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HIGH POWER ALLISON SCANNER FOR ELECTRONS* Aurelia Laxdal, Friedhelm Ames, Richard Baartman, Dan Brennan, Shane Rupert Koscielniak, David Morris, William Rawnsley, Peter Vincent, Graham Waters, Dimo Yosifov Abstract TRIUMF’s new Allison emittance scanner is designed to measure the emittance of a 100-300 keV electron beam at an average current of 10 mA (1 kW beam power cw) with a phase space area resolution of 0.032 µm, in a high vacuum environment 10 -9 Torr. The emittance scanner has been tested with a beam produced by a 100kV thermionic electron source. The source is modulated at frequency of 650 MHz and can be operated with a macro pulse structure to allow duty factors of less than 0.1 % up to cw operation. The expected transverse emittance is 4εrms ≤ 20µm. This paper discusses the engineering challenges of designing an Allison scanner for a high intensity electron beam, the components and materials used, as well as the technologies introduced. Also included are the experimental results of first tests with the 100 keV source up to 600W @ 99% duty factor. Emittance contours were measured at high resolution for a variety of focusing conditions and intensities.. INTRODUCTION An electron linac (e-Linac) is being designed and installed at TRIUMF as part of the ARIEL project to produce radioactive ion beams through photo-fission. The final specified energy and intensity are 50MeV and 10mA. The electron source for the e-Linac is a thermionic electron gun (e-gun) that will operate at 300 kV. An initial electron source test stand operating at 100 kV is installed for initial beam tests. A variety of beam diagnostics have been installed in the test stand to characterize the electron beam both at low duty cycle and the high duty cycle. REQUIREMENTS An Allison type emittance scanner was proposed for measuring the beam emittance of the 100 keV electron source, and later for the 300 keV electron source. TRIUMF’s previous experience with such scanners is for low power density beams i.e. for pulsed H- beams at 300 keV and 1 µA, and for c.w. heavy ion beams at 50 keV and a few nA. For the electron beam the scanner had to be re-engineered to take high intensity beams at higher beam power densities. The design requirements were: 10 mA full c.w. beam at 100 keV, for a beam size of 1cm. Also, since the e-Gun operates at 10-9 Torr, the technology and materials used for the emittance scanner must meet UHV standards. For this purpose the emittance scanner assembly was considered to be installed on a 6” CF flange. CONCEPTUAL DESIGN ___________________________________________ *Work supported by … THIS INFORMATION MUST BE WITHIN #[email protected] THE TEXT & COLUMN MARGINS Briefly an Allison scanner consists of a mechanical unit where a front slit is stepped across the beam while downstream deflecting plates scan the each selected beamlet across a downstream slit to a Faraday cup to measure the angular spread (Fig. 1)[ref]. Fig 1: Emittance scanner schematics For a non-relativistic Allison scanner the necessary scan voltage for a charged particle of energy E entering with an angle x’ through the entrance slit and exiting through the second slit is: ' V 4 gx E ( D 2 ) The maximum required scan voltage V m is: 2 2 2 8 Eg V ( D 4 ) m for a maximum analyzable angle: x'm2 gD 2 For 100 keV, the relativistic parameters are γ = 1.2, β = 0.55 and for 300 keV γ = 1.59, β = 0.79 respectively. The maximum analyzable angle and the maximum voltage must be scaled by a factor of: k2(11) For the 100 keV electron beam k = 1.09 and k =1.23 for the 300 keV electron beam. The (relativistic) maximum voltage and maximum angle are: V m _Re l V mk ' xm_Rel kx'm DESIGN In order to satisfy the design requirements without jeopardizing the quality of the scans, the length of the deflecting plates was chosen to be as long as possible within the boundary of the CF flange. This defines a length of 45mm, with a gap of 3.5 mm . Since the top plate is biased at high voltage, the distance δ between the electrostatic plates and the pair of slits upstream/downstream them is chosen at 2 mm. The slit gap for both entrance and exit slits is 0.038 mm. The design parameters are also optimized to allow an achievable maximum deflecting voltage and a reasonable maximum analyzable angle. See Fig.1 and Table 1 for a summary of the chosen design parameters. Beam Energy 100 keV 300 keV on the Tungsten plate and 300 deg C on the Tungsten slits. Table 1: ENGINEERING & MATERIALS The emittance scanner materials used inside vacuum chamber are: Oxygen free Copper, selected for its high thermal conductivity and for machining purposes, Tungsten, for its high meting point and low vapour pressure, SS and Aluminium Nitride (AlN) insulators, for their low porosity and low out-gassing rate at high temperatures in UHV. The machining technologies used are: CNC, electron beam welding of Copper, ceramic brazing to SS, electrical discharge machining (EDM) and wire EDM, explosive bonding of Copper to SS and explosive bonding of Copper to Tungsten. The emittance scanner body is made of Oxygen free Copper explosively bonded to SS, so it contains also the vacuum flange: see Fig 2. The seal of this flange is a very light ESI spring energized Metal C-Ring made of Silver plated Inconel. Less than 5000 pounds clamping force is required to compress the seal, so very light non-magnetic SS hardware is used. The water cooling of the emittance head is achieved through 2 parallel water line of 0.25 inch diameter. The parallel water lines have different lengths, so the flow percentage is slightly higher on the upper branch which is shorter compared with the lower branch. The water enters and leaves the emittance head from its top part, which is also the vacuum flange, through 2 VCRs welded directly on the emittance head body: see Fig.2. To preserve the UHV, the main supply and return cooling lines are placed into an inner tube, at atmospheric pressure. Here the assembly has a vacuum arrangement that is formed of: atmosphere-vacuum-atmosphere sandwich. The front slits, made of Tungsten, are covered by a plate, made of Tungsten explosively bonded to Copper, to stop the high power beam and efficiently remove the heat through thermal conduction. This plate has a 1.25mm slit width and collimates most of the beam.. ANSYS steady tate thermal simulations were conducted for a simplified 3D model of a 1cm2 beam spot and with a beam power of 1 kW. See Fig.3. The simulation show 526 deg C on the Tungsten plate, 270 deg C on the Copper bonded material Fig 2: Emittance scanner head and assembly FABRICATION The explosive bonded materials are made by High Energy Metals, Inc. in USA. The AlN insulators are manufactured by Omley Inc. in USA. The EDM and wire EDM of the Tungsten material is done by Innovative Tool & Die Inc in Canada. The rest of the machining and the assembly were performed at TRIUMF. The special ESI vacuum seal is manufactured by Parker Inc. in USA. INSTALLATION AND CONTROLS Prior to installation in the beam line the emittance scanner was cleaned to UHV standards with degreasing in an ultrasound bath. Vacuum and electrical tests were performed. Baking was done directly in situ flowing hot air at 200 deg C through the cooling lines. Emittance scans are controlled from EPICS running under Linux, on a VME based CPU. The following types of VME modules are employed: stepping motor controller for positioning the slit; DAC for controlling the voltage ramp; and variable gain current amplifier/digitizer for scanner current. The number and size of mechanical steps and the range and size of the voltage ramp are selected via a GUI interface. These settings can be saved and recalled. At each step the DAC sweeps the deflector plate voltage in Fig.3 ANSYS thermal simulations and the scanner current is read and stored. When the scan is complete, the scan parameters: position, voltage and current data are written to a file and made available to emittance analysis software. The slit position and voltage may be controlled independently of the emittance software and the current display. This allows the emittance scanner to quickly find the beam during the initial setup. The present readback device has a 10 Hz update rate so the emittance scanner has a variable delay with a minimum value of 100 ms, so a coarse emittance scan of 21 positions by 21 angles takes 58 seconds and a detailed scan of 41 positions by 41 angles takes 190 seconds. [TREK PS, current amplifier QSX?, accuracy?, pulsed beam?] PERFORMANCE AND RESULTS Scans were taken at 60keV for different average beam intensities and duty cycles ranging from 3 µA, 0.1% duty factor to 11 mA, 99.9%. The scan taken at the highest beam power density is for a beam rms size of 2.69 mm at 11 mA c.w. and 60 keV. Fig.4 is one such beam, roughly 700 Watts/cm^2. In a 0.03 mm-mrad pixel of phase space, this is 4 microA at the peak of the emittance figure. The noise is around 1 nA on this gain range. This allows detail down to the 98% contour. In principle, more orders of magnitude sensitivity are available: a similar scanner installed in our radioactive beam facility can measure currents to the fraction of a pA level. The data file, consisting of 6561 current readings, 81 positions and 81 voltages or angles in this particular case, is processed and contour-plotted using a MATLAB script. At high beam power levels, the processing includes background subtraction: the current in the pixels along the lower edge of the emittance plot, where the beam is entering the first slit but deflected too far to make it through the second slit, is used to characterize this background. It arises from a heating effect causing a few nA positive current reading. At the very highest current densities, the entry slit closes from thermal expansion, and this gives an anomalous dip in the centre of the emittance figure. It is readily verified by lowering the duty factor. The rms emittance for the data of Fig. 4 is found to be 10.1 µm, while the 39% emittance is 7.1 µm. For a perfectly gaussian beam, the rms emittance and the 39% emittance are equal. The enlarged rms emittance is due to the “bowtie”shaped distortion evident in the figure. The origin of this kind of distortion is thought to be space charge combined with a non-optimal Pierce geometry of the electron gun; it is under investigation. Fig. 4 . (1) REFERENCES [1] C. Petit-Jean-Genaz and J. Poole, “JACoW, A Service to the Accelerator Community,” EPAC’04, Lucerne, July 2004, THZCH03, p. 249 (2004); http://www.JACoW.org. [2] A. Name et al., Phys. Rev. Lett. 25 (1997) 56. [3] A.N. Other, “A Very Interesting Paper,” EPAC’96, Sitges, June 1996, MOPCH31, p. 7984 (1996); http://www.JACoW.org {no period after URL} [4] F.E. Black et al., This is a Very Interesting Book, (New York: Knopf, 2007), 52. [5] G.B. Smith et al., “Title of Paper,” MOXAP07, these proceedings.