Electron Gun using Coniferous Carbon Nano-Structure

We have measured the emission stability of a coniferous carbon nano-structure (CCNS) field emission electron source. Stable emission over the 1274 h measurement was observed at an emission current density of 20 mA·cm−2. The CCNS emitter can generate an emission current of more than 10 mA making it a practical choice for many applications requiring high electron current. For example, we are currently developing a CCNS based electron gun for electron accelerators and a portable X-ray source. Recent progress on both these applications is presented.


Introduction
High brightness electron guns for use with electron accelerators or for X-ray sources typically use a thermionic (hot) cathode.In the resent years, point source cold cathodes employing field emission (FE) have been used as electron sources in electron microscopy.FE based cold cathodes have many advantages over traditional thermionic cathodes, including lower power consumption and instant operation, i.e. no need for warm-up.However, point source cold cathodes have a limited emission current which is difficult to scale up given the nature of the source.The carbon nanotube, discovered in 1991 [1], can also be used as a FE electron source and much previous research has been focused on this area.The emission current can be dramatically increased for these carbon based emitters because multiple emitters can be arranged over an area with the emission current scaling with the total number of emitters.Many measurements of CNT as field emission electron sources were reported from 1995 [2][3][4] to recent years.A number of source lifetime and stability measurements have been reported for printed CNT emitters [5][6][7][8][9][10] and directly grown CNT emitters on substrates [11,12].For printed CNT emitters high emission current density has not yet been reported.On the other hand, directly grown CNT emitter can generate a high emission current density electron beam, however, long term stable operation of such sources has not been demonstrated.Such CNT emitters are therefore not practical for applications as X-ray sources.Recently, a new type of carbon based electron emitter, the so-called coniferous type carbon nano structure (CCNS), was discovered by Hiraki, Haba and Wang [13].This carbon nano structure can generate an electron current density of more than 100 mA • cm −2 [14].CCNSs can be grown on various substrates and to various sizes.The total cur- rent can be increased by increasing the size of the emitting area as the emission current is proportional to the emission area.The CCNS emitter has a possibility of high power electron source.However, the lifetime of CCNS emitter has not been reported.In this paper, an emission stability measurement at a high emission current density of 20 mA • cm −2 for CCNS emitter is reported.Our group at AIST is developing both a dedicated electron accelerator for positron production [15,16] and compact X-ray sources.We also report on our plan to use the CCNS emitter as an electron source for the accelerator and on the development of X-ray tubes for X-ray imaging and X-ray fluorescence spectrometry [17,18].
The structure of the report is as follows; The CCNS structure and a long term emission stability measurement at an emission current density of 20 mA • cm −2 using CCNS are described in Sec.

Coniferous Carbon Nano-Structure (CCNS) for electron source
The CCNSs were grown on substrates using the chemical vapor deposition (CVD) method [13].
CCNS consists of a complex of CNT, graphene, and nano diamond.Electron microscopy images of the CCNSs are shown in Fig. 1.The structure has a shape like coniferous tree from which the name is derived.CCNS has the same tip shape as CNT, and the structure becomes thicker towards the substrate side.If an electric potential is applied to CCNS, the electric field is concentrated at a tip and electrons are emitted by the field electron emission phenomenon.We measured the I-V curve and the emission stability of a 5-mm-diameter CCNS emitter to evaluate the emission property of CCNS.This measurement was performed at a pressure of below 1 × 10 −5 Pa in a vacuum chamber.The vacuum chamber was cooled by circulating insulating oil at about 15 • C. The experimental setup for this measurement consists of a CCNS cathode and grounded electrode and anode.A voltage from 0 to −65 kV can be applied to the cathode.The anode voltage was 50 kV and does not affect the electric field around the emission surface.For an applied cathode voltage of −60 kV, the averaged electric field on the surface of the 5-mm emitter was calculated to be 6.96MV • m −1 .Before the stability measurement, the CCNS cathode was aged for 100 h at an emission current of 4.5 mA.In the aging process, only the tip of the CCNS is destroyed by Coulomb force or Joule heating.The other parts of CCNS structure are resistant to high electric fields, and show the stability under an electric field.In the emission stability measurement, the emission current density (20 mA • cm −2 , emission current of 4 mA) was fixed and we measured the applied DC voltage of the CCNSs cathode as a function of time (see Fig. 2).An increase of applied voltage at the same emission current indicates reduced performance of the CCNS cathode.Even though a high current density beam was emitted continuously for more than 50 days (1200 h) we have not observed any large emission instability or drastic reduction in performance.We have observed only a small (6 %) increase in the applied voltage required to maintain constant current emission.Several previous mea-surements for various CNT emitters have reported emission for more than 1000 h [5,6,10], but these measurements were performed at a low emission current density of below about 1 mA • cm −2 .Fujii et al. reported stable emission for 200 h at a high emission density of 150 mA • cm −2 using a directly grown CNT emitter [12].However, this measurement was performed at a low emission current of 3 µA with a small surface area, point source.The present work is the first report using a carbon-based emitter combining both high emission current density (more than about 1 mA • cm −2 ) and long term stable operation (more than 1000 h).This result shows that the CCNS structure has excellent characteristics for a field emission electron source and may be practically used in applications requiring high current electron beams.To this end, we are developing an electron gun for use with an electron accelerator and compact X-ray sources.

The setup for electron gun
For high intensity positron production, an electron accelerator with both high pulsed rate and high duty, characteristics of a superconducting accelerator (SCA), is suitable.Since an SCA operates at cryogenic temperatures, the low thermal heat load from a cold cathode makes them a suitable choice for use with SCA.Previous simulations have shown that it is possible to incorporate a cold cathode RF gun directly into the SCA cavity [19], resulting in a compact design.In the future, such a combination of a high current CCNS electron source and SCA cavity could form the basis for a compact electron accelerator for positron production etc.As an initial step towards an SCA with a cold cathode RF gun, we are developing a new electron gun using a CCNS cathode for use with the AIST SCA, a dedicated facility for positron production currently under development.Previously we estimated a positron beam of 1 × 10 7 slow e + • s −1 is possible with an electron beam energy of 5 MeV an average electron current of about 500 µA [15].Since the AIST SCA can operate with a maximum duty of 10 % and considering the conversion from DC to pulsed beam has an efficiency of around 20 %, an emission current of 25 mA or more is needed.The present CCNS emitter can generate electron current density of 20 mA • cm −2 , therefore a 25 mA beam can be generated with a 12.6 mm diameter emitter.The present work was performed in a DC acceleration mode.If the emitter is installed in a RF gun, emission can be synchronized with the electron accelerator and resulting in a more efficient source.We have started to research the possibility of using CCNS cathodes in RF guns for electron accelerators.
The setup for present DC electron gun with CCNS cathode is shown in Fig. 3.The CCNS cathode was mounted on an ICF70 flange insulated from the grounded beamline by a ceramic.The emitted electron beam is focused and passed through a pre-accelerator cavity before entering the superconducting accelerator [15,16].
Electron Beam
The CCNS cathode was designed with the aid of an electric field simulation (see Fig. 4).The TriComp beam simulation code (Field Precision) was used for the simulation.In this simulation, contour lines show the strength of the electric field used in the numerical simulation.A uniform electric field (a few MV • m −1 ) is applied to CCNS by applying a high voltage (∼−40 kV) to the cathode.The CCNS cathode has a diameter of 16 mm and has a concave structure of R32 mm.The distance between the CCNS cathode and grounded electrode is 15 mm.

Electron emission property of electron gun
We measured the I-V curve of the CCNS electron gun at an applied voltage of up to 40 kV, and the result is shown in Fig. 5(a).Field emission was explained by quantum tunneling of electrons.The theory of field emission was proposed by R. H. Fowler and L. W. Nordheim [20] and typically the Fowler-Nordheim (F-N) equation is given by; where I is the emission current, E is the electric field on the surface of cathode, β is the field enhancement factor, φ is the work function, and m e is the mass of electron.In Fig. 5(b), the measured I-V data are re-plotted in an F-N plot.The linearity of this plot shows that electrons are emitted by field electron emission from CCNS.In Fig. 5(b), the values of the slope and intercept in the linear function of 1/E were determined in a least-squares fit analysis of the experimental F-N plot.The result from this analysis, after being converted back into I-V curve, is plotted as a solid line in Fig. 5(a).The values of the slope and intercept of the F-N plot are −8.66 and −2.80, respectively.The field enhancement factor of CCNSs is 27000, calculated from the slope of the F-N plot by assuming the work function of CNTs as 5.0 eV [12].Some groups have reported previously the field enhancement factors of CNT-based emitters such as a self-standing CNT fiber [21], vertically aligned CNT arrays on graphene layers [22], few-layer graphene on CNTs [23], and straight multi-walled CNTs [24] as 65000, 5340, 3980, and 7830, respectively.On the comparison of CCNS and other CNT-based emitters, the value for CCNS is between that of a CNT fiber and graphene.Since CCNS consists of CNT and graphene, this result is consistent.In this measurement, to the maximum applied voltage was only −40 kV.In the future, it is planned to use a higher voltage power supply and operate the AIST superconducting accelerator using CCNS emitter at an emission current of 25 mA.

The development of X-ray source using CCNS electron gun
We are also developing portable high-energy X-ray sources for non-destructive inspection using CCNS electron guns.There are several advantages of CCNS over thermionic filaments for portable X-ray sources including no warm up time, low standby power consumption, simple electrodes and a compact electric circuit.We have developed a compact and lightweight portable high-energy X-ray source using several AA-sized batteries as the power source, shown in Fig. 6(a).The present X-ray source consists of an X-ray tube using a CCNS cathode, a high voltage generating circuit and an operating circuit, shown in Fig. 6(b).The X-ray tube consists of a 5-mm-diameter CCNS cathode, a tungsten target and getter pump.This X-ray source can operate with a maximum voltage of about 100 kV, maximum current of 20 mA and an exposure time of 20 ms.The beam spot size is focused to about 1 mm using electrostatic lens.The size of the present X-ray source (width = 190 mm, height = 170 mm, depth = 70 mm) is below half that of comparable commercial X-ray sources.Preheating the filament is unnecessary so an X-ray image can be taken immediately.Therefore, the X-ray source using CCNSs is suitable not only for replacement of previous X-ray sources but for new applications.
We performed a demonstration of X-ray imaging using the portable X-ray source and an imaging plate (FujiFilm BAS-IP MS 2040, screen size: 20 cm × 40 cm, pixel size: 50 µm).Figure 7 shows the fine internal structure of a cellular phone.This X-ray image was taken with an energy consumption of less than 50 J.1000 of such images can therefore be obtained with 6 AA-sized batteries, as the standby power consumption is almost zero.The present X-ray source has a higher portability than commercial X-ray sources as it is battery operated and light (2.5 kg).Furthermore, due to the compact size of the source, it is possible to obtain X-ray images in narrow places down to gaps of about 100 mm, typical of the distance between parallel pipes in a factory or chemical plant.

Conclusion
An emission stability measurement of a CCNS field emission cathode at an emission current density of 20 mA • cm −2 was performed.Continuous electron emission was monitored for 1274 h (> 50 days) with only a small (6 %) increase in the applied voltage required to maintain constant current emission.CCNS has a higher stability than any previously reported high current density CNT based emitter.The measured performance of the CCNS emitter shows that such sources can be used in practical applications requiring high current electron beams.At AIST we are developing electron guns for an electron accelerator and portable X-ray sources using CCNS emitters.We plan to further develop both DC and RF electron guns and X-ray sources using CCNS, working towards the production of both intense positron beams and compact, portable X-ray devices for industrial applications.

Fig. 4 .
Fig. 4. The simulation of electric field for the electron gun.

Fig. 5 .
Fig. 5. (a) I-V curve and (b) F-N plot of electron gun using CCNS electron source.

5 ■■■Fig. 6 .
Fig. 6.(a) Picture and (b) schematic diagram of the portable high-energy X-ray source using AA-sized batteries as the power source.

Fig. 7 .
Fig.7.X-ray image of cellular phone using a portable X-ray source operating with a voltage of about 100 kV, current of a few mA and an exposure time of 20 ms.