CHARACTERIZATION OF MICROWAVE DISCHARGE ION SOURCE FOR HIGH PROTON BEAM PRODUCTION IN CW AND PULSED MODE Rosalba Miracoli Consegna del premio “Francesco Resmini” Rosalba Miracoli, Roma, 6 Febbrio 2012

Microwave Discharge Ion Source Rosalba Miracoli, Roma, 6 Febbrio 2012 MDIS-Microwave Discharge Ion Sources produce high current and high brightness of H + beams by means of 2.45 GHz microwave. These sources present many advantages in terms of compactness, high reliability, ability to operate in CW mode or in pulsed mode, reproducibility, and low emittance. The production of high current beams is a key point for different applications Industrial application: Etching: surface cleaning Ion beam sputter deposition: deposition of coatings Ion-assisted deposition: directional deposition Research projects TRASCO, MYRRA DEDALUS ESS

VIS characterization in continuous mode The total beam current, the proton fraction and emittance have been measured as a function of: The 2.45 GHz microwave power The hydrogen mass flow The permanent magnets position The Current depends linearly on the electron density: τ i is the lifetime of the ion The MDIS ion sources produce ion beams with low charges state ( 1 + and 2 + ). So in our sources the increment of in beams is connected mainly to the enhancement of electron density. Rosalba Miracoli, Roma, 6 Febbrio 2012

VIS: Beam current Rosalba Miracoli, Roma, 6 Febbrio 2012 The extracted ion current is plotted as a function of microwave power at various gas pressures. The ion beam trend put in evidence an increase of the ionization rate with the microwave power. If the electron density increases:

VIS: proton fraction Rosalba Miracoli, Roma, 6 Febbrio 2012 The analyzing magnet (30° Bending Magnet) allows to measure the species fraction of the whole beam The fraction of H 3 + molecular species has not been measured because its amount is negligible

VIS: proton fraction Rosalba Miracoli, Roma, 6 Febbrio 2012 The proton fraction of a beam extracted from VIS, at 500 W and at different pressure. H + is always higher than 70 % The H + was measured for different P RF, at 1.9 · 10 −5 mbar. The beam current increases when enhancing the microwave powers, conversely the proton fraction saturates at medium high power levels.

VIS: proton fraction Rosalba Miracoli, Roma, 6 Febbrio 2012 This trends can be understood if the important physical process occurring in a hydrogen plasma have been considered. In VIS source the electron temperature is about 5-20 eV, so high proton content is mainly generated by the two-step process: When we applied higher microwave power values, the electrons gained larger amount of energy. For higher electrons temperature, the cross section enhances slightly for the production of

Emittance Rosalba Miracoli, Roma, 6 Febbrio 2012 DTL ε n. rms < 0.23 π mm mrad RFQ ε n. rms < 0.2 π mm mrad  y’max = 150 mrad   V = 9200 V  Gap between the plates= 6 mm  Effective length of deflection field = 69 mm  Total length ≈ 100 mm  y’max = 150 mrad   V = 9200 V  Gap between the plates= 6 mm  Effective length of deflection field = 69 mm  Total length ≈ 100 mm Emittance Measurements Unit: EMU

VIS: Emittance measurements Rosalba Miracoli, Roma, 6 Febbrio 2012 The rms emittance increases with the P RF because for higher currents there are stronger space charge effects. The beam emittance changes from 0.12 to 0.17 π mm mrad at 60 kV The beam emittance changes from 0.12 to 0.23 π mm mrad at 55 kV The variation of the beam emittance depends on the puller voltage value The space charge field can decrease by applying higher acceleration potential ε n,RMS < 0.25 π mm mrad

VIS: Emittance measurements Rosalba Miracoli, Roma, 6 Febbrio 2012 The beam divergence has its lowest value when the pressure is about 2.5 · 10 −5 mbar. But the corresponding values of the beam current are lower than the other measurements, so low beam intensity means low beam divergence because of the space charge effects decrease. The best experimental condition has been obtained at a pressure of 2.3 · 10 −5 mbar: for this configuration the source performed the highest current with lower emittance values. ε n,RMS < 0.2 π mm mrad

VIS: Emittance measurements Rosalba Miracoli, Roma, 6 Febbrio 2012 The ion beam intensity and ion beam emittance change as a function of the magnetic induction in the injection side. The permanent magnets configuration with the highest divergence beam has been achieved when the permanent magnets were at home. In all the other configurations the emittance values are lower than 0.2 π mm mrad and change slightly with microwave power.

SILHI characterization in pulsed mode Rosalba Miracoli, Roma, 6 Febbrio 2012 Source parameters High voltage 85 kV Intermediate electrode ~ 25 kV Repeller electrode ~ 2 kV Working pressure ~ 1.4 · Torr P RF = 600 W Source parameters High voltage 85 kV Intermediate electrode ~ 25 kV Repeller electrode ~ 2 kV Working pressure ~ 1.4 · Torr P RF = 600 W PULSED MODE D C = duty factor t on = pulse duration f= frequency t r = rise time s t f = fall time 0.16·10 -3 s PULSED MODE D C = duty factor t on = pulse duration f= frequency t r = rise time s t f = fall time 0.16·10 -3 s The time dependence of the ion current is measured for different pulse length at a constant repetition rate of 30 Hz. In pulsed mode the beam currents were more intense than those ones obtained in cw mode operations

Rosalba Miracoli, Roma, 6 Febbrio 2012 SILHI: emittance measurements Emittance [π mm mrad] 10 ms5 ms3 ms2.5 ms2 ms1.5 ms1 ms 10 Hz Hz Hz Hz Duty cycle = 0.04

How could we increase the performances of the Microwave Discharge Ion Sources? Rosalba Miracoli, Roma, 6 Febbrio 2012

There are two BN disksThere are two BN disks and a thick allumina tube AL 2 O 3 Inner diameter: 79 mm. Outer diameter: 89.5 mm. Lenght: 95 mm. BN disk in injection side.

Rosalba Miracoli, Roma, 6 Febbrio 2012 The improvement of the performance with the use of the alumina tube and of the disk of BN disk can be explained by means of two different theories. For the first one the BN and the alumina are electron donors that influence the plasma essentially by emitting cold electrons. The second one explains this phenomenon by introducing the Simon currents within the plasma.

Rosalba Miracoli, Roma, 6 Febbrio 2012

Methods to improve the density of the MDIS Rosalba Miracoli, Roma, 6 Febbrio 2012 Tungsten probe tip: 4 mm length, 0.15 mm diameter Electronics for data coming from the Langmuir probe Tungsten probe tip used as microwave antenna to be connected to power meter and S.A. Spectrum analyzer for inner plasma electromagnetic waves detection

Methods to improve the density of the MDIS Rosalba Miracoli, Roma, 6 Febbrio 2012 At 3.76 GHz there is a suprathermal (ST) electron population at probe position equal to 20 cm (corresponding to B=B ECR /2) The appearance of hot electrons at the first cyclotron harmonic is a signature of possible BW ABSORPTION Suprathermal electrons are much warmer (T ST >20eV) than that ones observed at 2.45 GHz. At P RF =40 W was T ST ~40 eV.

Methods to improve the density of the MDIS Rosalba Miracoli, Roma, 6 Febbrio 2012 The overdense plasma formation in the central part of the cavity, with densities well above the critical value at the corresponding frequency (1.5·10 17 m -3 ). The evident pronounced oscillations of n e are due to EM wave cutoffs and resonances displacements. The establishment of an overdense plasma is another signature of MODE CONVERSION. The establishment of an overdense plasma is another signature of MODE CONVERSION.

Conclusion Rosalba Miracoli, Roma, 6 Febbrio 2012 VIS: CW MODE I MAX ≈ 40 mA H + > 75 % ε< 0.2 π mm mrad SILHI: Pulsed mode I MAX ≈ 100 mA H + > 80 % ε< 0.17 π mm mrad The X-B mode conversion has been successfully demonstrated on a small linear plasma device at extremely low power density (<40 W). Future experiments will be attempted on ECRIS and MDIS, especially in the framework of the beamline developments for the forthcoming European Spallation Source facility.

Rosalba Miracoli, Roma, 6 Febbrio 2012 Santo Gammino Luigi Celona David Mascali Giovanni Ciavola Giuseppe Castro Luciano Allegra INFN-LNS