Measurements of an Electron Beam Assisted Laser Ion Source by a76m823ik


									                 Measurements of an Electron Beam Assisted Laser Ion Source
                      V. Orsic Muthig1,3, Bianca Cudalbu2, D.H.H. Hoffmann3,4 , J.Jacoby1, S. Krebs4,
                                             Gonzalo Rodriguez Prieto3 U. Ratzinger1, A. Tauschwitz3
       JWG-Univ. Frankfurt, Germany; 2NILPRP, Magurele, Bucharest, Romania; 3GSI, Germany; 4TU Darmstadt, Germany;

In order to study the effects of space charge neutralization in                                      The measurement of the electric current is performed with a
a laser ion source (LIS) [1], a high current electron source                                         faraday cup that cannot distinguish between electron and ion
[2] was combined with a LIS, as shown in Fig. 1.                                                     beam. The separation of these two beams is realized with the
                                                          Spectrometer                               magnetic field of a permanent magnet which deflects the
Electron Source Gas Inlet          10-30kV
                                                                                                     electron beam and leaves the ion beam mainly unaffected
                                                Target                    Chamber          Faraday   (Fig. 3).
                                                                             Mirror                                    2


                                                                                                        Current (mA)
                                                                                                                                                 without magnet
                                                         Planar Mirrors                   C
                                                                                                                       -4                        with magnet
                                                                                        C CD
                                                                                              a                        -6

Fig. 1:Set-up of the neutralization experiment. The part of                                                            -8
                                                                                                                            -10   0   10    20     30   40   50   60
the figure depicted in blue represents the electron source.
                                                                                                                                           Time (µs)
Since the working pressure of the electron source is much                                            Fig. 3: Faraday cup measurements of both electron and ion
higher (approx. 10-3 mbar) than the pressure needed in the                                           beams with (green line) and without (black line) magnetic
ion source (approx. 10-6 mbar), a gas inlet into the hollow
cathode of the electron source was built. This change in the
experimental set-up, and increasing the length of the capil-                                         The plasma expansion with (right part of Fig. 4) and without
lary tube (by a factor of two) connected to the hollow cath-                                         (left part of Fig. 4) electron beam was recorded with a gated
ode, separates the vacuum systems of electron and ion                                                CCD camera.
source and thus allows the operation of the electron source
connected to the ion source. Due to the highly decreased
pressure at the end of the capillary tube, the electron source
produces a current of 100 A only, which is a factor 10 below
the current under best pressure conditions. The electron
beam diameter increases slightly caused by the non-ideal
pressure in the LIS.
The operation of the electron source leads to the following
effects in the LIS:                                                                                  Fig. 4: Picture of the plasma cloud 6 µs after laser impact on
       •    spark discharges between the extraction electrodes,                                      the carbon target (left side without electron beam, right side
    • a partial voltage break-down at the extraction.                                                with electron beam). The white line on the right side repre-
                                                                                                     sents the axis of the electron beam, which has a maximum
To circumvent these problems, the total capacity connected
                                                                                                     diameter of 2 cm.
                                        to the extraction
                                        system was in-
                                                                                                     In a spectroscopic measurement of the emitted plasma light
                                        creased from 7.2
                                                                                                     in the wavelength region between 100 nm and 500 nm
                                        nF to 42 nF and
                                                                                                     which was performed parallel and perpendicular to the di-
                                        the shape of the
                                                                                                     rection of plasma expansion, no significant differences in
                                        extraction     elec-
                                                                                                     line intensities of the charge states C1+, C2+ and C3+ could be
                                        trodes was opti-
                                                                                                     observed with or without using the electron source.
                                        mized to reduce
                                        voltage       break-
                                        down problems,                                               References
                                        (Fig. 2).                                                     [1] V. Orsic Muthig et al. GSI-2003-1 Report, 136 (2003)
                                                                                                      [2] V. Orsic Muthig et al. GSI-2004-3 Report, 23 (2004)
Fig. 2: Shape optimized extraction electrodes

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