Studies on space charge neutralization and emittance measurement of beam from microwave ion source

Abstract

A 2.45 GHz microwave ion source together with a beam transport system has been developed at VECC to study the problems related with the injection of high current beam into a compact cyclotron. This paper presents the results of beam profile measurement of high current proton beam at different degrees of space charge neutralisation with the introduction of neon gas in the beam line using a fine leak valve. The beam profiles have been measured at different pressures in the beam line by capturing the residual gas fluorescence using a CCD camera. It has been found that with space charge compensation at the present current level (∼5 mA at 75 keV), it is possible to reduce the beam spot size by ∼34%. We have measured the variation of beam profile as a function of the current in the solenoid magnet under the neutralised condition and used these data to estimate the rms emittance of the beam. Simulations performed using equivalent Kapchinsky-Vladimirsky beam envelope equations with space charge neutralization factor are also presented to interpret the experimental results.

Transport of intense proton beam in the presence of subdominant species in a low energy beam transport system

“…The results of test simulations are found to converge when the number of macroparticles is more than 40000 and step size is less than 2 mm. We have performed the simulation for the following choice of parameters: total beam current I = 10 mA, mesh size N x = N y = 128, number of macroparticles for each species N = 77000, step size ∆s = 1 mm in the axial direction, percentage species fractions [p, H + 2 , H + 3 ] as [80, 15,5] and initial rms beam size 2016 JINST 11 P04020 X rms = 1.25 mm at s = 0 for all the species. The charge on the macroparticles of each species is determined from the species fraction and the total number of macroparticles.…”

“…The initial distribution of the proton beam is chosen as KV in all the cases. The percentage species fractions [p, H + 2 , H + 3 ] are chosen as [80, 15,5] in a total beam current of 10 mA. Figure 5 shows the results of simulation.…”

“…However, in reality the situation is not so favorable. The typical proton fraction from the microwave ion sources operating presently at various laboratories is of the order of 60-85% in the extracted beam [4][5][6]. The presence of large fraction of unwanted species alters the dynamics of the primary beam during the transport and causes degradation in the beam quality particularly in the cases where the beam intensity is high.…”

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

“…By increasing the residual gas pressure, it is possible to reduce the emittance growth in the system to levels where it is comparable to the zero SC case [14,18] because as we increase the residual gas pressure the final beam potential decreases to levels where its contribution in the beam dynamics is negligible.…”

“…Even when the behavior indicates that the solution to the SCC emittance growth problem is to increase the residual gas pressure, it is not always possible due to pumping constraints or other processes like beam striping that starts to play an essential role in the beam losses or even increase the emittance [18,19].…”