Operating the LaB6 cathode at a pressure of 3 · 10−11mbar leads to a reduced elec-
tron emission, which is also observed when leaving the cathode in the heated state without applied extraction potential for approximately six hours at this pressure. [Avdienko and Malev 1977] reports on the in uence of the partial pressure of var- ious residual gases, for instance O2, N2, CO, CO2, H2O, H2 and hydrocarbons of
dierent lengths, on the cathode emission. The equilibrium pressure of the reaction of hydrocarbons with the LaB6 crystal depends exponentially on the hydrocarbon's
molecular length. While the equilibrium pressure for a reaction with hydrocar- bons, shown in Eq. 5.4.1, is 1 · 10−6mbar for methane, the equilibrium pressure is
1 · 10−10mbar for ethane.
LaB6+n2CnH2n+2+3CO = 32B4C + LaC2+32CO2+2n + 1n H2 (5.4.1)
Since a scan with a residual gas analyzer revealed that none of those previously mentioned hydrocarbons are present in the observed gun volume with a higher par- tial pressure than 10−14mbar, long-chained alkane-molecules evaporating from the
pumps are suspected to be causing the cathode-poisoning. The carbon pads of the cathode holder, which evaporate at the heated state of the cathode, can be consid- ered as another source. The following investigations focus on the cathode-reviving process rather than on the mechanisms avoiding cathode poisoning.
5.4.1 Cathode revival using oxygen
Oxygen has proven to be the most eective element for cleaning a LaB6 cathode
surface from carbon compounds. Because the poisoning is a permanently ongoing and cathode-temperature dependent process, the partial pressure of oxygen has to be adjusted carefully to keep the cathode at a state of maximum emission. The optimal pressure is usually 2-3 times higher than the base pressure of the cathode at a certain temperature Tc. In [Goldstein and D.J.Szostak 1978] it is reported that
an oxygen partial pressure above the optimal pressure reduces the emission because the O2-LaB6 reaction dominates according to Eq. 5.4.2. Below and at the optimal
pressure Eq. 5.4.3, describing the reviving process from a carbon poisoned cathode, dominates and balances the oxygen poisoning.
2LaB6+212 O2 =La2O3+6B2O3 (5.4.2)
6B4C + 4LaC2+7O2 =4LaB6 +14CO (5.4.3)
In Fig. 5.4.1(a) a reviving process of a LaB6 cathode is shown for dierent tem-
peratures. The distributions are aligned to the event of opening the leak valve into the gun volume and thus increasing partial O2 pressure. The reaction speed of the
reviving process depends on the state of initial poisoning of the cathode and of the cathode temperature. Higher cathode temperatures require higher oxygen ux
due to the temperature dependency of the chemical reaction. A slightly poisoned cathode with a reduced emission reacts immediately to the oxygen ux.
Here the emission increases by more than 30 % within 10 minutes. If oxygen is introduced to an partially revived cathode, the emission increases marginally as shown in Fig. 5.4.1(a) by thered distribution. When the oxygen ux is stopped, the cathode emission decreases according to the cathode temperature and the residual partial pressure of the remaining oxygen inside the gun volume as shown in Fig. 5.4.1(b). In the case of the full lines, the leak valve was closed and the oxygen supply stopped. The emission reduction increases with the cathode temperature as expected from the temperature dependency of the chemical poisoning process. The dashed lines show the poisoning process of a cathode after varying the temperature by changing the heating power when the leak valve is closed. The current was acquired after the cathode reaches its new emission level. Here the black dashed line is the measured current after decreasing the cathode temperature by 24 K, the red dashed line shows the measured current after increasing the cathode temperature by 96 K. Icoll [mA ] 100 150 200 250 0:00 2:00 4:00 t [h:mm] Tc,t=1738K Tc,t=1666K Tc,t=1642K
(a) Slopes of current increase for a revived LaB6 crystal. The partial oxygen pressure is
6.5 · 10−11mbar for the red and black distribu-
tion and 8 · 10−11mbar for the orange distribu-
tion. Icoll /I0 [%] 1.00 0.98 0.96 0.94 0.92 t [mm:ss] 00:00 15:00 30:00 45:00 p0=2.0 · 10−10mbar p0=4.0 · 10−11mbar p0=1.5 · 10−10mbar p0=2.5 · 10−11mbar I0=81mA I0=234mA I0=207mA I0=112mA
(b) Temperature dependent poisoning process measured after closing the leak valve (solid lines) and varying the cathode temperature (dashed lines). I0 is here the emission current when the
recording starts.
Figure 5.4.1: Reviving and poisoning process of the LaB6 cathode installed at
REXEBIS. The color code is shown in the left gure. The slope depends on the degree of poisoning and cathode temperature. The pressures in the right gure indicate the pressures before the measurement starts.
5.4.2 Eects of hydrocarbons on the cathode
As previously discussed, long-chained hydrocarbons are considered as a source of the poisoning reaction. In order to evaluate the eects of those molecules, methane was injected into the gun volume at dierent partial pressures. Fig. 5.4.2(a) shows the development of the emitted current after the leak valve has been opened for dierent partial pressures and cathode temperatures. As with the reviving process
using oxygen, the poisoning depends on pressure and cathode temperature. Higher pressure accelerates the poisoning mechanism for equal temperatures. Increasing the cathode temperatures at constant pressure also increases the poisoning due to a shift of the equilibrium pressure of the chemical reaction towards the educts. When interrupting the methane- ux to the cathode, the cathode immediately recovers, as shown in Fig. 5.4.2(b). All shown distributions were acquired with a cathode operating near the background pressure and are normalized to the emitted current before closing the leak valve.
Icoll /Icoll,0 [%] 1.02 1.00 0.98 0.96 0.94 0.92 0.90 t [m:ss] 0:00 2:00 4:00 6:00 Tc,t=1637K 6.0 · 10−11mbar 8.0 · 10−11mbar 1.1 · 10−10mbar Tc,t=1677K 4.5 · 10−11mbar 6.0 · 10−11mbar 1.3 · 10−10mbar
(a) Poisoning process after injecting methane at dierent partial pressures and cathode temper- atures. Icoll /Icoll,0 [%] 1.04 1.03 1.02 1.01 1.00 0.99 0.98 t [m:ss] 0:00 2:00 4:00 6:00 Tc,t=1637K Tc,t=1677K
(b) Self-reviving process of the cathode after closing the methane- ux into the gun volume. Figure 5.4.2: Eects of methane on the emission performance of the LaB6cathode
installed at REXEBIS.
5.4.3 Eects of neon on LaB
6Icoll [mA ] 110 108 106 104 102 t [mm:ss] 00:00 10:00 20:00 30:00 40:00 1.4 · 10−10mbar 1.2 · 10−9 mbar 2.7 · 10−9 mbar
Figure 5.4.3: Current distribution of the LaB6 cathode during the injection of neon at
dierent pressures in the gun volume.
In order to verify that the poisoning is caused by chemical reactions on the cathode surface and not by sputter- ing, neon, an inert reactant with simi- lar mass to methane, was injected into the gun volume. Neon ionizes in the electron beam between cathode and an- ode, accelerates towards the cathode and sputters the surface. Fig. 5.4.3 shows the emission current distribution at dierent neon partial pressures. The sputtering process induced by neon pro- jectiles has no degrading eect on the cathode emission as shown in the plot. Therefore sputtering of light residual noble gases on the cathode surface can be excluded as a source for cathode poi- soning.