ELECTRON BEAM PROBE FOR BEAM POSITION AND BEAM SPACECHARGE MEASUREMENTS# L.Mishra*, P. Roychowdhury, H. Kewlani, Dr. K.C Mittal, Accelerator and Pulse Power Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Abstract Space charge (SC) compensation facilitates the transport of low energy intense Ion Beam. Thus determination of the radial distributions of the beam potential, compensating electrons (CE) and particle densities in the proton ECR Ion Source  which has been developed for low energy and high intensity beam for ADS  application is necessary. The source also finds other applications in various areas of science and technology. Such a high power beam system has to perform as per the required design parameters and thus the performance has to be monitored with reference to the designed parameters. The ECR Ion Source designed delivers a 40 mA Hydrogen beam current at 50 KeV. The beam from the source enters in the LEBT section consisting of the RFQ, Linac and finally the target where neutrons will be generated to control the power generation in a Nuclear Reactor. The complete system integration involves regulated and stable beam. The beam emittance has been measured and found to be less than 0.19 pi mm mrad rms normalized at 25 keV beam energy and proton fraction has been found to be better than 80 percent of the total beam current. While the beam propagates the emittance growth due to nonlinear radial distribution of the residual space charge forces of the system beam ions and CE. EBP offers a possibility to measure preciously the SC neutralization of the ion beam and also beam position could be inferred with this technique. The EBP measurement does not perturb or disturb the Ion beam and hence EBP gives advantage over conventional ways to measure the beam current including Faraday cup, Langmuir probe, slit wire scanners and often sometimes with scintillating screens. INTRODUCTION The SC of Low-energy (25 - 50 keV) beam transport (LEBT) of high-current (25 - 40 mA) positive-ion beams in background gas pressures of order 10-5 mbar is predicted to be effectively neutralized by electrons in a beam plasma . The degree of SC neutralization is an important parameter in beam transport calculations and simulations. Various techniques are developed for the experimental investigation of SC compensation factor for positive ion beams. It involves the measurement of the slow ion energy distribution using a Four Grid Energy Analyzer  which measures the secondary-ion energy distribution to determine SC compensation of the Ion _____________________________________ *Department of Atomic Energy, India. # firstname.lastname@example.org Beam. In this paper we describe an Electron Beam Probe (EBP) to measure the Ion Beam Position and Space Charge Compensation. This non-disturbing and nondestructing determination of SC neutralization and beam positioning has become of common interest rather than the other methods. RF Langmuir Probe has been used to perform such diagnostics but the measurement differ from the actual charge distribution as it perturbs the actual space charge compensation by emitting secondary electrons and even capturing the CE’s. Rowgoski coil and similar other such inductive probes could only measure total beam charge but cannot measure the beam crosssectional charge distribution. Hence EBP finds the best place for beam space charge neutralization and beam position monitoring. The beam availability during the measurement process is what differentiates the measurement using EBP than the other techniques, though the apparatus and experimental set-up has resulted from high advancement in involved electronics and components. MEASURING TECHNIQUE The electron beam of 1µA and of 1 keV energy will traverse the ion beam transversally. Assuming the EBP field does not perturb the ion beam, in fact which is true as far as electron probe current is less than the ion beam current, the EB gets deflected due to the space charge field of the ion beam, without influencing the ion beam. The deflection of the EB is then used to determine the space charge neutralization of the ion beam and its position. Under the assumption that the Ion Beam particles are confined within an infinite long cylinder extending symmetrically on both of the EBP, the analysis is simple. The deflection of the electron trajectories is described in terms of deflection angle α (𝑎) , (Fig.1) defined as 𝛼(𝑎) = 𝑒.𝑎 𝑈 𝑟 𝐸(𝑟)𝑑𝑟 ∫𝑎 𝑑 (𝑟 2−𝑎2 )1/2 (1) Here 𝑎 is the impact parameter, 𝐸(𝑟) is the electric filed at distance ‘𝑟’ from the beam axis, 𝑟𝑑 is the distance of e-gun from the ion beam axis which in the designed apparatus is the beam pipe radius, 𝑒 is the charge of the electron. The charge of the ion beam which we have assumed is cylindrical gives rise to the field 𝐸(𝑟). The magnitude 𝐸(𝑟) depends on the charge of the ion beam and the distance from the beam axis. Around Fig.1 Trajectory of electrons due to the space charge of the Ion beam Halo the Ion beam the field only depends on the charge seen at that point. The charge of the ion beam is neutralized by the electrons produced during the residual gas ionizations by the ion beam. These electrons shield the electric field of the beam particles from each other and prevents the beam from blowing up due to the its space charge forces. The beam space charge increases with the ion beam current. In this paper we have simulated the deflection of the probe electrons as a function of the impact parameter 𝑎, for fixed energy of the Ion beam. In Fig 2, the deflection 𝛼 is plotted as a function of impact parameter 𝑎 for various levels of space charge neutralization. Fig. 2 Simulated deflection characteristics at various level of charge compensation. Fig. 2 shows the simulated characteristics for different degrees of space charge compensation for a 40mA H+ 50 KeV ion beam. Practical measurements of the deflection angles are difficult to make rather than the deflection parameter 𝜆 which is easily measured experimentally. For small deflection angles, the deflection parameter is related with the deflection angle. 𝜆(𝑎) 𝛼(𝑎) ≈ 𝑟𝑑 ⟹ 𝜆(𝑎) = 𝛼(𝑎)𝑟𝑑 The radial space charge potential distribution 𝑉(𝑟) can be obtained from the deflection characteristics with an Abel inversion of the integral equation (eq.1), as the space charge potential. 𝑉(𝑟) = 𝑟 2𝑈 𝜆(𝑎)𝑑𝑎 ∫𝑑 𝑒𝜋𝑟𝑑 𝑟 (𝑎2 −𝑟 2 )1/2 (2) The distribution determines the charge compensation of the Ion beam by the ionized electrons of the residual gas. Thus one can measure experimentally the deflection parameter of e-beam and from above analysis determine the degree of space charge compensation of beam. The EBP could be used to measure s.c compensation at various residual gas pressures which then directly determines the density of ionized gas molecules/ions to compensate for the ion beam charge. As EBP gives the potential distribution from which charge density profile of the ion beam could be found, the technique would also give the beam location with the beam tube and its transverse profile . In principle EBP could also be used to perform time of flight measurement to determine the time frame in which the charge of the ion beam is space neutralized. The time resolution would be of ns order which is the time of traversal of the electrons from the e-gun to the edetector scintillator screen, while the beam s.c neutralization event is of ms duration . EXPERIMENTAL TECHNIQUE In spite of the straight forward analysis, experimental performance of the EBP is challenging. The set-up designed is shown in Fig. 3. The test setup requires an compact e-gun, emitting Gaussian profile electron beam with energy 1keV and very low thermal energy spread (~3eV), the e-beam should be stable within the experimental time frame. As the energy of the electrons from the probe is low and being very low mass particles, electrons deviate from straight trajectories due to background weak electric and magnetic fields apart from the deflection due to the space charge force of the ion beam. The effect of geomagnetic field responsible for producing electron Fig.3 The e-beam diagnostic experimental setup. deflection is corrected by the electrostatic deflecting magnets such that the offset due to earth’s magnetic field is compensated by the field of the chicane system. The electrons after traversing the ion beam are detected at the low energy scintillator screen. The electron position on the screen is measured at various electron beam offsets and stored for data analysis. Thus at different impact parameters the deflection parameter is measured and by Abel inversion potential distribution of the ion beam is determined. Also the position could be inferred from the analyzed data. 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