Emittance growth due to space charge compensation and beam intensity instabilities in negative ion beams

The need to extract the maximum beam intensity with low transversal emittance often comes with the drawback of operating the ion source to limits where beam current instabilities arise, such fluctuations can change the beam properties producing a mismatch in the following sections of the machine. The space charge compensation (SCC) generated by the beam particles colliding with the residual gas reaches a steady state after a build-up time. This paper shows how once in the steady state, the beam ends with a transversal emittance value bigger than the case without compensation. In addition, we study how the beam intensity variation can disturb the SCC dynamics and its impact on the beam properties. The results presented in this work come from 3-D simulations using tracking codes taking into account the secondary ions to estimate the degree of the emittance growth due to space charge and SCC.

Figure 1

Laplace solution for beam simulations with boundary conditions (top) and Poisson solution once the beam contribution has been included (bottom).

Figure 2

Input beam distribution (top) and beam at the end of the drift (bottom) after experimenting the SCC for an ${\mathrm{H}}^{-}$ beam at 30 mA.

Figure 3

${\mathrm{H}}^{-}$ emittance and beam size evolution in time due to SCC using the scc-spi code for a 30 mA beam with 50 keV energy.

Figure 4

${\mathrm{H}}^{-}$ emittance growth vs beam current without compensation for a beam with 30 and 50 keV energy in a 0.5 m drift.

Figure 5

Emittance growth vs beam current in a 0.5 m drift section using the secondary particles (scc-spi) to compensate the 50 keV negative beams.

Figure 6

Emittance time evolution for the 50 keV negative beams in the 0.5 m drift section using the secondary particles to carry the compensation.

Figure 7

Beam potential along the line for 30 mA of ${\mathrm{H}}^{-}$, ${\mathrm{D}}^{-}$, and ${\mathrm{Li}}^{-}$ beams (top) and compensation levels for the same ion beams (bottom).

Figure 8

Transverse emittance at the end of the beam line using the solenoid field for different ${\mathrm{H}}^{-}$ beam currents: zero current (blue), 10 mA (red), and 30 mA (yellow).

Figure 9

Secondary ions within the drift (top) and emittance time evolution at the end of the beam line for the 30 mA ${\mathrm{H}}^{-}$ beam with the solenoid on and off (bottom).

Figure 10

Beam pulses taken to simulate beam intensity instabilities instead of constant current.

Figure 11

Beam emittance evolution in time recorded at the end of the line when the beam current is growing.

Figure 12

${\mathrm{H}}^{-}$ beam size evolution when the beam current grows linearly.

Figure 13

${\mathrm{H}}^{-}$ transversal emittance evolution under SCC when the beam intensity decays linearly.

Figure 14

Emittance evolution under SCC when the beam intensity decay for a short period of time.