# Negative Ion Surface Production: a comparison between the C12A7 electride and gadolinium

Broude Juan-Pablo, Raval Om, Inoue Shinsaku, Inokuchi Yuya, Nakano

Haruhisa, Wada Motoi, Sasao Mamiko, Layet Jean-Marc, Cartry Gilles

► To cite this version:

Broude Juan-Pablo, Raval Om, Inoue Shinsaku, Inokuchi Yuya, Nakano Haruhisa, et al.. Negative Ion Surface Production: a comparison between the C12A7 electride and gadolinium. 2026. hal-05489515

HAL Id: hal-05489515

https://hal.science/hal-05489515v1

Preprint submitted on 2 Feb 2026

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Negative Ion Surface Production: a comparison between
the C12A7 electride and gadolinium

Juan-Pablo BROUDE¹, Om RAVAL¹, Shinsaku INOUE², Yuya INOKUCHI²,
Haruhisa NAKANO³, Motoi WADA⁴, Mamiko SASAO⁴, Jean-Marc LAYET¹, Gilles CARTRY¹

¹Aix-Marseille Université, CNRS, PIIM, UMR7345, F-13013 Marseille, France

²Graduate School of Science and Engineering, Doshisha University, Kyotanabe, 610-0394, Kyoto, Japan.

³National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan.

⁴Organization for Research Initiatives and Development, Doshisha University, Kyoto 602-8580, Japan

Abstract

In the context of ion sources for Neutral Beam Injection (NBI) in magnetically confined nuclear fusion plasmas, negative ion production on two different surface materials exposed to low-temperature, low-pressure deuterium plasma is investigated. A novel electride material, promising for its potential to enable highly efficient negative ion production, is compared to gadolinium, a low work function metal, with the aim of highlighting both similarities and differences in surface-driven negative ion formation. For both materials, negative ion yields and surface work functions are measured. The lowest work function obtained for the electride material is 2.7 eV, higher than the previously reported value of 2.4 eV. No clear correlation is observed between the negative ion yield and the work function for the electride highlighting a behavior with respect to negative-ion production which is not the one of a metal. In contrast, gadolinium exhibits the expected behavior with a clear correlation between yield and work function. The lowest work function measured for gadolinium is 2.2 eV, a value comparable to that of caesium, the material currently used in NBI sources, and significantly lower than the tabulated value for pure gadolinium (2.9 eV), likely reflecting the influence of surface impurities.

# 1 Introduction

Negative ions are used in a variety of applications, including plasma propulsion [1], semiconductor processing [2], and particle accelerators [3]. Their use in controlled nuclear fusion, through Neutral Beam Injection (NBI) systems for tokamaks, constitutes a particularly demanding application [4].

In a high energy NBI system such as the one needed for ITER (1 MeV), negative ions play a crucial role. The large current requirements and long operation time expected of such a system mean that negative ion production is a key design consideration. Current designs use a Caesium (Cs) coating on the source surface. The low work function (2.1 eV) metal enhances negative ion surface production when in contact with the hydrogen plasma inside the source. Nonetheless, the use of Cs comes with a number of drawbacks. The high chemical reactivity of Caesium imposes its continuous injection in order to keep a fresh clean Cs layer on the source walls. It leads to a huge Caesium consumption (~5 - 10 µgs⁻¹ [5]) and to further diffusion and pollution of the acceleration stage which has a detrimental effect on the accelerator components and can result in parasitic beams and voltage breakdowns. The use of Cs implies a regular, restrictive and expensive maintenance of the ion source and beam line in a nuclear environment [6]. These issues complicate the operation of the high energy negative-ion based NBI systems for present day tokamaks such as ITER, and prohibit their use for future fusion power plants. Caesium free negative ion sources are therefore a topic of current interest and new materials are being investigated as a possible alternative to Cs coated metals.

Negative ions are produced on a surface when neutral or positive ions from the plasma come into contact with it, capturing an electron and becoming negatively charged. The effectiveness with which this process happens is mainly governed by two parameters. The difference in energy between the affinity level of the incoming particle and the Fermi level of the surface material, and the coupling between these levels. As a neutral atom approaches a metallic surface the affinity level is lowered due to the mirror potential, up to the point where it is close enough so that the affinity level is below the Fermi level of the surface material, and an electron can tunnel from the surface to the incoming atom and become captured by it. However, as the newly formed negative ion retreats from the surface, the affinity level raises again coming to the same level as the conduction band of the metallic surface where
there are empty spaces for the electron to tunnel back, thus destroying the negative ion. On an insulator, although the valence band is lower and the coupling must be stronger to achieve the electron capture, there is the band gap between the valence and conduction band. This means that the recently captured electron cannot tunnel back to the surface, resulting in a diminished negative ion destruction rate (Fig 10.2 of [7]). Some dielectric materials have been shown to be efficient negative ion producers, even compared to metals. Diamond, with a large band gap, has been proven to produce an efficient negative ion yield, comparable to that of graphite, a metallic material [6][8]. The current work was performed on a novel material, The [Ca24Al28O64]4+ (hereinafter referred to as C12A7). Its band structure (Fig. 1c drawn according to [9]) makes it the ideal candidate for negative ion production. Its intrinsic nanostructure where the normal lattice containing O2- has been replaced by free electrons, produces an occupied cage conduction band (CCB) above its valence band. The electride has an effective work function of just 2.4 eV [10] from the Fermi level in its cage conduction band, and also benefits from the insulating material structure due to the band gap between the cage and frame conduction bands. Hence, the electride material may combine the advantages of both metals and insulators: a high electron capture yield, similar to that of low work function metals, and a low electron loss rate, as observed in dielectric materials. This unique combination suggests a strong potential for achieving a high net negative ion production yield at the surface. In this paper, negative-ion yields and surface work-functions are measured for various experimental conditions for both the electride material and Gadolinium (Gd), a low work-function metal (tabulated work-function of 2.9 eV). The goal is to compare the electride material behavior with respect to the negative-ion surface production to the behavior of a low-work function metal. The experimental device and the methods are first presented with a focus on a newly introduced pulsed bias approach. Then negative-ion yields variations are studied versus work-function values for the electride material and gadolinium.

# 2 Materials and methods

## 2.1 Theoretical background

Negative ion production occurs through two distinct processes, backscattering of incoming particles and sputtering of previously implanted particles (Fig. 1 of [11]). In both these processes electron capture occurs as the particle is leaving the surface. The ionization probability is defined as the probability that an outgoing particle escapes the surface as a negative-ion. It has been largely studied in the past for metals. The first description, given by Hiskes [12], was presenting the ionization probability as the product of a negative-ion production term (electron transfer from the metal to the atom) times a loss term on the outgoing trajectory (electron transfer back from the negative-ion to the metal), both depending on the perpendicular velocity of the outgoing particle :

$$P_{iz} = (1 - e^{-\alpha/v_{\perp}})e^{-\beta/v_{\perp}} \quad (1)$$

For lower perpendicular velocity ($v_{\perp}$) the production term is equal to one and the probability to keep a negative-ion at an infinite distance from the surface (the ionization probability) is determined by the loss term:

$$P_{iz} = e^{-\beta/v_{\perp}} \quad (2)$$

Despite this model was rather empirical and its theoretical background was later shown to do not be correct, the function $P_{iz}$ as presented above proved to efficiently fit experimental data. Two models describing more accurately the resonant electron transfer from the metal to the atom were developed at that time, namely the probability model [13] and the amplitude model [14]. Both models were giving very close results except at low perpendicular velocity. The amplitude model, the more complete one, predicted a simple exponential dependence of the ionization probability on the work-function ($\Phi$), the electron affinity (EA) and the perpendicular velocity, in the limit of the low perpendicular velocity [14]: the dependence of $P_{iz}$ on the surface work-function ($\Phi$) could be simplified to:

$$P_{iz} \propto e^{-\frac{(\Phi - EA)}{\epsilon v_{\perp}}} \quad (3)$$

Where $\epsilon$ is a constant. Some authors [15] have shown that in some situations there is no dependence of the ionization probability on the perpendicular velocity, in particular in the case of a $M^-$ negative ion sputtered from an M metal:

$$P_{iz} \propto e^{-\frac{(\Phi - EA)}{\epsilon_0}} \quad (4)$$

The dependence of the ionization probability on the perpendicular velocity has been clearly demonstrated for $H^-$ produced from surfaces, but in the limit of low perpendicular velocity this dependence may fall and the ionization
probability might be better described by equation 4 than 3. This has been shown by Bernheim et al [15] for sputtered H⁻ ions from a cesiated NiCu alloy surface. For perpendicular velocity above 50km/s a clear exponential dependence on the inverse of the perpendicular velocity was established, but the results suggested that for lower velocity the ionization probability resembles the one of equation 4 with an ε₀ value of about 0.4 eV. Let us note here that other groups have demonstrated a perpendicular velocity dependence even for velocities below 50 km/s [16], the dependence might therefore be case dependent.

Coming back to the general case of equation 3, the perpendicular velocity v⊥ depends on the angle θ and energy E of the emitted negative ion. Therefore the flux of negative-ion emitted from the surface at E and θ for a given surface bias Vsurf can be written as:

$$ \phi_{NI}^{V_{\text{surf}}}(E, \theta) = [\phi_{B}^{V_{\text{surf}}}(E, \theta) + \phi_{S}^{V_{\text{surf}}}(E, \theta)] \times P_{iz}(E, \theta, \Phi(V_{\text{surf}}), EA) \quad (5) $$

Where φ<sub>B</sub><sup>V<sub>surf</sub></sup>(E, θ) and φ<sub>S</sub><sup>V<sub>surf</sub></sup>(E, θ) are defined as the the flux of backscattered and sputtered particles at an energy E and an angle θ [17] for a given surface bias V<sub>surf</sub> (the surface bias sets the positive ion impact energy and therefore the sputtering and backscattering yields). Note that the work-function is depending on the surface bias due to the possible changes of the surface state with positive-ion impact energy. Note also that the ionization probability dependence with the electron-affinity EA can be omitted since we are only dealing here with hydrogen negative-ions. In previous papers [18][19][20], the collection of the negative ions emitted by the surface was studied and it was shown that the collection probability P<sub>coll</sub> was depending on the negative-ion angle and energy of emission as well as on the surface bias voltage V<sub>surf</sub> defining the characteristics of the sheath in front of the sample. The total flux of measured negative-ions by the mass spectrometer can then be written as:

$$ \phi_{NI}^{V_{\text{surf}}} = \iint_{E,\theta} [\phi_{B}^{V_{\text{surf}}}(E, \theta) + \phi_{S}^{V_{\text{surf}}}(E, \theta)] \times P_{iz}(E, \theta, \Phi(V_{\text{surf}})) \times P_{coll}(E, \theta, V_{\text{surf}}) dE d\theta \quad (6) $$

Once divided by the incoming positive ion flux, one gets the measured negative-ion yield Y<sub>NI</sub> as a function of the backscattering Y<sub>B</sub>(E, θ) and sputtering yields Y<sub>S</sub>(E, θ):

$$ Y_{NI}^{V_{\text{surf}}} = \iint_{E,\theta} [Y_{B}^{V_{\text{surf}}}(E, \theta) + Y_{S}^{V_{\text{surf}}}(E, \theta)] \times P_{iz}(E, \theta, \Phi(V_{\text{surf}})) \times P_{coll}(E, \theta, V_{\text{surf}}) dE d\theta \quad (7) $$

The variations of the measured relative negative-ion yield have been studied in the past as a function of several experimental parameters, such as surface temperature, isotope type (H₂ or D₂ plasmas), and the impact energy of positive ions on the surface (set by the surface bias V<sub>surf</sub>). For the latter parameter in particular, it is difficult to separate the influence of the ionization probability P<sub>iz</sub> on the negative-ion yield from the effects of backscattering and sputtering changes and variations in collection efficiency. Indeed, the angular and energy distributions f(E, θ) of backscattered and sputtered particles change with the positive-ion impact energy, as do the collection and ionization probabilities of negative ions, a situation already complicated even without considering the changes in the surface work function, and thus on the ionization probability P<sub>iz</sub>; caused by ion impact at different energies. Therefore, changing the sample bias can affect all factors simultaneously, making interpretation challenging. The effects can be unraveled but assumptions must be made and is a time consuming process [17] making it difficult to study the influence of surface state on the negative-ion yield.

In this paper we propose an original method to simplify negative-ion surface production studies. As in previous studies, different DC bias voltages V<sub>bias</sub> are applied to the surface, modifying its work function as a result of changes in the energy of incident positive ions. However, in the present study, a short measurement pulse is superimposed onto the DC bias (Fig. 2). This pulse is sufficiently brief to minimally disturb the surface state, which remains primarily determined by the applied DC bias. Negative ions are measured exclusively during this short pulse. Importantly, the pulse voltage V<sub>ext</sub> is kept constant regardless of the value of V<sub>bias</sub>, thereby ensuring identical extraction and production conditions. This results in the same collection efficiency as well as identical sputtering and backscattering yields—across all DC bias values V<sub>bias</sub>. This pulsed bias approach enables the surface state, and therefore ionization probability, to be changed while keeping the other parameters influencing surface production constant. The equation above can be re-written to introduce V<sub>bias</sub> and V<sub>ext</sub>:

$$ Y_{NI}^{V_{\text{bias}}, V_{\text{ext}}} = \iint_{E,\theta} [Y_{B}^{V_{\text{extr}}}(E, \theta) + Y_{S}^{V_{\text{extr}}}(E, \theta)] \times P_{iz}(E, \theta, \Phi(V_{\text{bias}})) \times P_{coll}(E, \theta, V_{\text{ext}}) dE d\theta \quad (8) $$
Let us note that in the present experiments and in most of our previous experiments [21, 20, 18, 17, 11, 6, 9], the negative-ion perpendicular velocities hardly exceed 50 km/s. Let us note also that our previous modeling of negative-ion energy distribution functions [20, 19, 18] formed from carbon surface did not show any dependence of the ionization probability with the perpendicular velocity. Equation 4 might therefore be more appropriate for analyzing the present experiments than equation 3 and the previous equation can be rewritten as:

$$Y_{NI}^{V_{bias}, V_{ext}} = P_{iz}(\Phi(V_{bias})) \iint_{E,\theta} [Y_{B}^{V_{extr}}(E,\theta) + Y_{S}^{V_{extr}}(E,\theta)] \times P_{coll}(E,\theta, V_{ext}) dE d\theta \quad (9)$$

In equation 9 when changing the surface DC bias only the first term is modified, the integral term stays the same, therefore allowing for a simple analysis of the correlations between work-function and negative-ion yield. Studying the correlation between negative-ion yield and the work function is of interest to determine if the material is behaving as a dielectric-like or a metal-like material [8]. For metals the correlation between work-function and negative ion yield is well established both theoretically and experimentally [22][23]. For dielectric, due to different surface production mechanisms and in particular to the strong importance of the band gap, such correlation is not expected. This kind of study is particularly well suited in the case of the electride material which is presenting original electronic properties.

## 2.2 Experimental Setup

The setup shown on figure 1a has been previously described in detail [24]. It consists of an inductively coupled plasma produced by a radio frequency (RF) generator (Huttinger PFG 1600, 13.56 MHz) coupled to a three turn planar coil through a matchbox (Huttinger PFM 3000 A matchbox). The produced plasma diffuses through a grounded mesh (285 µm spacing, 85 µm hole size), to reduce electron temperature and prevent fluctuations on the plasma potential [24] into the diffusion chamber where the sample is placed. The base pressure inside the chamber is held at about $3 \times 10^{-8}$ mbar. Deuterium gas is inserted at 7.4 sccm using a mass flow controller (Brooks 5850 TR), plasma was produced at 2 Pa, the pressure is controlled by an adjustable gate valve placed between the main chamber and a turbo molecular pump (Alcatel ATP 400) connected to a scroll pump (Edwards nXDS10i). A Hiden EQP 300 mass energy spectrometer placed in front of the sample at a distance of 3.7 cm was used to measure negative-ions as well as positive-ions. At 2 Pa and 150W, the main ion is $D_3^+$ (~80%) followed by $D_2^+$ (~18%) and traces of $D^+$ (2%) [17]. When a bias is set onto the sample ($V_s$), the positive ions bombard the sample at an energy equal to the difference between the plasma potential and the sample bias. However, $D_3^+$ dissociates at impact and the fragments share the energy thus reducing the impact energy by a factor 3. A Langmuir probe from Impedans Ltd was used to determine the plasma parameters. At 2 Pa and 150W, the Langmuir probe placed close to the sample holder, but not in the exact region of interest between the mass spectrometer and the sample, gives $n_e = 10^9$ cm$^{-3}$, $T_e = 0.2$ eV and $V_p = 5$ V. These values can be considered as a rather good estimation of the plasma parameters in front of the sample. Finally, Photoemission Yield Spectroscopy (PYS) was used to measure the sample's work function under the different experimental conditions. The set-up consists of a tunable light source (Zolix TLS, 200-2200nm) whose photons are focused onto the sample through a quartz viewport. The set-up has been described more in details previously [8] and the analyzing methods are discussed in this paper.

## 2.3 Experimental campaign

The first experimental campaign was conducted in the summer of 2023. During the 2023 campaign, two electride samples from two different batches were tested. The second campaign was conducted in summer 2024 with a sample from a third batch. Results from both campaigns and all three batches are compared. The experiments on gadolinium were conducted in 2023.

## 2.4 Pulsed and DC Bias Approach

The negative ions produced at the sample surface are accelerated toward the plasma by the sample sheath and, owing to the low operating pressure, can reach the opposite sheath in front of the grounded mass-spectrometer aperture. When a negative DC bias is applied to the sample, the ions gain sufficient energy to overcome the potential barrier of the mass-spectrometer sheath and can thus be collected. Changing the surface bias allows setting different surface states through the modification of the positive-ion impact energy, and thus allows studying the influence of the surface state on the ionization probability. However, as explained in the introduction, changing
<figure><img src="image_2.png" /><figcaption>Figure 1: Schematics of the experimental setup and electride band structure</figcaption></figure>

the DC bias affects not only the surface state but also the collection efficiency and the backscaterring and sputtering yields [17], making it difficult to disentangle the various parameters influencing the negative-ion yield.

Short pulsed biases have also been used in the past to study negative-ion production. Their purpose was to extract negative ions while preserving the surface state: the pulse was long enough to enable detection with the mass spectrometer, yet short enough (20 µs at 1 kHz) to perturb the surface only minimally [24]. However, the surface state could not be varied this way.

In the present work, we combine both approaches. A DC bias is applied most of the time (98 %) to establish a well-defined surface state, while a short pulse (2 %) is superimposed to extract and synchronously measure the negative ions without altering the surface properties primarily set by the DC bias (Fig. 2).

DC biases have been varied from +20 to -60V while only two different extraction pulse voltages have been used, -20V and -60V. The first one gives a situation in which negative-ion production through sputtering is minimized (the sputtering threshold was estimated to be close to 20 eV for carbon materials [17] and should be higher in the present situation), while for the second one a sputtering contribution to the negative-ion production can be expected [9]. From a practical point of view, a high voltage summator circuit sums up the DC and pulse voltages coming from two voltage power supplies and its output is applied to the sample holder. The extraction pulse is lasting 20 µs at a frequency of 1 kHz (duty cycle = 2%). Only negative-ions produced during the last 10 µs of the extraction pulse are measured by triggering appropriately the mass spectrometer [21].
<figure><img src="image_3.png" /><figcaption>Figure 2: Pulsed bias approach scheme (drawn to scale). Left column shows a -20 V extraction pulse for a -10 V (top) and -60 V (bottom) DC bias, while right column shows a -60 V extraction pulse for a -10 V (top) and -40 V (bottom) DC bias. Insert is a zoom of the pulse area. The period is 1000 µs while the extraction pulse lasts for only 20 µs. Negative ion detection happens only during the last 10 µs of the pulse to avoid the period of the sudden voltage change.</figcaption></figure>

## 2.5 Positive Ion Current

To compute the negative ion yield, it is necessary to determine the positive ion flux (or current) impinging on the sample. This ion current depends on the applied bias which influences the sheath expansion around the sample holder and alters the overall plasma equilibrium. Simply measuring the sample current under DC bias conditions is insufficient to accurately assess the ion current during a short bias pulse at the same voltage. Indeed, during a short pulse, a transition occurs between an undisturbed state (pulse off) and a disturbed state (pulse on), resulting in an average ion flux that differs from that observed under steady DC bias. For example, previous studies have shown that the positive ion flux onto the sample is higher during a 20 µs short pulse at a bias of -20 V or -130 V (with 0 V between pulses) compared to a continuous DC bias of -20 V or -130 V [25]. This difference was attributed to the perturbation of the plasma by the DC bias which creates a sheath in front of the sample significantly reducing the plasma volume between the sample and the mass spectrometer and lowering the plasma density. Of course, if the pulse duration is extended, the positive ion current eventually reaches the value corresponding to the DC bias case, typically after a few tens of microseconds.

For all these reasons, a measurement of the sample positive ion current during the extraction pulse has been performed for all bias schemes by using a copper sample isolated from the sample holder by kapton tape as shown in figure (1b). The bias is applied both on the sample and the sample holder but the current is only measured on the
sample. The sample holder can be seen as a guard ring commonly employed for planar probe measurement [26]. This method is ensuring that the sample current is measured in a situation where the sheath in front of the sample is identical to the negative-ion measurement situation. The method was previously introduced and described in [24]. The current is measured using a home made high voltage trans-impedance converter, and output is observed on a scope. Measurements are performed both with and without plasma in order to identify and subtract the capacitive current contribution. The resulting current is then averaged over the last 10 µs of the pulse, corresponding to the negative-ion measurement period.

Due to the Kapton tape used for the copper sample insulation from its holder and clamp, this method cannot be applied at high surface temperatures. To assess if the positive ion sample current is depending on the sample temperature, the time resolved positive ion saturation current density of a Langmuir probe was measured. The probe was placed as close as possible to the sample while maintaining enough distance so that the sample sheath would not interact with the Langmuir probe. Fig. 3 displays the ion saturation current density profile at room temperature measured. It can be clearly observed that for a given DC bias on the sample, the ion saturation current density is disturbed by the application of the extraction bias pulse. It shows again that the incoming positive ion flux onto the sample during the measurement pulse at $V_{ext}$ is not equal to the one that would be measured for $V_{bias}$ = $V_{ext}$ (compare for instance the case -40/-60 case, corresponding to $V_{bias}$ = -40V, $V_{ext}$ = -60V with the case -60/-60 corresponding to $V_{bias}$ = -60V with no extraction pulse). The mean of the data acquired from 10 µs to 20 µs gives the average ion saturation current per unit area [$A/m^2$] during the negative-ion measurement period.

Note that the "sample current" is equal to the "ion saturation current density" times the surface collecting the ions reaching the sample. This surface is defined by the sheath shape around the sample and sample holder. As this sheath might not be perfectly planar, the collection surface might change with the experimental conditions and is not easy to determine. Therefore the "ion saturation current density" measured by the Langmuir probe can only give an estimate of the "sample current" variations with the experimental conditions. For that reason the ion saturation current density (measured by Langmuir probe) and the sample current (measured by the isolated sample method) variations are compared at room temperature after normalizing both quantities at 1 4. The Langmuir probe ion saturation current density during the measurement pulse displays the same trend as the sample current measured with the isolated sample technique. This validates the use of the Langmuir probe to monitor possible changes of the sample current with surface temperature. The measurements versus surface temperature have been performed under only two biasing schemes ($V_{bias}$ = 0V and $V_{ext}$ = -20V or $V_{ext}$ = -60V). Tab. 1 summarizes the measurements. When the temperature is increased the positive ion saturation current density measured by the Langmuir probe shows a slight tendency to decrease, by no more than 15% over the whole temperature range. This decrease will be taken into account when displaying the negative-ion yield.

<table><caption>Table 1: Positive-ion saturation current density [$10^{-2}$ A/m$^2$] measured by Langmuir probe during the extraction pulse for various surface temperatures, a DC bias of 0V and two different extraction pulse bias $V_{ext}$ = -20V and $V_{ext}$ = -60V.</caption><thead><tr><th>Temperature [°C]</th><th>0/-20</th><th>0/-60</th></tr></thead><tbody><tr><td>40</td><td>19.8</td><td>13.7</td></tr><tr><td>150</td><td>18.5</td><td>13.4</td></tr><tr><td>300</td><td>17.9</td><td>13.2</td></tr><tr><td>450</td><td>17.9</td><td>12.9</td></tr><tr><td>600</td><td>17.5</td><td>13.1</td></tr><tr><td>750</td><td>16.8</td><td>12.6</td></tr></tbody></table>

## 2.6 Negative-ion energy and angular distribution of emission

The mass spectrometer collects negative-ions according to their mass and to their energy, giving as output the $D^-$ negative-ion energy distribution function. Due to the mass spectrometer reduced angle of acceptance this setup does not allow to probe all the negative-ion angular distribution of emission, in particular high emission angle surface produced negative-ions [6] cannot be measured. In order to ensure that the negative-ion counts measured by the mass spectrometer are representative of the negative-ion flux, and not only low emission angle negative-ion flux, negative-ion energy distribution functions were measured at different tilt angles (Fig. 5). Indeed, by tilting the sample it is possible to probe a different part of the negative-ion angular distribution [6]. As the tilt angle is increasing one can observe that the low energy ions cannot be measured anymore showing indirectly that a different
<figure><img src="image_4.png" /></figure>

Figure 3: Ion saturation current density of the Langmuir probe versus time for different sample biasing conditions ($V_{bias}$, $V_{ext}$). Top plot shows the change in ion saturation in -20V extraction bias pulse, and bottom shows -60V extraction bias pulse.

<figure><img src="image_5.png" /></figure>

(a) Extraction bias = -20V. Data is normalised at -20V (b) Extraction bias = -60V. Data is normalised at -60V data point.

Figure 4: Comparison of the sample ion current measured by the isolated sample technic with the ion saturation current density measured by the Langmuir probe. Both measurements are averaged of the last ten micro-seconds of the extraction pulse
<figure><img src="image_6.png" /><figcaption>Figure 5: Negative ion energy distribution for different sample tilt angles with biasing scheme (0/-60V) (no DC bias applied, -60 V extraction pulse)</figcaption></figure>

part of the distribution is measured. The energy distributions have been integrated to produce a total counts of negative-ions. The results are displayed on Fig. 6. It can be observed that the same trend is present at normal incidence as well at the other angles. Therefore, it is enough to consider only normal incidence, there is no loss of information when neglecting higher emission angle negative ions under the present experimental conditions.

## 2.7 Photoemission yield spectroscopy

Work function measurements were performed using Photoemission Yield Spectroscopy (PYS) [8]. Briefly, white light from 200 nm to 2200 nm is emitted by a 300 W xenon lamp and directed to a monochromator (Zolix Omni-λ 300i), which selects a specific wavelength (λ) with a ±5 nm resolution. The monochromatic light then passed through an optical fiber, a 35 Hz chopper, focusing optics, a quartz viewport installed on the diffusion chamber, an aperture into a collection electrode before reaching the sample.

The photon wavelength is scanned from low to high wavelengths, and as long as the photon energy exceeds the work function, photo-electrons are emitted from the surface, generating a measurable sample current. The collection electrode is biased at 5 V to collect electrons emitted from the sample surface. This electrode also helps limit illumination to the analyzed area, allowing for accurate sample photo-current measurements [8]. The electrode is moved along a rail inside the chamber and is positioned as close as possible to the sample without contact.

The sample current is converted into a voltage signal using a Keithley 6482 ammeter and fed into a Zurich Instruments MFLI lock-in amplifier to detect its 35 Hz component, representing the photoemission signal, while suppressing external noise.

PYS measurements is carried out immediately after plasma exposure, since acquisition during plasma operation is impossible due to the strong background current and detector saturation. Each scan consists of photocurrent measurements taken every 2 nm and averaged over one second. Typically, about 10 s elapses between the end of the plasma discharge and the start of the wavelength scan, one complete scan across 250 nm lasts 125 s, and two successive scans are separated by at minimum by 25 s. The background current is measured at the end of each scan by closing the lamp shutter and recording the remaining 35 Hz signal. A typical background current value is around 80 fA.

Raw measurements are obtained as a function of wavelength and then converted as a function of photon energy $E = hν$. The averaged measured background current is then subtracted to the total current resulting in the photoemission current which is then divided by the spectrally dependent incident light flux onto the sample (in units of photons per seconds). The latter was determined using a combination of a photodiode and a thermal detector placed at the location of the sample. The resulting quantity ($Y_{ph}$) is the number of electrons per incident photon on the sample (given in arbitrary units in the present study):
<figure><img src="image_7.png" /></figure>

(a) Electride material. At each sample DC bias V<sub>bias</sub> the negative-ion counts have been measured versus sample tilt angle and are displayed in a normalized way for comparison purpose. The extraction pulse bias used was V<sub>ext</sub> = -60 V and the sample was at room temperature

<figure><img src="image_8.png" /></figure>

(b) Gadolinium material. At each sample temperature the negative-ion counts have been measured versus sample tilt angle and are displayed in a normalized way for comparison purpose. The biasing scheme is (0/-60 V) (no DC bias applied V<sub>bias</sub> = 0, V<sub>ext</sub> = -60 V extraction pulse)

<caption>Figure 6: Influence of the sample tilt angles with respect to mass spectrometer axis (0°, 5°, 10°, 15°) on measured negative-ion total counts. D<sub>2</sub> plasma, 2 Pa, 150 W.</caption>

$$Y_{ph}(E) = \frac{I(E) - B}{\phi_{inc}(E)} \qquad (10)$$

Where B is the mean background, E is the energy of the incident light, and $\phi_{inc}$ is the incident flux of photons.

The material work function was determined using two methods; first by plotting log(Y<sub>ph</sub>(E)) versus photon energy, thereafter identifying the fall of the photocurrent below the background current by a discontinuity in the plot. This method is named as graphical method since the work-function is simply determined on the plot log(Y<sub>ph</sub>(E)). This method is very efficient to get rapidly the value of the work-function. The second method is using Fowler theory [27] describing the current rise versus photon energy for materials with free electron on the valence band, that is metals. The C12A7 material has such free electrons on its cage conduction band, as well as Gd of course. Data were fitted according to a truncated Fowler law:

$$Y_{ph}(E) = A \left( \frac{\pi^2}{6} + \frac{1}{2} \left( \frac{E - \Phi}{k_B T} \right)^2 - e^{\frac{E-\Phi}{k_B T}} \right) \qquad (11)$$

where E is the photon energy, T the surface temperature and Φ the work-function. The fit's lowest bound is taken as close to the threshold as possible but still sufficiently above to ensure that the photocurrent is high enough and dominates the amplitude of the background fluctuation. The energy span for the fit is always lower than 0.8eV to remain close enough to the threshold in the validity region of the Fowler law [27].

Fig. 7 shows the data comparison between both methods. The difference in the values is lower than 4% and the trends are similar. This is giving confidence in the work-function estimation since both methods have inherent (and different) limitations. The determination of the photo-current threshold in the graphical method is affected by the minimum measurable current (ie the background current). On the other hand, Fowler's theory was originally developed for the analysis of photoelectric sensitivity curves of clean metals [28]. Therefore, its application to the electride material is not fully justified, and it might also be questionable for gadolinium if the surface is contaminated by oxygen, hydrogen, or hydroxyl radicals, for instance. Moreover, Lange et al. studied work-functions of metals with oxygen adsorption and suggested to fit the photon yield retaining the dominant term of equation 11 ($\frac{E-\Phi}{k_B T}$)² but changing its exponent by a fitting parameter n [29].
<figure><img src="image_9.png" /><figcaption>Figure 7: Work function determination from photoemission measurements: comparison of Fowler fit and graphical determination. 2023 electride campaign</figcaption></figure>

$$Y_{ph}(E) = A' \left( \frac{E - \Phi}{k_B T} \right)^n \qquad (12)$$

They showed good agreement with the measurement results using Kelvin method. In our previous paper, we explored fitting photoemission curves of electride samples with exponent factor n ranging from 2.4 to 2.8 [30]. A slightly better agreement was obtained with an exponent n=2.7. In the present paper, as we are mostly interested by studying the work-function changes and their influence on the negative-ion yield rather than determining with high accuracy the work-functions, and for a sake of consistency between both materials, all work-functions have been determined using the simple graphical method, except for the time resolved measurements of (Fig. 9) for which details are given later.

## 2.8 Sample Preparation

Samples were introduced into the plasma chamber through a load lock system. Samples are first annealed at 400 °C to outgas impurities thanks to a heated tungsten filament at the back of the sample. The Electride samples have followed a slightly different annealing cycle according to the manufacturer recommendations for sample preparation and according to the previous experimental campaign [31][9][32] The electride sample was heated up to 700° C in 100° intervals, letting the sample cool between each heating phase.

The samples then undergoes Argon (Ar) sputtering (-60 V, 0.2 Pa, 400 W) to clean away the top layer and obtain a clean, pristine surface. Argon sputtering was used also to return to the same clean surface state after several experiments with the sample. The optimal work function for both the electride and Gadolinium samples were obtained after Ar sputtering.

The work function for the electride obtained after Ar sputtering was ~2.6 eV (Fig. 8a). A subsequent exposure to a D₂ plasma 150 W 2 Pa with the electride sample left at ground potential led to a work-function of about 3.3 eV. This value was used as a starting point for all measurements presented here. The previously reported 2.4 eV work function could not be obtained under these experimental conditions.

The work function measured for the Gadolinium sample after argon sputtering, approximately 2.25 eV, was consistently lower than the tabulated value of 2.9 eV [33]. This value is remarkably close to that of caesium, the metal with the lowest known work function (2.1 eV). The origin of this reduced work function is not yet understood and will be the subject of future investigations. For Cs, it is known that the presence of a surface oxide can produce a significantly lower work function than that of the pure metal [34, 35]. A similar effect may also occur for Gd.
<figure><img src="image_10.png" /><figcaption>Figure 8: Photoemission current curves obtained after Argon sputtering at -120 V, 0.2 Pa, 400 W</figcaption></figure>

# 3 Results and Discussion

## 3.1 Work Function Evolution

The work function was measured through PYS directly after plasma exposure. Several successive full wavelength scan acquisitions were taken to track a possible time evolution of the work function after plasma stop. The measurements showed a time evolution of the threshold (Fig. 9a). As the photoemission scan takes about two minutes to complete, and the threshold (i.e. work function) is only measured at the end of the scan (scan are performed from high to low photon energies), the work function measured might be different from that during plasma exposure and negative ion production. In order to know the scale of the difference between the value during plasma exposure and the first measurement, the time evolution of the work function was evaluated. To that aim the change of photocurrent in real time after plasma exposure was measured at three specific wavelengths, namely 340, 350 and 360 nm, in three consecutive identical experiments and used to determine the work function time evolution using a simplified Fowler theory [30].

$$Y_{ph}(E) = A' \left( \frac{E - \Phi}{k_B T} \right)^2 \qquad (13)$$

The wavelengths were chosen to be close enough to the threshold so that the simplified Fowler theory is still relevant but still far enough so that the signal dominates over the background for the duration of the measurement. Fig. 9b shows the results obtained. The red circles represent the work function as obtained by consecutive PYS full wavelength scans (Fig. 9a) while the blue asterisks represent the result of the fit obtained from the three-wavelengths method which obviously overestimates the work function. This is most likely a result of the use of only three wavelengths for the Fowler fit, but can also be partially attributed to the use of a simplified function with a fixed exponent factor equal to 2 (see discussion in section 2.7). The results of three-wavelengths method shifted by -0.3 eV are shown by red asterisks in Fig. 9b. Although a discrepancy is observed for the absolute value of the WF, obviously both methods show good agreement in the overall time evolution. The work function shows little change between the end of the plasma exposure and the first threshold measurement performed using the full wavelength scan method. This first threshold measurement can therefore be considered representative of the WF value during plasma exposure.

## 3.2 Negative Ion Yield

Fig. 10 shows the surface bias dependence of the work function of C12A7 electride (a) and the negative ion yield (b). The measurements were carried out in two different experimental campaigns in 2023 and 2024 and repeated in each campaign. The error bars indicate the standard error of data reproducibility. The work function of the electride sample was varied by applying a DC surface bias $V_{bias}$. The lowest work function was observed at around the floating potential with a value of about 3.1 eV, higher than the 2.4 eV lowest achievable work-function for
<figure><img src="image_11.png" /><figcaption>(a) Photoemission yield from consecutive PYS scans after plasma exposure. Scan acquisition starts $t$ seconds after the end of plasma exposure.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>(b) WF time evolution. Blue asterisks: WF determination by the three wavelength method and Fowler fit. Red circles: WF determination by full wavelength scans and Fowler fit. Red asterisks = blue asterisks shifted by -0.3 eV</figcaption></figure>

Figure 9: C12A7 electride (sample 1) work function time evolution after 10 min $D_2$ plasma exposure (150 W, 2 Pa).

the electride material [10], and higher than the value obtained after argon sputtering (2.6 eV). This reveals the influence of deuterium adsorption or implantation on the electride work-function. In the positive bias region, the work function increased a lot and the results present much more dispersion (as can be seen from the larger error bars). In the negative bias region, increasing the surface bias on the sample was found to increase the work function from about 3 to 3.5 eV or even above. Fig. 10 (b) shows the negative-ion yield for an extraction voltage of -60V ($V_{ext} = -60$V applied on top of the DC bias $V_{bias}$ displayed on the x axis, as per the pulsed bias approach previously described). The negative ion yield does not present any clear trend with the surface bias. Referring to equation 9, the only parameter that affects the negative-ion yield here is the ionization probability since only the $V_{bias}$ value is changed. According to equation 4, and assuming $\epsilon_0 = 0.4$ eV [15], an increase of the surface work function from 3 eV to 3.5 eV should reduce the negative-ion yield by approximately a factor 3.5 for a metallic surface. Obviously, the electride surface exposed to the deuterium plasma is not behaving as a metallic surface. Figure 11 (a) is presenting the results of figure 10 in a different manner where the negative-ion yield is plotted as a function of the work-function, this time for both extraction voltages investigated ($V_{ext} = -20$ and $-60$V). No clear correlation between negative-ion yield and work-function can be observed. Figure 11 (b) displays again the negative-ion yield versus the work-function but this time for the gadolinium sample. For gadolinium we did not observe any dependence of the work-function with the surface bias. However, we noticed that exposure at floating potential allows to progressively increase the work-function. So the following experimental protocol was followed. First the sample was sputtered in argon, the lowest work-function was obtained and a negative-ion measurement was performed in deuterium plasma at a DC bias of 0 V and an extraction voltage of -60V (or -20V). Then the sample was exposed to deuterium plasma at floating potential for a certain period of time and a new measurement of negative-ion yield was performed at a DC bias of 0 V and an extraction voltage of -60V (or -20V). In between each steps, the surface work-function was measured. The change of work-function under floating potential exposure is not understood but might be attributed to impurity sticking onto the surface. Figure 11 (b) shows a clear correlation between the negative-ion yield and the work-function, the first one is decreasing when the second one is increasing, as expected for a metallic surface. Referring to equation 9 the only parameter changing during this set of experiments at a given extraction voltage is the ionization probability which is depending on the work-function. The data of figure 11 have been fitted according to equation 9 using the ionization probability expression of equation 4. The pre-exponential factor of the fit represents the integral term of equation 9. An overall rather good agreement is obtained with a value of $\epsilon_0 = 0.64$, although we can see that for the highest work-function obtained the fit is not correct. Nonetheless, the purpose here is not to accurately describe the theory behind the negative-ion surface production but rather to
highlight the fact that the surfaces are behaving very differently, the gadolinium surface behaving as expected for
a metallic surface while the electride is not.

<figure><img src="image_13.png" /><figcaption>Figure 10: Surface bias dependence of the work function of C12A7 electride (a), and negative ion yield (b). Deuterium plasma 2 Pa, 150 W.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 11: Negative ion yield versus surface work function. Deuterium plasma 2 Pa, 150 W. Solid lines are fits according to Eq. 9</figcaption></figure>
# 4 Conclusion

We investigate negative ion surface production in low pressure low temperature hydrogen plasmas. In our usual measurement configuration, the sample on which negative-ion are produced is DC biased so that negative ions produced at the surface are self-extracted through the plasma sheath toward the detector facing the sample. However, changing this DC bias simultaneously alters the backscattering and sputtering probabilities of the incoming ions, the ionization probability at the surface—because the electronic properties are highly sensitive to ion bombardment energy, and the extraction probability of the emitted negative ions, so that all key parameters are modified at once and the individual effects are difficult to disentangle. In this work, a dedicated experimental methodology was developed to investigate negative ion surface production on candidate materials for caesium-free ion sources. The key feature of the approach is the use of a short extraction pulse superimposed on a DC bias applied to the sample. The DC bias defines the positive-ion impact energy and thus the surface state and work function, while the extraction pulse is kept identical for all conditions. Under these conditions, the backscattered and sputtered yields, as well as the collection efficiency, are effectively frozen, and only the ionization probability is modified through changes in the work function. This pulsed-bias scheme therefore provides a simple and robust way to probe the correlation between work function and negative-ion yield, which is usually difficult to extract when using only DC bias variations.

Applied to the C12A7 electride, this strategy revealed a behavior that clearly departs from that of a conventional metal. Under deuterium plasma exposure, the electride work function could be tuned in the range 3–3.5 eV by changing the DC bias, yet the negative-ion yield showed no clear dependence on the work function for either extraction voltage investigated. Similarly, the temperature scans showed that large changes in work function are accompanied only by modest variations of the negative-ion yield, while a strong increase in yield is observed for relatively small changes in work function. Taken together, these results indicate that, for the electride, the work function alone is not the main parameter governing negative-ion production. The particular electronic structure of C12A7, with its cage conduction band and possible trapping or localization effects, likely leads to surface production mechanisms that cannot be captured by a simple metallic picture based solely on ($\Phi - EA$).

In contrast, the gadolinium sample behaves as expected for a low work function metal, the negative-ion yield exhibits a clear anti-correlation with the work function. When the work function is progressively increased by plasma exposure at floating potential, the yield decreases in a way that can be reasonably fitted by an exponential dependence of the ionization probability on ($\Phi - EA$), in line with simplified models derived for metallic surfaces. Interestingly we have measured a work-function of the gadolinium surface of 2.3 eV after argon sputtering, close to bulk cesium (2.1 eV), while the tabulated one for pure gadolinium is 2.9 eV. We think that this might be due to oxygen impurities on the surface creating an oxide with a lower work-function than the pure material such as already observed for caesium. Although the possible effect of D/H atoms embedded near the Gd surface cannot be excluded.

Beyond the specific case of C12A7 and gadolinium, this work shows that the pulsed-bias approach is a powerful tool to decouple parameters affecting negative-ion measurements, and can be readily extended to other candidate for fusion negative-ion sources. On what concerns C12A7 and gadolinium, future work will focus on clarifying their electronic structure and their surface chemical composition when exposed to hydrogen plasma to better understand their work-function variations.

# 5 Acknowledgments

The authors would like to thank H. Hosono for the use of C12A7-Electride. The C12A7-Electride was supplied from AGC Inc. Technical support by Naomichi Miyakawa, and Satoru Watanabe of AGC Inc. as well as deep discussion with them are greatly acknowledged. The authors would like to Aix-Marseille University for the appointment of Pr. Mamiko Sasao as a visiting researcher at PIIM laboratory in 2023. This work has been carried out within the framework of the French Federation for Magnetic Fusion Studies (FRFCM). The project that gave rise to this publication received funding from the Initiative of Excellence of Aix-Marseille University — A\*Midex, Investissements d'Avenir program AMX-19-IET-013. This project has also been supported by the Région Sud PACA through the "Appel à projets recherche 2021 - Volet exploratoire" — project SARDINE. This research was funded by the Agence Nationale de la Recherche (ANR) under project "ANR-23-CE30-0025-01".
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