Electron emission induced by singly charged species

Useful reviews of the subject have been provided by Krebs [35], by Baragiola et al. [36] by Hasselkamp [37] and much earlier by Medved and Strausser [38].

In Table III we list a selection of the data sources for this area. We find rather significant coverage of the light projectile impact, but very limited data on emis-sion induced by impurity ions (C⁺, N ⁺ , 0⁺) or by the ions of metals that may find use as wall materials. For reference we also list in Table III data sources for rare gas impact cases; these are quite numerous and provide guidance on the general behaviour of the phenomenon.

Table III concentrates on relatively recent data sources for which one expects proper attention to surface conditions and that the data are reliable. For earlier, but generally less reliable data, one might refer to the various reviews cited above and to our earlier paper [6]. One should also note the extensive work at ener-gies above 100 keV by Hasselkamp et al. [52-55]. All data sources listed are for polycrystalline targets and generally involve normal incidence. We do not here review data for insulators and instead refer the reader to the review by Krebs [35].

A general indication of behaviour under proton bombardment is provided by Fig. 7, taken from Hasselkamp [37]. The coefficient is a maximum at around 100 keV H⁺ energy and drops by a factor of five to keV energies. Some data are available for D +

ions and are identical to those for H+ on a velocity scale [42]. A very pleasing feature is that the data from different groups are generally in rather good agreement at overlapping energies. The extensive data sets of Zalm and Beckers [41] from 5 to 20 keV energies agree with those of Baragiola et al. [42] from 2 to 50 keV, to better than 10% in most cases; both publications give their data in convenient tabular form [41, 42]. Hippler et al. [39] have studied how the emission coefficient for H + impact varies with the atomic number of the target and find results very similar to those in Fig. 6 for the electron case; the coefficients are small for almost full or almost empty atomic shells and maximize for an approximately half filled shell.

Interestingly Schou [8] shows that the coefficients of Fig. 9 mirror rather well the electronic stopping power of these targets. This is expected from Eq. (1) because the factor D is proportional to stopping. The factor D of Eq. (1) does not vary much between targets. We suggests that, in the absence of a complete data set for 8, one could make a reasonable estimate by using the

TABLE III. DATA ON ION INDUCED ELECTRON EMISSION FOR SELECTED PROJECTILES AND TARGETS, FOR ENERGIES BELOW 100 keV ONLY

TABLE III. (cont.)

Target Z Projectile Energy range

(keV) Refs

Target Z Projectile Energy range

(keV) Refs

Reference [48] contains excellent data as a function of incidence angle (Cu targets).

electron stopping power data of Andersen and Ziegler [58] to provide the energy dependence and normalizing to an available data point.

At high impact energies ( > 4 0 keV) it has been shown that the dependence of the emission coefficient on the angle of incidence 9 is given [48] by

7(0) = 7(0 = 0)/cos^z 0 (9)

where the power z is unity for light ions (H + , He+) and between 1 and 1.2 for heavier species (Ne + , Ar + , Kr⁺). These equations are valid to angles of about 80°, after which the coefficient falls rapidly. Note the similarity to Eq. (8), which is for electron impact.

Let us turn now to the question of electron emission induced by heavy ion impact. There are significant data for rare gas ion impact (see references in Table III) performed for the fundamental study of the emission phenomenon. The data from various sources are gene-rally in good agreement, and useful tabulations over wide energy ranges can be found in the work of Zalm and Bekkers [41] and of Baragiola.et al. [42]. Such data are, of course, of little direct value to the under-standing of fusion situations and we will not discuss them in detail here. There is only sparse coverage of impurity ion (C + , N + , 0+ and metals) impact on metals and carbon. Useful indication of how the coeffi-cient varies with projectile Z for various targets and with target Z for various projectiles is shown in Figs 8 and 9 from the work of Baragiola et al. [36]. Similar graphs for various projectiles on stainless steel [59, 60],

1 0 " 10' 1 0 * 10°

Proton Energy IkeV)

10"

FIG. 7. Electron yield as a function of ion energy for H+ impact on Al, Cu, Ag and Au; a compendium from Hasselkamp [37].

Data from (a) Baragiola et al. [42]; (b) Hasselkamp et al. [52];

(c) Koyama et al. [56]; (d) Veje [57]; (e) Svensson and Holmen [43]

and Holmen et al. [47]. The dashed lines are interpolations to guide the eye.

3.0

7 2.0

-1.5

I I

- * KZT

i i i — r

r?J?\--V

i i i i

1 1 !

D C T Mo O Al A Ag A Cr • Au V Cu

-12 16

FIG. 8. Dependence of electron yield on projectile atomic number Z, for targets of C, Al, Cr, Cu, Mo, Ag and Au at a projectile energy of 30 keV. From Baragiola et al. [36].

on Al and on Ni [45] are also available, but the surfaces are likely to be covered with adsorbed gases.

Regrettably, these are all at far too high an energy to be valuable in fusion situations and the coverage does not give a clear picture of the variation with shell structure. Figure 10 shows the variation with velocity for emission from Al bombarded by a wide variety of

incident projectiles; this is perhaps the only recently published example of a broad ranging data set. The coefficients all increase generally linearly with the inci-dent particle velocity. In Fig. 11 we have collected the data on emission induced by metallic and C + ions on the same material as a function of energy. The general picture is that the data are fragmentary and confined to energies of incidence far in excess of those anticipated in fusion devices. There is no systematic method for extrapolation to lower energies nor for interpolation to other combinations.

It may be valuable to note that there should be a distinct threshold for kinetic ejection processes. An estimate due to Baragiola et al. [42], based on the

4.0 3.0 2.0

1.0 0.7

I I I I I

-

nf

I I I I I I I

O He V N +

i i

l l l

oo++

A Ne D Ar +

i I l l

-10 20 30 40 50 60 ^{70 80}

FIG. 9. Dependence of electron yield on target atomic number Z2

for projectiles H+, He + , N+, 0 +, Ne +, Ar+ at a projectile energy of 30 keV. From Baragiola et al. [36].

0.5

Velocity ( 1 0 ' c m / s )

FIG. 10. Electron yield for various projectile ions, D +, He*, B + , C*. N+, 0 + , F+, Ne + , S+, Cl + , Ar+, Kr*, Xe + , incident on Al, as a function of velocity (energies 1.2-50 keV).

From Alonso et al. [61].

7.0 6.0 5.0 4.0 3.0 2.0 1.0

Cu++Cu ,

/

/ /

Mo* + Mo /

ONi*+Ni

- L i - l J . 1 , 1 1 1 I 1 1 1

/ c++c

/ 1-2)

M\ kl

l 1 1 1

10 100

Energy (keVl

FIG. 11. Electron yield for C+ and metallic ions on like materials; C+ + C (shown divided by two) from Hasselkamp and Scharmann [40]; Al+ + AI from Svensson and Holmen [43], Ni+ + Nifrom Fehn [45], Cu+ + Cu from Holmen et al. [47]

and Mo⁺ + Mo from Telkovskij [67].