Effect of double frequency heating on the lead afterglow beam currents of an electron cyclotron resonance ion source

The effect of double frequency heating on the performance of the CERN GTS-LHC 14.5 GHz Electron Cyclotron Resonance (ECR) ion source in afterglow mode is reported. The source of the secondary microwave frequency was operated both in pulsed and continuous wave (CW) modes within the range of 12–18 GHz. The results demonstrate that the addition of the secondary frequency can significantly impact the extracted beam currents and the temporal stability of the beam during the afterglow discharge. For example, up to a factor of 2.6 increase was achieved for ${{}^{208}\mathrm{Pb}}^{35+}$ and a factor of 3.1 for ${{}^{208}\mathrm{Pb}}^{37+}$ compared to single frequency afterglow currents. It is shown that these effects are dependent on the choice of the secondary frequency with respect to the primary one and on the temporal synchronization between the two microwave sources. Overall, the results provide new insight into the afterglow discharge supporting the prevailing understanding of the physical processes behind the phenomenon.

Figure 1

The on-axis magnetic field of the GTS-LHC and the conventional scaling law limits for 14.5 GHz ECRIS (dashed lines). The resonance field for 14.5 GHz is $B_{\mathrm{ECR}}=0.52\mathrm{T}$. The axial field variation due to different solenoid settings used in this work falls within the presented range of field strength. The biased disc and the plasma electrode locations represent the axial boundaries of the plasma chamber.

Figure 2

The GTS-LHC ECR ion source injection plug, viewed from the plasma chamber. At the center is the biased disc (A), which wraps around the two WR62 type rectangular wave guides (B), seen in the figure on top left and right. The gas inlet is at the bottom (C), surrounded on both sides by the two circular oven ports, one shown empty (D) and the other with the oven in place (E).

Figure 3

A schematic presentation of the measurement setup used in the double frequency heating experiments.

Figure 4

Layout of Linac3 low energy beam transport section, from the ion source to the entrance of the RFQ. The main ion optical elements and beam diagnostics are also indicated.

Figure 5

The effect of the GTS-LHC extraction voltage modulation on the measured ${{}^{208}\mathrm{Pb}}^{29+}$ beam current during the afterglow. In case (a) the modulation is not applied and consequently the steady state beam current at the end of the klystron rf pulse is observed ($t<-0.72\mathrm{ms}$), and the afterglow current continues until the end of the measured time window. In case (b) the modulation is used and these parts are discarded before the beam reaches the Faraday cup.

Figure 6

${\mathrm{Pb}}^{29+}$ and ${\mathrm{Pb}}^{35+}$ afterglow beam currents as a function of the secondary frequency operated in CW mode normalized to the single frequency beam currents ($190\mu \mathrm{A}$ for ${\mathrm{Pb}}^{29+}$ and $33\mu \mathrm{A}$ for ${\mathrm{Pb}}^{35+}$). The main frequency of 14.5 GHz is pulsed with 1920 W and the secondary frequency is operated at a constant 200 W power.

Figure 7

${{}^{208}\mathrm{Pb}}^{29+}$ beam current during the afterglow with pulsed primary and CW secondary frequency. The nominal single frequency case (“Pulsed nominal”) is presented for comparison. The primary 14.50 GHz frequency is supplied at 1930 W in all cases. The secondary frequency powers for 12.00, 12.70, 14.50, 16.34, and 18.00 GHz are 161, 204, 200, 175, and 200 W, respectively.

Figure 8

Lead charge state distributions measured during afterglow with pulsed main frequency mixed with a secondary frequency operated in CW mode. Primary frequency operated at 1920 W and the secondary at 200 W. Three cases are presented, (1) pulsed primary frequency only at 14.50 GHz, (2) pulsed primary at 14.50 GHz with 12.36 GHz secondary in CW, and (3) pulsed primary at 14.50 GHz with 16.71 GHz secondary in CW. In addition the change in the beam currents of different charge states with added secondary frequency is presented (points connected with dashed lines).

Figure 9

${\mathrm{Pb}}^{29+}$ and ${\mathrm{Pb}}^{35+}$ afterglow beam currents as a function of the secondary frequency operated in pulsed mode normalized to the single frequency beam currents ($192\mu \mathrm{A}$ for ${\mathrm{Pb}}^{29+}$ and $26\mu \mathrm{A}$ for ${\mathrm{Pb}}^{35+}$). The main frequency of 14.5 GHz is pulsed with 1920 W and the secondary frequency is operated at a constant 200 W power.

Figure 10

Lead charge state distribution during afterglow with two different secondary microwave frequencies. Klystron operated at 1920 W, TWTA at 180 and 185 W for 12.36 and 16.71 GHz, respectively. Both microwave sources are pulsed at 10 Hz. Three cases are presented, (1) klystron only at 14.50 GHz, (2) klystron at 14.50 GHz with 12.36 GHz secondary frequency from TWTA, and (3) klystron at 14.50 GHz with 16.71 GHz secondary frequency. In addition the change in the beam current of different charge states with secondary frequency is presented (points connected with dashed lines).

Figure 11

Lead charge state distribution during afterglow with varied power balance between the two injected microwave frequencies. Klystron is operated at 14.50 GHz and the TWTA at 12.36 GHz. Three cases are presented, (1) klystron only with nominal power level, (2) klystron and TWTA simultaneously with total power set to be comparable to the single frequency case, and (3) klystron and TWTA with increased total power. In addition the change in the beam current when using two frequencies instead of one is presented for the different charge states (points connected with dashed lines).

Figure 12

${{}^{208}\mathrm{Pb}}^{29+}$ afterglow beam currents with varied secondary frequency (columns) and trailing edge delays between the klystron and the TWTA RF pulses (rows). The presented delays are 0 ms for the first row, 0.5 ms for the second, 1.0 ms for the third and 1.3 ms for the fourth. The dashed vertical lines indicate the trailing edges of the klystron (K) and the TWTA (T) RF pulses.

Figure 13

Beam currents of selected lead ion charge states with single and double frequency heating. With double frequency heating a fixed delay of 0.65 ms (0.35 ms for the ${\mathrm{Pb}}^{35+}$ at 12.36 GHz case) was used between the trailing edges of the primary (K) and secondary (T) frequency pulses. The first and second columns presents charge states $27+$, $30+$, and $33+$ with 13.50 and 16.70 GHz secondary frequencies. The third column presents charge state $35+$ with frequencies 12.36, 13.50, and 16.70 GHz. The TWTA output power with the frequencies 12.36, 13.50, and 16.70 GHz were 180, 200, and 185 W, respectively.

Figure 14

The GTS-LHC magnetic field profile on axis around the minimum $B$ field and resonance fields for nonrelativistic electrons at selected frequencies. The magnetic fields resulting from the different solenoid settings used in the experiments fall within the presented field band.