Field-Assisted DC-Pulsed Cathodes for next generation light sources and accelerators

International audienceThe scope of that contribution is to present the challenges of the next particle sources for accelerators. It is admitted that emittance near Interaction Point (IP) is strongly dominated by the emittance of the low power source. To minimize theBremstrahlung effects in the Interaction Point (IP), we also need extremely low emittance bunches, ultra high brillance, very low charges sub fC, near depopulated attosecond electronic bunches. These produced bunches should fit the entrances of Dielectric Laser Accelerators (DLA) and Laser Plasma Accelerators (LPA).A 20kV DC pulsed sub nanosecond Field Emission Array source with extremely low emittance is considered in order to obtain such results. Firstly, we will describe the DC-pulsed experimental source by blocks. Following that, we will raise more general problems induced by DC-pulsed configuration: thermal transient behaviour of nanostructures, enhancement of plasmons coupling in relation to nanostructured networks, then fast prototyping of cathode geometry will be undertaken using different models. These cathodes are to be fabricated at Orsay.We present the method of curvilinear coordinate calculus, adapted to major classes of nanostructured tips. We define 3 major classes of 3D analytical profiles to emulate experimental conditions (multi-segment, quadratic and exponential one) and apply curvilinear analytical Maxwellsolving to find electrostatic potential around the profile. Our method is concurrent to T-splines for instance, but it is expected to converge faster. Cathode physics will be modelled integrating different phenomenons: photo/thermal/field/emission... Results will be compared to electromagnetic simulations with CST and Astra tools. To conclude, we shall then evaluate the emittance performances planned for a 20keV cathodic source, and its acceptance to the next stages, with the help of some electrostatic focusing. Numerous experimental and theoretical aspects are to be solved

PHIL photoinjector test line

LAL is now equiped with its own platform for photoinjectors tests and Research and Developement, named PHIL (PHotoInjectors at LAL). This facility has two main purposes: push the limits of the photoinjectors performances working on both the design and the associated technology and provide a low energy (MeV) short pulses (ps) electron beam for the interested users. Another very important goal of this machine will be to provide an opportunity to form accelerator physics students, working in a high technology environment. To achieve this goal a test line was realised equipped with an RF source, magnets and beam diagnostics. In this article we will desrcibe the PHIL beamline and its characteristics together with the description of the first two photoinjector realised in LAL and tested: the ALPHAX and the PHIN RF Guns

Optimization of flat top of high pulsed power waveform by wavelet analysis

Accelerator physics for elementary particle colliders : topics linked with time frequency domain tuning of Pulse Forming Network in High Pulsed Power sources Particle physics needs high quality beams for reaching a minimum of energy/position dispersion (low emittance)colliding bunches. It means in first, constant flat top of pulsed power sources, like High Voltage modulators. Conventional micro seconde one, already lie on Pulse Forming Network (PFN), which proper tuning influence the minimum ripple on wave form. That tuning is not trivial. tuning of Pulse Forming Network in High Pulsed Power sources The goal of the machines like Large hadron Collider LHC at CERN, is to generate collision of packets of particles with a high definition in energy/momentum, hence a small dispersion. In fact, the ''emittance'', which is the volume of X,P in phase space, must be the smallest as possible. For that, the impulse power sources must deliver an electrical, ideally ''flat top'' (inside that plateau, the energy of emitted particles is constant), so unlike signal recognition or transmissions for example, for today accelerators the performances are not yet axed on rising or falling time (however it could be the case in future high frequency machines). The resulting spectrum of power waveforms is then dominated by its pass-band character. To improve the flat top, we need to develop and tune some passive Pulse Forming Network PFN, and their performance must reach some per mill of the High voltage, regarding the relative amplitude of the ripples in the flat top. The problem of optimal tuning seems to lie in ''control and optimization'' class. In our machine, it depends on 12 continuous variables, who are the values of tunable power inductances. The PFN is a 12-strongly coupled harmonic oscillator (many bodies problem), and the optimal tuning state is not described by a trivial algorithm. What we need is to find a control and optimization method, either by analytical -if possible- approach, either by analysis of measurement during tuning. As classically, the time or frequency waveforms do not help us to improve the tuning, essentially because in time measurements, 2 distant cells of PFN may interfere with unknown phases. Naturally the Frequency transform doesn't allow us to catch that hidden information. We then used a wavelet toolbox, with scilab, and as we met some difficulties in the dynamic range of our waveforms with that toolbox, we are presently conducting some direct investigations with for example complex morlet wavelets. We shall describe the first results. In alternative approach, we try to solve the correspondent evolution equation, which belongs to Hill family, because it is linked with a pseudo transmission line with inhomogeneous characteristic impedance. We have extrapolated the Flocquet method by infinite determinants, but in synthesis like algorithm. We shall also discuss these first progresses. Considerations on mixed/intricate time frequency analysis Inside the same problem, we try to answer to another different question : is there a class of transform which would use a ''mixed variable'' which we call for example w. For sake of simplicity, w is complex and we have w= g(f,t) (or g(a,t) where a is the scale parameter of wavelet formalism). All our measurement V(t) or V(f) should be then transformed/mapped to V(w) where V is the High voltage image of the power source. To make our question or formalism useful, we have to restrict the features of g. We try to discuss these restrictions regarding the general aspects of g, (symmetries, deterministic category then metric abscissa ?...) and also investigation on the ''visibility'' of V(w), as measurement tool in signal recognition. It is not mandatory to closely link that question to precedent theme. We intend to discuss/list some -linear or not- representations different from the couple translation/rotation. That discussion and these ideas could help us in many other topics of the physic, like Electromagnetic field measurements (photo emission assisted by Schottky effect) for femtosecond electrons bunches. That issue could also be beneficial in other scientific researches

Controlled electron emission and vacuum breakdown with nanosecond pulses

International audienceVacuum electron sources exploiting field emission are generally operated in direct current (DC) mode. The development of nanosecond and sub-nanosecond pulsed power supplies facilitates the emission of compact bunches of electrons of high density. The breakdown level is taken as the highest value of the voltage avoiding the thermo-emission instability. The effect of such ultra-fast pulses on the breakdown voltage and the emitted electron current is discussed as a result of the thermo-emission modelling applied to a significant protrusion. It is found that pulsing very rapidly the vacuum breakdown occurs at higher voltage values than for the DC case, because it rises faster than the heat diffusion. In addition, the electron emission current increases significantly regardless of the theoretical approach is used. A comparative study of this theoretical work is discussed for several different forms of the protrusion (elliptic and hyperbolic) and different metals (hence varying the melting point), particularly refractory (tungsten) versus conductor (titanium). Pulsed mode operation can provide an increase on breakdown voltage (up to 18%) and a significant increase (up to 330%) of the electron extracted current due to its high non-linear dependency with the voltage, for the case for the case with a hyperbolic protrusion

Low electrical resistivity in thin and ultrathin copper layers grown by high power impulse magnetron sputtering

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