Gmail - RE: Meeting yesterday Yine Sun <yinesun@gmail.com> 

RE: Meeting yesterday

      Power, John <jp@anl.gov> Sun, Feb 18, 2007 at 9:17 PM 
      To: "Kim, Kwang-Je" <kwangje@aps.anl.gov>, "Gai, Wei" <wg@hep.anl.gov>, 
      "Harkay, Katherine C." <harkay@aps.anl.gov>, philippe.piot@gmail.com, Yine 
      Sun <yinesun@gmail.com> 
            Hi All,
             
            Here’s a short summary of the meeting.  Please feel free to correct 
            or add anything that needs fixing. (I’m sure I left out a few 
            things!)
             
            *) Attending: Yin-e, Philippe, John, and Kwang-Je
             
            *) E-x Conceptual understanding.  Yin-e is writing up her Matlab 
            simulations that track individual particles through the first-order 
            beamline.  The goal of the simulations is to develop a simple 
            explanation of the e-x process.  The difficulty is in choosing the 
            initial particle coordinates so that the results are clear.  In 
            particular, this means choosing initial coordinates that do not 
            couple transverse and longitudinal coordinates.
             
            *) Deflector Cavity Sims: John showed MathCAD simulations of the 
            TM110.  The most significant result is that the cavity gives a 
            transverse offset of the beam centroid on passing through the 
            cavity. (Philippe used a Matlab-based code to verify this result so 
            we now have high confidence that the effect is real.)  The 
            simulations also reveal that while the size of the transverse 
            momentum kick does not depend on frequency for a given field, the 
            size of the offset does.  For example, a 4 mrad kick has a 5 mm 
            offset at L-band, but a 40 um kick at X-band. This is likely the 
            reason that this effect is either unknown or ignored by other 
            researchers.
             
            Three options were discussed for dealing with the offset.  1) Design 
            the back-half of the e-x beamline so that it can be translated.  
            This is will not be trivial. Aside from the practical difficulties 
            of using bellows and a translation system, we would also need to 
            worry about re-aligning after each translation which may make the 
            beam tuning difficult.  2) Use a pair of magnets to recenter the 
            beam after the deflector cavity.  This is probably the least 
            attractive option since it will actually change dispersion which 
            will affect the exchange. 3) Tilt the deflector cavity to cancel out 
            the offset.  This would have similar practical difficulties as ‘1’ 
            since we would need to install bellows at the front and back of the 
            cavity, develop a way to rotate the cavity in a controlled manner, 
            and check the alignment after every rotation.
             
            *) Initial Phase Space.  Kwang-Je suggested that we use an 
            elliptical laser spot to generate a initial normalized phase space 
            of (10, 3, 3) and e-x it to (3, 3, 10); thus avoiding the additional 
            complexity of flat-beam generation for this PoP experiment.  
            Ideally, we will do this at as low of a charge as possible (say, 100 
            pC) in the hopes of avoiding a space-charge dominated beam during 
            the e-x.  The only constraint of the lower limit of the charge is 
            detection.
             
             
            *) Diagnostics.  We agreed that the longitudinal and transverse 
            emittance must be measured before and after the e-x beamline.  
            Discussion of diagnostics is still in the very early stages and 
            should be taken with a grain of salt at this point. While transverse 
            emittance measurements should be manageable, the longitudinal 
            emittance measurements are problematic.  For transverse emittance we 
            may even be able to use a quad scan since the charge is so low.  
            Otherwise, a pepper pot or a slit is still straightforward. Options 
            for longitudinal are:
            +The AWA has never done a measurement of the longitudinal phase 
            space, but has planned (for several years) to mount a Be window on 
            the spectrometer exit and use a streak camera to look at either 
            aerogel or frosted-quartz plate for a true energy-time correlated 
            meaurment. 
            +Sweeping the phase of a 2nd accelerating linac (not currently 
            installed) and measuring the energy spread to accomplish a 
            “longitudinal quad-scan” ala Dave Dowell.
             
            Lastly, Philippe suggested that we use TOF monitors at the front and 
            back of the e-x beamline in order to create a map of the 
            longitudinal transfer matrix.  Philippe suggested that we use an e-o 
            based scheme for the TOF measurements.  While we should consider 
            this, we must consider that this technique is still in development 
            at the AWA.  John suggested that we get help from Bob Lill at APS to 
            implement a stripline BPM-based system that uses the Analog Devices 
            phase comparator chip which is capable of ~1 degree RF.  Philippe 
            will investigate what resolution the TOF system needs.
             
            *) The time of the e-x meetings have now been changed to Thursday’s 
            since Philippe does have teaching obligations on this day.
             
            *) Next week’s agenda:
             
            Yin-e: look into the diagnostics
            John: look into the possibility of tilting the deflecting cavity in 
            order to avoid the offset.  Compare the first-order cavity kick 
            (x,x’, z, delta) to the exact trajectory.  Calculate the transverse 
            shunt impedance of the MWS cavity to the TM110 pillbox.
            Philippe: Will make an estimate of the timing resolution needed from 
            the TOF monitors in order to create a longitudinal transfer map.
             
             
            Cheers,
            John