Laser-aided photodetachment is a well-established [119] technique for determining neg- ative ion densities and temperatures, although emphasis is placed on the former here. Photodetachment methods were initially developed in response to the desire to under- stand hydrogen negative ion sources, particularly for their use in energetic neural beam generation for thermonuclear fusion applications [140], but have been applied to other de- vices containing negative ion species, such as magnetron sputtering sources [120,137,141]. The fundamental principles of operation are discussed below.
4.2.3.1 Principles of operation
As the name suggests, laser-aided photodetachment involves firing a pulsed laser beam through a plasma discharge such that electrons are detached from negative ion species. In the system presented here, these photoelectrons are subsequently collected by a positively biased probe (with respect to the plasma potential) located in the vicinity of the laser beam and aligned parallel to the beam axis. Figure 4.13 shows a simplified version of this geometry. Here, the eclipse photodetachment method is employed, which involves ‘shadowing’ the probe using a blocking wire as shown in figure4.13. This prevents ablation of the probe surface by the laser beam, which can result in spurious current measurements [142].
As stated above, the collection probe is biased positively with respect to the plasma potential such that in the absence of a laser pulse the recorded probe current Ip is the
electron saturation current and directly proportional to the electron density, ne. The
incident laser pulse and the subsequent detachment of electrons from negative ions causes a temporary and local increase in ne. This increase in electron density, ∆ne, results in a
corresponding increase in the measured probe current, ∆Ip. Under the assumptions that
all negative ions in the collection volume are destroyed, all photodetached electrons are collected by the probe, and that contributions to ∆Ip come exclusively from photode-
tached electrons, then ∆ne = n−. The local value of electronegativity, α, can then be
determined directly: ∆Ip Ip = ∆ne ne = n− ne =α. (4.15)
4. EXPERIMENTAL APPARATUS
Figure 4.13. A schematic diagram showing the shadowing of the collection probe in a photodetachment set-up. The current collected by the positively biased probe can be obtained by measuring the potential drop across a known resistorR.
Furthermore, ne can be determined from the I-V characteristic of the probe allowing a
value for the local negative ion density,n−, to be obtained. Of course, the accuracy of this
measurement relies upon the validity of the assumptions mentioned above. It is crucial, therefore, that the correct choice of laser parameters are chosen.
The photon energy (hν) of the laser beam must be chosen such that photodetachment of electrons from relevant negative ions occurs but single photoionization of neutrals is avoided. Hence, Eaff < hν < Eiz where Eaff is the electron affinity of the species of
interest and Eiz is the ionization energy of any neutrals in the discharge. For atomic
oxygen, Eaff = 1.46 eV and Eiz = 13.6 eV, as such the second harmonic of a Nd:YAG
laser (λ = 532 nm, ν = 5.6×1014 Hz) can be used for the purpose of photodetachment of electrons from O− ions as (hν)Nd:YAG= 2.33 eV. It is worth noting thatEiz−Ar = 15.75
eV, hence this choice of laser is also valid for an Ar/O2 gas mixture as used in this thesis.
Detachment of O−2 ions would also occur, however, it has been reported that O−2 ions account for less than 2% of the total negative ion concentration [141].
It is also assumed that all negative ions in the beam path are destroyed, however, this is dependent upon not just the photon energy, but the energy density of the laser pulse, E/S, and the photodetachment cross section, σpd. The fraction of negative ions that
experience photodetachment following the laser pulse, as a ratio of the local negative ion density prior to the laser pulse is given as [119]
∆n− n− = 1−exp −σpd hν E S (4.16)
where, E is the laser pulse energy, S is the cross-sectional area of the beam and for the case of O− ions σpd = 6.5×10−18 cm2 [143,144]. For sufficient laser energy density,
4. EXPERIMENTAL APPARATUS
Nd:YAG laser detecting O− ions, the required laser energy density is approximately 300 mJ cm−2.
Figure 4.14. From left-to-right, the laser diameter,DL, is increased to cover a greater
proportion of the collection diameter, 2rc, and the measured ∆Ip in-
creases untilDL>2rc.
Another important laser parameter to consider is the beam diameter, DL. The elec-
trostatic probe has a finite collection radius, rc, given by [145]
rc≈rp+As (4.17)
whererp is the probe radius,sis the sheath width andAis a multiplication factor between
2 and 3. If DL < 2rc, increasing DL results in an increase in the measurement of ∆Ip
as more negative ions in the collection area undergo photodetachment (see figure 4.14). When DL = 2rc, the cross-sectional area of the discharge undergoing photodetachment
is equal to the probe collection area and so the measured value of ∆Ip saturates and no
longer increases for larger DL. Values of rc can be experimentally determined; Devynck
et al. [100] found that rc = 1−2 mm in a laser photodetachment set-up for measuring
negative ions in a volume H− source. It was found that any increase in DL beyond 2 mm
had no effect on the amplitude of ∆Ip. The laser-aided photodetachment diagnostic used
in this thesis was applied to a HiPIMS discharge operating in an Ar/O2 gas mixture, the
details of which are described in chapter 6.