Field emission properties of nano-tendril bundles formed via helium plasma exposure with various additional impurity gases

Nano-tendril bundles (NTBs) were formed on tungsten via helium (He) plasma exposure with various additional impurity gases, such as neon (Ne), nitrogen (N2), and argon (Ar). The sizes of the NTBs showed different distributions with different additional impurity gases. The field emission property of the NTBs formed with various additional impurity gases was measured. The field-emission property was significantly affected by the morphology of the NTBs, especially the tips of the fibers. In the Ne- and Ar-seeded cases, the NTBs were formed with sharp tips, and the onset electric field for field emission was ∼1 kV mm−1 for all the NTB samples. The Ne-seeded samples showed the most rapid increase in the emission current. In the N2-seeded case, two types of NTBs were formed. The NTBs were formed with sharp tips when the ratio of N2 impurity gas was 2.1%. With an increase in the ratio to 3.0% or higher, the fibers of the NTBs became thicker and the tips became rounder. In the Fowler–Nordheim (F-N) plot analysis, the field enhancement factors were approximately 6000–7000 without significant differences, for all NTBs with sharp tips. NTBs with round tips showed totally different field emission property, as the emission current was only several μA, which is one-tenth of that for the other samples. This suggests that the local morphology of NTBs, especially the geometrical shape of the tips, and not the general size of NTBs, is the main factor in determining the field emission property of NTBs.


Introduction
In a nuclear fusion reactor, the divertor exhausts helium ash, which is a product of the deuterium-tritium fusion reaction. The divertor plate is exposed to high-temperature plasma and is subjected to a large heat load. To decrease the heat load on the divertor plate, the seeding of impurity gases such as nitrogen (N 2 ), neon (Ne), and argon (Ar) is performed in the divertor region to increase the radiation cooling [1] and facilitate ion recombination [2]. Tungsten (W) is selected as the material for the divertor plate because of its high melting point, low sputtering rate, and other unique property [3]. With a mixture of He, W, and impurity gas, an interaction between W and the plasma occurs, leading to some morphological changes on the W surface. One well-known morphological change is called fuzz, which can be formed on W by He plasma exposure [4]. In recent years, a cone-like, fiber-form structure called nano-tendril bundles (NTBs) has been found to be formed on W via radio frequency He plasma exposure [5]. NTBs can also be formed by helium plasma exposure along with seeding of additional impurity gases such as Ne, N 2 , and Ar in the sputtering regime [6]. Therefore, the NTB structure is potentially formable on the divertor plate. The formation mechanism of NTBs is not yet fully understood. The balance between sputtering and line-of-sight deposition [7] of W atoms is considered to be an important process [8], and the growth modes depend on the grain orientation [9].
The thickness of the fuzz is usually less than 5 μm, while the height of the NTBs can attain values of several tens of μm to 100 μm [8,10]. Furthermore, the fuzz always uniformly covers the bulk surface as a layer, while the NTBs appear as isolated 'islands' on the surface [6]. Owing to the long distance from each other and high heightto-radius ratio, the field emission from the NTB is significantly higher than that from the fuzz [11,12]. The NTB structure can trigger arcing [13], which leads to many adverse effects on the divertor plate, such as erosion increment and W dust formation [14,15]. Therefore, the formation of the NTB is considered unfavorable for the divertor plate.
When a strong electric field is applied to a metal surface, the potential barrier becomes low and electrons are emitted from the surface, which is called field emission. The initial report of field emission was done by Wood [16]. The observation on the effect of field emission was reported by Lilenfeld [17]. The connection between field emission and electrical breakdowns was investigated by Dyke, et al [18]. When the current density within emitter increases to a particular level, Joule heating in the emitter causes rapid melting of the material and consequent electrical breakdown. The field emission causes erosion craters to form on a clean surface, and these craters can be new cathode spots for electron emission [19]. The explosive electron emission then becomes the source for unipolar arcing in plasma [20]. The field emission can also be applied in industries such as electron microscopes [21]. It was confirmed with experimental evidence that the microscopic surface protrusion structure can serve to multiply the average field by a field enhancement factor of one hundred or more [22]. In recent years, many materials such as carbon nano-tube (CNT) [23,24], have been used in field emission applications.
It has been known that the field emission property of NTBs is correlated with their morphology [25]. Furthermore, the morphology of NTBs is affected by many parameters such as the impurity gas type and its ratio [8]. If the morphology of NTBs can be modified by changing the formation conditions such as the type of impurity gas and its ratio, it is possible to reduce the field emission of NTBs and its adverse effects to divertor plate. However, it is still unclear how the field emission property of NTBs change with different impurity gases and ratio. Therefore, it is important to investigate the field emission property of NTBs formed with various impurity gases, and find the formation condition for NTBs with low field emission. The effect of changing the incident ion energy, impurity gas type, and impurity gas ratio on the morphology and field emission property of NTBs was investigated. The size distribution and field emission property of the NTBs were measured. The differences in the field emission property and geometry of the morphology of NTBs have been discussed.

Plasma exposure
Plasma exposure experiments were conducted using a linear plasma device NAGDIS-II (NAGoya Divertor Simulator II). Figure 1 shows the schematic of the experimental setup. A 2 m-long He plasma column was produced in a steady state with a magnetic field of 0.1 T. The plasma discharge current was set to 15 A. Impurity gases, such as N 2 , Ne, and Ar, were puffed from the end of the device and mixed with He in the plasma device. The neutral gas pressure was measured using two capacitance manometers and was ∼0.5 Pa (∼3.5 mTorr) during the plasma exposure. The impurity gas ratio was defined as the ratio of partial pressure of the impurity gas to the total gas pressure, and it was in the range of 2%-16% by adjusting the flow rate of the impurity gas. With N 2 impurity gas puffing, the neutral gas pressure was increased to ∼0.6 Pa (∼4.6 mTorr) to obtain a good NTBs formation.
A 10 × 5 × 0.2 mm 3 W thin plate sample was installed in the plasma column at a distance of ∼1.2 m from the plasma source. The sample was placed vertically at ∼12 mm away from the center of the plasma column with a typical plasma potential of ∼−10 V. For all the samples, the electron temperature and electron density were ∼4.5 eV and ∼2 × 10 18 m −3 , respectively. The sample was biased by a power supply, which provided an incident ion energy in the range of 120-250 eV. The surface temperature of the sample was measured using an infrared pyrometer with a wavelength of 1.6 μm assuming that the emissivity was 0.3 and adjusted to ∼1450 ± 50 K, which is the appropriate range for NTB formation [6]. Temperature measurements were performed at the initiation of plasma exposure. The plasma irradiation time was controlled in the range of 3600-4800 s to give a total fluence of ∼2.5 × 10 25 m −2 with a typical flux of ∼6 × 10 21 m −2 s −1 . A 100 Ω resistor was installed in the circuit to prevent arc ignition during plasma exposure. After plasma exposure, NTBs were formed on the sample surface. The surface conditions and morphology of the NTBs were first observed by scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM) was then used to clarify the threedimensional (3D) profile, including the height, projected area, and location of the NTBs. To distinguish the NTB structure from the fuzz layer, structures that were higher than 6 μm and boarder than 65 μm 2 (9 pixel 2 in CLSM micrographs) were recognized as NTBs [26].

Field emission measurements
After surface observations with SEM and CLSM, the field emission current measurement was conducted in a vacuum chamber. Figure 2 shows a schematic and digital photo of field emission measurement device. A W sample with the formed NTBs was installed on a sample holder to act as the cathode. A copper plate was placed above the sample and used as the anode. The surface of the copper plate was parallel to the sample surface. The distance between the electrodes was set to 1 mm by adjusting the screw connected to the sample holder. The sample holder can be removed without adjusting the screw so that the gap distance was maintained to be equal for each measurement.
During the field emission measurement, a high-voltage power supply of up to 12 kV was connected to the sample to generate an electric field between the electrodes. While increasing the electric field, the field emission current was measured using a digital multimeter (Tektronix, DMM4050). The current in circle suddenly increased when an electric breakdown occurred. A 1 MΩ resistor was installed in the circuit to prevent the overload current at electrical breakdown. A turbo molecular pump was installed to maintain the background gas pressure at the order of 10 -5 Pa during the measurements.

Results and discussion
3.1. NTB formation After plasma exposure, NTBs were formed on the W surface. Important parameters of the four samples, denoted W1-W4, are listed in table 1. To investigate the changes in field emission property with different impurity gas ratios, W3 and W4 were both formed with N 2 seeding, where the ratio of N 2 impurity gas was slightly lower for W3. Figure 3 shows SEM micrographs of W1-W4. The gray background is the substrate of W, and the white porous structures are NTBs. Figure 3(a) shows that there is a large NTB with a radius of ∼30 μm, and some small NTBs are distributed around the larger one. In figure 3(b), the NTBs formed on the surface were smaller than those shown in figure 3(a). In the N 2 -seeded samples, the NTBs formed on the surface were comparatively larger than those formed with Ne or Ar seeding, as shown in figures 3(c) and (d). The sizes of the NTBs differed for different impurity gas types and ratios. Furthermore, similar to the Ne-seeded samples, the morphology of the NTBs depended on the location even on the same sample surface, as shown in figure 3(a). It has been shown that the {101} grains were easy for NTBs to grow, and the growth condition is highly dependent on individual grain orientation [9] which could be one of the reasons for the individual differences in morphology between NTBs formed on the same surface.
To quantitatively analyze the size of the NTBs, the projected area and mean height were further measured by CLSM. Figures 4(a)-(d) show the height and area distributions (insets in the figures 4(a)-(d)) of NTBs for W1-W4, respectively. As shown in figure 4(a), W1 exhibited the widest height distribution. The maximum mean  height of NTBs was ∼60 μm. Conversely, fewer NTBs were formed on W2, and the height of the NTBs was lower than 25 μm. Although W3 and W4 were exposed to He plasma with N 2 seeding, the size distribution of the NTBs showed a large difference with only a small change in the ratio of N 2 impurity gas. For example, the number of NTBs for W4 was almost twice that of W3. The height distribution of the NTBs for W3 was in the range of 6-50 μm, while that for W4 was in the range of 6-30 μm. The area distributions of W1, W3, and W4 were similar attaining values up to ∼3000 μm 2 , while that of W2 showed a narrow distribution up to ∼1000 μm 2 . This implies that the area of the NTBs of W2 was smaller than that of the others, which corresponds well with the results of the SEM micrographs. It is likely that heavier Ar atoms lead to more sputtering, which suppresses the growth of NTBs.

Field emission measurements
The dependence of the field emission current on the electric field was measured. The typical evolution of the field emission current and electric field is shown in figure 5. The emission current increased with increasing electric field, while some spikes were observed in the evolution of the electric field. This was thought to be due to a local short-time electrical breakdown. In a previous study [12], the current became unstable when it was increased to ∼100 μA. In this study, the electrical breakdown events occurred at a lower emission current of ∼40 μA. It could be assumed that the NTBs were more porous in this study due to the higher ratio of Ne was used, which means overheating of NTBs and electrical breakdowns could occur easier. Electrical breakdown events destroied the emission sites, resulting to a decrease in the emission current. Therefore, there is a difference in the emission current at the same electric field between the increment and decrement of the electric field.
To obtain the field emission property, only the data points measured from the increment of the electric field were used. The field emission property are shown in figure 6. The onset electric field for W1-W3 was found to be ∼1 kV mm −1 . The field emission current for W1 increased rapidly to ∼100 μA at an electric field of 2.5 kV mm −1 . With a further increase in the electric field, the emission current became unstable and electrical breakdown occurred frequently. With serious breakdowns, the field emission current suddenly dropped to ∼ 50 μA from ∼100 μA. For the W2 sample, the emission current dropped at an electric field of ∼1.6 kV mm −1 due to the electrical breakdowns. With a further increase in the electric field to ∼4 kV mm −1 , the current increased to 150 μA. For the N 2 seeded cases, W3 exhibited a field emission property similar to that of W2. A series of electrical breakdowns occurred at ∼3 kV mm −1 , which make the field emission current dropped from~90 μA to ∼60 μA. However, the field emission current for W4 was very low. Even when the electric field was as high as ∼3 kV mm −1 , an emission current of only a few μA was detected, which is one-tenth of the values of the other samples.
For quantitative evaluation of the field enhancement effect, the Fowler-Nordheim (F-N) equation [27] was applied, and the field enhancement factor (β) was calculated according to equation (1). where i is the emission current in A, E is the electric field in V m −1 , Φ is the work function of bulk W, β is the field enhancement factor, and S is the emission area. In this study, we used Φ of 4.5 eV [28]. Figure 7 shows the F-N plot for W1-W3. When the electric field was higher than ∼1600 V mm −1 , the data points deviated from the line. Some of the reasons were changes in the emitter temperature, strong field fall-off, and changes in emitter geometry, emission area, or local work function [29]. The main reason was thought to be the destruction of emission sites due to breakdown. The evolution of the three samples was evaluated and it was found that the first breakdown occurred at ∼1600 V mm −1 for all the samples (E W1 = 1500 V mm −1 , E W2 = 1800 V mm −1 , E W3 = 1700 V mm −1 , where E is the electric field when the first breakdown occurs). Therefore, only data points below 1600 V mm −1 were used to fit the linear regression. The field enhancement factors (β) for W1, W2, and W3 were determined to be 7900 ± 800, 6400 ± 300, and 7500 ± 590, respectively. Considering the error range, no clear dependence on the size distribution was observed among these three samples.

Discussion
Previous research has shown that the enhancement factor of NTBs is much higher than that of the fuzz layer, and NTBs are the main emission sites as determined by luminescence detection [11,12]. Therefore, it can be thought that the field emission property of samples mainly depended on the morphology of NTBs. In section 3.2, the W4 sample obtained the lowest field emission current in 4 NTB samples, as shown in figure 6. However, the CLSM  data did not show clear dependence on the mean height or the projected area of NTBs. The CLSM can only measure the average height of NTBs, but it is difficult to accurately measure the height profile of tips of NTBs due to the diffusion of the laser at the tips. As an alternative, SEM micrographs were used to discuss the morphology of tips of NTBs. Figure 8 shows the morphology of the NTBs formed on W3 before and after the field emission measurements. Initial NTBs with sharp tips formed on the surface as shown in figure 8(a). Figure 8(c) is an enlarged view of figure 8(a). Figures 8(b), (d) shows the same position as figures 8(a), (c) after breakdown events. However, it can be observed that the tips of the NTBs disappeared due to the melting and evaporation at electrical breakdown, while the main body of the NTBs remained intact in figure 8(b). This morphology change suggests that these thin tips are the main emission sites for electric field emissions. A numerical calculation shows that the temperature of NTBs can increase to thousands of K in a very short time (several nanoseconds) at explosive emission, and an electric field of 2 kV mm −1 is sufficient to destroy the tip structure [25], which is consistent with the results of this experiment.  Changing the impurity gas species and its ratio would change the sputtering yield on W sample, which is an important factor to determine the morphology of NTBs. Figure 9 shows the morphology of the NTBs formed on the W1 ∼ W4. It can be confirmed that the NTBs formed on W1 have sharp tips, as shown in figure 9(a). As shown in figure 9(a), the length of tip was ∼ 13.6 μm while the width was only ∼ 0.22 μm. Similar structure also formed on W2 and W3, as shown in figures 9(b) and (c), respectively. One can see the length of the tip were ∼24.5 μm and ∼ 25.1 μm for W2 and W3, respectively. The width of the tip of NTB on W2 and W3 were ∼ 0.92 μm and ∼ 0.7 μm, respectively. Note the length of fiber was 2 times longer than it looks on the SEM micrographs because the SEM micrographs were taken with an angle of 45 • tilted. Compared to those sharp tips formed on W1-W3, no sharp tip can be found on W4 as shown in figure 9(d). Figure 9(d) shows the enlarged SEM micrograph that focus on the tips of the NTBs formed on W4. It can be clearly confirmed that the tip structures on W4 were rounder than those on W1-W3. It is difficult to calculate the width of the fiber by analyzing the SEM micrographs while it can be confirmed that the length of the tips was ∼1.3 μm, which is one tenth value of other samples. Even though the number of NTBs formed on W4 was approximately twice that of the other samples, the lowest field emission current for W4 indicated that the ability of field emission of the thicker fibers was so weak that the field emission current was reduced to several μA. In [25], it was shown that the aspect ratio, which was calculated by using the length of the fiber to divide its radius, has a significant effect on the enhancement factor by simulation. In another previous study [30], the NTBs were annealed in an infrared furnace. After the annealing treatment, most of NTBs lost its sharp tips and became rounder. The field emission current decreased to a few several hundred nA as the morphology of the tips changed. This suggests that the local morphology of NTBs, such as the shape of tips, is the main factor determining the field emission property. Conversely, the size distributions of NTBs showed a large difference for W1-W3, while the field enhancement factors were similar, which suggests that the number or general size of NTBs is not a dominant factor. Figure 10 shows the enlarged SEM micrographs of 4 samples which were made by He plasma exposure with N 2 seeding. The ratio of N 2 impurity gas was 2.1% (W1), 3.0% (W2), 3.4%, and 5.7% for figures 9(a)-(d), respectively. As shown in figure 9(a), the typical thickness of single fiber was marked in the figures 9(a)-(d). With increasing the ratio of N 2 , the thickness of fiber increased significantly. As shown in figures 9(b)-(d), with increasing the ratio of N 2 to 3.0%, 3.4% and 5.7%, the typical thickness of fiber increased to ∼270 nm, ∼300 nm and ∼440 nm, respectively. The results indicated that the fibers of the NTBs tended to become thicker with increasing the impurity ratio when N 2 was used. The increase of radius of fibers could be another reason for low field emission current of N 2 -seeding cases. Interestingly, similar morphological changes on the tips were not observed in the case of Ne or Ar seeding, but were only observed in the case of N 2 seeding. This special property of N 2 is beneficial for reducing the field emission from NTBs and the possibility of arcing on the divertor plate. It is considered that a higher ratio of N 2 will lead to stronger sputtering on the NTBs themselves, thereby suppressing the formation of sharp tips. Because many types of ions coexist, including N + , N 2+ , and N 2 , a small change in the ratio of N 2 can significantly alter the degree of ionization. Spectroscopic measurements are expected to be used in the future to investigate the composition of ions in the plasma.

Conclusions
In this study, NTBs were formed on W by He plasma exposure along with various additional impurity gases, such as Ne, Ar, and N 2 . The size distributions showed that the NTBs formed with these gases had different morphologies; for example, NTBs formed with Ne seeding exhibited the widest distribution, whereas most of the NTBs formed with Ar seeding were smaller and shorter. Two samples were formed with N 2 seeding at different N 2 mixture ratio. NTBs formed with a lower N 2 impurity gas ratio had sharper tips, while those formed with a higher N 2 impurity gas ratio had thicker fibers and rounder tips. The field emission current measurement showed that the onset electric field for the NTBs with sharp tips (W1-W3) was ∼1 kV mm −1 , and the Ne-seeded sample (W1) exhibited the most rapid emission current increment. The field enhancement factor was calculated using the Fowler-Nordheim equation. The results showed that the enhancement factors were ∼7900 ± 800, ∼6400 ± 300, and ∼7500 ± 590 for the Ne-seeded (W1), Ar-seeded (W2), and N 2 -seeded samples (W3), respectively. No clear dependence of the size distribution measured by CLSM was observed for the samples. With an increase in the ratio of N 2 -seeding gas (W4), the NTBs with thick fibers and rounder tips showed a completely different field emission property compared to the other samples. On W4, the emission current was only several μA, which is one-tenth of the values of the other samples, at an electric field of 3 kV mm −1 . A comparison of the morphology of NTBs before and after field emission measurements shows that the sharp tips of NTBs were easily destroyed after the field emission measurement owing to electrical breakdowns. We believe that the local morphology of NTBs, especially the geometric shape of the tips, should be the main factor in determining the field emission property.
The morphology of the NTBs depended on the ion sputtering and re-deposition of W atoms. The NTBs formed with a higher ratio of N 2 had thicker tips and rounder tips, suggesting that heavier sputtering suppressed the tips from becoming sharp. Although increasing the sputtering yield would enhance the erosion of the divertor material, it would also suppress the formation of sharp NTBs tips, thereby reducing the possibility of electrical breakdown or arcing. Only NTBs formed with N 2 impurity had these round tips, suggesting that N 2 Figure 10. SEM micrographs of fibers of NTBs formed with N 2 impurity gas seeding. The ratio of N 2 was (a) 2.1%, (b) 3.0%, (c) 3.4%, and (d) 5.7%. The blue square represents the fiber which used to calculate the thickness. The thickness of fiber were marked in (a-d).
was unique for NTB formation. Further investigation of the relationship between the morphology of the NTBs and the composition of ions by spectroscopy is expected in the future to control the morphology of NTBs.