For example, SIMS has been espe- cially successful in measuring depth profiles of hydro- gen isotopes in pure Si [10,14].

2.3. Rutherford backscattering spectrometry

RBS is a well established method based on energy loss measurements of high-energy ions elastically back- scattered from target atoms. These measurements give quantitative data about impurity levels within the range of the analyxing beam in the target under study 1151. Analysis depths can be up to several microns and depth profiles can often be inferred from the data. Good sensitivity to high Z elements is achieved particularly on low Z targets. Like the following methods, depth

resolution is itself a function of the depth z, varying in proportion to 6.

2.4. Elastic recoil detection

This high-energy method is used to measure near- surface hydrogen levels [16]. ERD works by ejecting hydrogen isotopes from a sample using a He+ analyzing beam at grazing incidence. These recoils are collected and energy analyzed for mass identification. Yields are established using calibration standards and enable quantitative measurements to be made. For thick sam- ples (> 5 pm), low incident and exit angles to the surface require a smooth sample for accurate measum- ments.

2.5. Nuclear reaction analysis

The nuclear reaction most utilized for probe sample analysis is D(3He, cr)H, which produces a particles of about 3.2 MeV and protons of 14.6 MeV when a 750 keV 3He beam is used to initiate the reaction (171. The yield of a particles or protons per incident 3He can be calibrated and is used to determine the absolute amount of deuterium in a sample. The depth distribution of deuterium can be estimated from the energy spectrum of the emitted particles if the stopping power of the target is known.

Resonant reactions, such as H(‘Li, y)sBe, H(15N, a)12C, or H(19F, uy)160, can be used to mea- sure hydrogen depth profiles by varying the energy of the incident particle beam to achieve resonance condi- tions at various depths [18]. Depth resolution is limited by the energy width of the resonance. Because of the higher energies and heavier ions required, their use is less frequent than D or He excited reactions.

3. TMX and TMX-U probe studies

The first use of accelerator-based diagnostics in tandem mirror fusion experiments took place on the TMX and TMX-U devices located at the Lawrence Livermore National Laboratory [19]. Two specially de- signed probes were used to expose carbon and silicon samples to the plasma in the center-cell and end-plug. The results of this study, which demonstrate the use of several accelerator-based analysis methods, are sum- marized in the following sections.

3.1. Flux measurements

The amount of deuterium trapped in a probe sample can be measured in a straightforward manner by ERD or NRA. Prior to saturation, which occurs at approxi- mately a 50% deuterium concentration in C and Si

R. Bostosz et al. / Accelerator- based diagnostics 489

substrates held at temperatures less than lOO*C, the amount of trapped deuterium is directly proportional to the incident fluence [20]. The proportionality constant is 1 - R where R is the reflection coefficient, which is a function of particle energy. Consequently, if either R is measured or the incident particle energy is known and R can be calculated, then the amount of trapped de- uterium can be related to the incident fluence. It is then a simple matter to determine the incident deuterium flux by dividing by the total duration of exposure to the plasma.

Fig. 1 shows the deuterium flux measured from C probe samples exposed to TMX plasma at various ra- dial locations and operating conditions [lo]. The fluxes were calculated assuming a Maxwellian incident particle temperature of 50 eV (energy rne~~ern~t described in section 3.2) and using calculated values of R. As the probe is moved away from the plasma, the flux de- creases and provides an indication of the sharpness of the plasma boundary. The effect of various modes of fueling the plasma upon the particle flux at the boundary is clearly seen and these results have been used as a check for the gas fueling code used to model TMX performance [21]. General agreement is found and has led to increased confidence in the plasma physics as- sumptions of the model.

‘o’6* 0

l

TMX Centnl-eell 1 kG field

Fig. 1. Deuterium flux to C probe samples exposed to the center-cell plasma in TMX as determined by NRA for three

fueling modes. (a) Gas box fueling at normal plasma density, (b) gas puffer fueling, and (c) gas box fueling at low plasma density.

Fig. 2. NRA measurements of retained deuterium in C exposed to the center-cell plasma in TMX along with calculated satura- tion curves for D incident on C at various MaxweIlian energies.

3.2. Energy measurements

The incident energy of deuterium trapped in probe samples can be determined from both NRA and SIMS measurements. The saturation behavior of deuterium in carbon probe samples provides an indication of the incident particle energy. This is done by exposing sep arate samples to different numbers of identical plasma shots and then measuring the amount of retained deu- terium in each sample with NRA. The incident deu- terium energy is estimated by comparing the data with established deuterium retention curves [lo]. Since this method requires knowledge of R, which is itself a function of energy, some ~bi~ty may occur. For- tunately, R does not vary strongly in the energy range typical of the plasma boundary layer (50-200 eV), so the method gives useful results with an assumed R.

The result of a series of measurements to determine the plasma-edge temperature in the center cell of TMX

Depth ,nm,

Fig. 3. Deuterium depth profile in Si exposed to the center-cell plasma in TMX measured with SIMS and compared to TRIM calculations of Maxwellian and monoenergetic deuterium en- ergy ~st~butions.

IV. CONTROLLED THERMONUCLEAR RFACTIONS

490 R. Basiasz et al. / Accelerator-based diagnostics

is shown in fig. 2. Saturation is evident and the particle distribution is best described by a Maxwellian distribu- tion with a 50 eV temperature.

Another way to determine the incident energy is to directly measure the depth distribution of trapped deu- terium with SIMS. The distribution can then be corn- pared with calculated profiles for various incident par- ticle energies. Accurate calculations can be obtained from the TRIM model, which takes into account nuclear and electronic stopping powers to determine the depth distribution of ions implanted in solids [22].

An example of this approach is shown in fig. 3, which is a depth profile of deuterium in a Si sample that was exposed to 26 successive plasma shots in the center cell of TMX [lo]. In this figure, the SIMS profile is compared with TRIM calculated profiles for both Maxwellian and monoenergetic energy distributions. The data clearly match a Maxwellian distribution and indi- cate that the incident particles had a 25 to 50 eV temperature, in agreement with the NRA results. Calcu- lations of the ion temperature profile based on indirect measurements [23] gave similar results for the central plasma temperature, suggesting that the particles inci- dent on the probe arose from charge-exchange neutrals generated in the plasma core.

3.3. Impurity analysis

RBS measurements can be used to identify impurity atoms trapped in samples exposed to the particle out- flux from a plasma. The goal of such measurements is to relate impurity retention in probe samples to impurity levels in the plasma.

Results are shown in fig. 4 for carbon samples ex- posed to various numbers of successive plasma shots in the TMX center cell. Analysis indicated that 0, Ti, Fe and sometimes Cu were deposited on the samples. The 0 and Ti levels increased with the number of plasma shots, indicating that the impurity atoms were deposited during the probe’s exposure inside TMX. Two explana- tions are possible: the impurities originated in the plasma or the impurities resulted from Ti deposition by getter wires used to sublime Ti onto wall surfaces for vacuum pumping. In the second case the observed oxygen may have resulted from residual gas (mainly H,O) adsorp- tion on the deposited Ti. The high levels of Ti ahd 0 seen support the second explanation. Also, 0 observed in the TMX plasma by ultraviolet spectroscopy was at a low concentration (0.4%) [24]. The origin of the Ti and 0 was later verified by Auger electron spectroscopy which showed deposition of Ti at a rate of = 0.8 nm/shot and uniform incorporation of 0 in the de- posited Ti films [25].

In general, the presence of oxygen on a probe sample is not strictly related to oxygen impurity levels in the plasma, since oxygen containing residual gases in the

% 2 IZZ 10 shois fn 15

Eo I 41 shots

z

z 0 c 10

9.7

0 0 Ti Fe (x10) Cu (x10)

Fig. 4. Impurity levels in C exposed to the center-cell plasma in TMX as determined by RBS for samples exposed to 10 and 41 successive plasma shots.

vacuum vessel or oxygen from the atmosphere may adsorb onto the sample before exposure or analysis. This problem can be mitigated through proper selection of sample materials and by use of a vacuum transfer station to avoid exposure to air.

Stainless steel and Cu components in the vicinity of the probe samples were likely sources for the observed Fe and Cu. The presence of these impurities on the probe samples indicated that plasma ions bombarded the components at energies above the sputtering threshold (= 35 ev).

100 200

DEPTH (nm)

Fig. 5. Deuterium depth profile in Si exposed to chargeex- change flux at east end-plug of TICK-U measured with SIMS and compared to a calculation of the expected sloshing ion distribution.

R. Bastasz et al. / Accelerator-bared diagnostics 491

3.4. Sloshing ion energies

Energetic ions confined within each endplug, re- ferred to as sloshing ions, are necessary for the estab- lishment of a thermal barrier to contain the center ceB plasma in TMX-U, the successor to TMX [26]. Their existence was confirmed by the use of a collector probe placed in a strategic location in TMX-U [27].

A Si probe was oriented so as to collect charge-ex- change neutrals generated from sloshing ions. After exposure to many plasma shots, the probe was removed and a SIMS profile was made of the implanted de- uterium. The results are shown in fig. 5 along with a model calculation based on the expected sloshing ion energy distribution. The measured distribution contains a subsurface peak representing implanted deuterium having an incident energy of about 3 keV superimposed upon a tail extending to 200 nm, indicating the presence

of energetic particles with energies reaching up to 10 keV. The profile has a mean depth of 105 nm which corresponds to a mean ion energy of 6 keV [27]. The expected distribution closely resembles the measured profile. This evidence, along with measurements using carbon film resistance probes, confirmed the existence of sloshing ions in TMX-U.

4. Conclusions

Accelerator-based methods for probe analysis pro- vide data useful in diagnosing the plasma boundary layer. At the current level of development there is a tradeoff between quantitation and depth resolution. High-energy techniques provide the quantitation needed for impurity level and flux measurements, but have limited surface specificity and depth resolution. The opposite situation applies to low-energy techniques, which excel in measuring depth profiles needed for determining particle energies. Utilizing several methods thus increases the information obtainable from collector probes. With the aid of these techniques, in combina- tion with other diagnostics, a more complete view of the plasma boundary layer in tandem mirrors is emerging.

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IV. CONTROLLED THERMONUCLEAR REACTIONS