Plasma-induced field emission study of carbon nanotube cathode

An investigation on the plasma-induced field emission (PFE) properties of a large area carbon nanotube (CNT) cathode on a 2 MeV linear induction accelerator injector is presented. Experimental results show that the cathode is able to emit intense electron beams. Intense electron beams of $14.9–127.8\mathrm{A}/{\mathrm{cm}}^2$ are obtained from the cathode. The CNT cathode desorbs gases from the CNTs during the PFE process. The fast cathode plasma expansion affects the diode perveance. The amount of outgassing is estimated to be $0.06–0.49\mathrm{Pa}·\mathrm{L}$, and the ratio of outgassing and electron are roughly calculated to be within the range of 170–350 atoms per electron. The effect of the outgassing is analyzed, and the outgassing mass spectrum of the CNT cathode has been studied during the PFE. There is a significant desorption of ${\mathrm{CO}}_2$, ${\mathrm{N}}_2(\mathrm{CO})$, and ${\mathrm{H}}_2$ gases, which plays an important role during the PFE process. All the experiments demonstrate that the outgassing plays an important role in the formation of the cathode plasma. Moreover, the characteristic turn-on time of the CNT cathode was measured to be 39 ns.

Figure 1

Schematic diagram of experimental setup of the 2 MeV LIA injector. 1, 2, 3: turbomolecular pumps; 4: quadrupole mass spectrometer and IVG; 5: CCD camera; 6: insulator; 7: cathode; 8: anode; 9: bakeout device and CNT cathode; 10: sampling resistor; 11: pulse power system; 12: Faraday cup; 13, 14: oscillograph; 15, 16: resistance.

Figure 2

Cathode of the 2 MeV LIA injector diode: (a) cathode, (b) CNT cathode and bronze subassembly.

Figure 3

CCD images: (a) background image of CNT cathode, (b)–(g) images of PFE of CNT cathode under diode voltages of 1.33–1.64 MV.

Figure 4

Typical waveforms of the voltages and the currents: (a) diode voltages, (b) anode currents, and (c) cathode currents.

Figure 5

Temporal variation of the voltage-current characteristics for the CNT cathode. Conditions: $U=1.61\mathrm{MV}$, $I_k=1.9\mathrm{kA}$.

Figure 6

Temporal variation pressure in different diode voltages: (a) $U=1.33\mathrm{MV}$, (b) $U=1.37\mathrm{MV}$, (c) $U=1.46\mathrm{MV}$, (d) $U=1.48\mathrm{MV}$, (e) $U=1.61\mathrm{MV}$, and (f) $U=1.64\mathrm{MV}$.

Figure 7

Emission current before and after bakeout of the CNT cathode: (a) anode current, (b) cathode current.

Figure 8

OMSs of the CNT cathode: (a) before discharging, (b) on discharging, and (c) after discharging.

Figure 9

CTT of the CNT cathode is 39 ns.