gek 06/041 ITRP Comments on Question 7 GEK 1/5/04 7.Describe the steps in the scheme to align the rf structures/cavities, quadrupoles, BPM’s and beam delivery elements needed to obtain the ab initio gold orbit and subsequent corrections on the time scales of intrabunch, train to train and slower timescales from seconds to days. What tests assure that this procedure will work and what R&D remains? Describe the time requirements for the tuning procedures and distinguish between intercepting and non-intercepting techniques. NB: “GOLD ORBIT” means beam based alignment. (“The orbit that preserves the emittance”) “BALISTIC ORBIT” means shooting beam through a section of the machine at a time with the rf and quads OFF, then powering the quads on and moving them until no deflection of beam is observed. Then turning on rf section by section.

gek 06/042 COLD OPTION (LINAC) i)Initial mechanical (optical) alignment of RF cavities done warm.  Relationship between warm and cold position MUST be maintained  Large aperture (70mm) makes it easier to transmit beam through linac. ii)BPM in every quad (355 SC quads with H and V dipole windings on H and V focussing elements)  Makes it possible to move the beam horizontally and vertically without moving position of quad. iii)Weak wakefields result in relaxed cavity alignment tolerances which can be achieved by mechanical alignment techniques. iv)Weak wakefields require relatively weak focussing, significantly reducing number of quads that need to be beam-based aligned. v)A full LINAC tuning procedure is described, starting with low charge, low energy beam with rf off.

gek 06/043 Cold option (linac)cont. Comments:  Predicted performance of the beam-based alignment algorithms is entirely based on simulation. For the ballistic alignment algorithm the influence of stray fields (assumed negligible for an SC linac) or residual fields of the quads when they are turned off still need to be studied.  How is the earth’s magnetic field dealt with? This is never mentioned in either technology. Depending on the orientation of the linacs, it can cause a 100GeV beam to deviate from the straight by up to ~35m! This seems to “complicate” the ballistic method??

gek 06/044 Cold Option, Beam Delivery System i)The alignment of the elements of the BDS is more critical than in the linac due to the tighter tolerances. ii)The procedures likely to be adopted by TESLA are essentially the same as those used by the NLC and are based extensively on experience gained at the SLC and FFTB iii)All magnets will contain high precision BPM’s (1 micron) mechanically located in poles of magnets. All magnets will be positioned on independent precision movers. (1micron over 1mm range) iv)Beam based alignment using magnet shunt method. Ie Quad strength is changed by % and magnet mover is then adjusted to null the reslting kick. v) As with the linac the BDS will be aligned using one or a few bunches per train with a non-damped or spoiled emittance beam.

gek 06/045 Cold Option, Beam Delivery System vi)“ Collisions can be readily established with the aid of fast intra-train feedback and short bunch train (ie ~100 bunches). The beam will be detuned at the IP to enable initial collisions to be achieved. ……………” vi)“ In practice, fast luminosity tuning may be made almost continuously by using automated “dither” tuning techniques as pioneered at SLC, which are virtually non-invasive.” Comment:This seems rather optimistic! I would expect the initial alignment and luminosity optimization of the linac and BDS to be a major challenge.

gek 06/046 Cold Option, Stability and Tuning times i)Slow Ground Motion (ATL)  Simulations show that the effects of slow diffusive ground motion (minutes) on the vertical emittance from the linac can be removed by a simple one-to-one steering algorithm (keeping the BPM’s readings fixed).  “The entire linac commissioning and ballistic alignment procedure can be automated. Once iterated a few times and optimized, the entire procedure can be performed within a single shift. The major time requirement is probably the cycling of the quads for ballistic alignment……..”  THIS ALIGNMENT IS INVASIVE  Effect of ATL on the BDS delivered luminosity, using a pessimistic ground motion model, shows that a combination of of fast intra-train beam-beam feedback and much slower orbit correction in the upstream BDS is sufficient to stabilize the luminosity up to ~1 day. This can be compensated for using dispersion tuning knobs.  THIS CAN BE DONE QUASI NONINVASIVELY

gek 06/047 Cold Option, Technology, R&D etc. i)Much of the luminosity (emittance) tuning and stabilization relies on high resolution BPM’s.  Linac requires 10 microns RMS pulse to pulse. Cold BPM’s in the TTF linac have achieved ~50 microns. R&D required to get to 10 microns (not considered to be a major problem).  BDS requires 1 micron, this has been achieved by stripline devices in both SLC and FFTB. ii)Hardware for the fast intra train feedback exists and has been tested at TTF. Kickers having band widths of ~Mhz have been tested. The BPM resolution for this beam-beam feedback is 5 microns, (needs R&D using either stripline or cavity technology). These must be very reliable due to the primary monitors will be inside the detector and therefore relatively inaccessible. iii)The electronic drift of the BPM’s must be well controlled and this would require the electronics to be placed in temperature contolled environment.

gek 06/048 WARM OPTION (LINAC) i)Use conventional (optical and mechanical) techniques to align components to a few mms on distance scales of a km or more. Ie long compared to the betatron wavelength. ii)On onset of operations the beam will be steered to beam dumps thereby establishing a trajectory with long wave alignment. Beam based alignment will then be used to smooth the the trajectory. iii)Key tolerance in Linac, about the smooth trajectory, is 2microns beam to quad and 6 microns beam to RF structure. iv)These tolerances lead to emittance growths of 11% from quad offsets and 8% from RF structure off sets. The total budget for vertical emittance growth in the main Linac is assumed to be 50%, which includes single bunch emittance from quad rotation and a number of time-dependent effects.

gek 06/049 Warm Option (Linac) cont. v) Four 60 cm RF structures mounted on a girder. vi) Each girder has a “mover” at each end. vii) There is a BPM rigidly attached to each quad. viii) The RF accelerator structure will also be used as a BPM, by measuring the power and phase of the induced dipole wakefields, a sensitive measurement of the H and V position of the beam in the structure is obtained. Comments:  This gives an almost continuous beam position indication down the whole length of the Linac.  Although the alignment down the Linac needs to be better (by about a factor of 3, 3microns cf 10microns) than for the cold option the instrumentation is almost continuous.

gek 06/0410 Warm Option, BDS i)The magnet alignment tolerances are actually looser in the BDS than in the main Linac. This is due to the larger number of quads and the larger energy spread in the Linac. In the BDS the capture tolerances, ie tolerances which reduce the emittance growth to a level that can be addressed with “global” tuning knobs is 20microns. Comment:The comparable BDS tolerance for the cold option is 1-5 microns?? I am not sure if these are comparable numbers.

gek 06/0411 Warm Option, Stability and Tuning YEARLY: During initial installation or during long shutdowns, components would be mechanically aligned using conventional techniques to +/- 100microns over lengths of 100m and +/- 1,5mm over lengths of 1 km and greater. Then use beam based tuning. MONTHLY: Need to reestablish the BPM to quad offsets and the resulting gold orbit. The gold orbit will need to be redefined if the electrical centres of the BPM shift by more than 2microns relative to quad magnetic centre. THIS IS INVASIVE. HOURLY: Due to slow ground motion. The severity of this problem depends on the site chosen. The quieter sites, eg US or some Japanese, it will be necessary to correct positions of magnets and RF girders once every few hours. This can be done NON INVASIVELY.

gek 06/0412 Warm Option, Stability and Tuning Cont. Seconds: Over timescales from 8msec (120Hz) to many seconds the orbit and beam energy will be corrected using beam-based feedbacks. These were Used at SLC and FFTB and are used at B-factories and Light sources. Nanoseconds: Claims that X-band design does not require any corrections on this time Scale to meet luminosity goals, techniques have been developed for fast Intra-train feedback at the IP. (FONT and FEATHER expts at SLAC and KEK) Noisy Sites: (eg DESY and KEK) High frequency (>few Hz) can be suppressed using active suppression techniques on the quads. These are routinely used in industry?? Low frequencies are more difficult, need to use beam based feedback and beam based alignment techniques. This would need to be applied much more frequently eg every 10 sec as at KEKB. Additional studies urgently needed if a cold machine were to be put on a “noisy” site.

gek 06/0413 Warm Option, Tests and R&D  The key hardware, eg movers, BMP’s, use of RF structure as a BMP has been tested. Techniques such as high precision quad shunting has been tested, at SLC and FFTB. Dispersion Free Steering (DFS), has been studied at the SLC and LEP.  All other hardware needed to implement the beam based alignment and emittance tuning has been prototyped.  Simulation studies of tuning and operations have been performed.  Further test on rf BMP’s  More realistic tuning simulations, combining many of the static simulations with dynamic studies in order to accurately model the impact of the time dependent effects on the tuning instrumentation.

gek 06/0414 Question 7 My Conclusions and Comments:  In principle the alignment of the COLD linac should be easier. The tolerances are looser, however this is mitigated by two factors in favour of the WARM linac, the more direct accessibility of the positions of the BPM’s and magnets, and the ability of using the rf structure as V and H BPM’s.  More work has been done by the proponents of the WARM technology on beam position technology, this can in some areas be also used by the COLD technology. However, there are some aspects that need still to be developed for the COLD technology.  One of the differences between the two linacs is that the COLD technology has dipole windings (both H and V) while the WARM does not.  This brings me back to the question of the Earth’s magnetic field.

gek 06/0415 My Conclusions and Comments cont.  In the case of the BDS it would appear that the tolerances of the COLD are somewhat tighter than in the WARM. But beam size at IP is larger in COLD than WARM.  The COLD machine is less sensitive to ground motion than the WARM machine. It therefore has a wider choice of sites. However, there are candidate sites that are satisfactory for the WARM option. .

gek 06/0416 My Conclusions and Comments cont.  MY MAIN CONCERN IS THE ALIGNMENT AND TUNING PROCEDURE OF BOTH MACHINES, BOTH INITIAL AND DURING RUNNING. THE PROCEDURES DESCRIBED BY BOTH PROPONENTS ARE LARGELY BASED ON MODELLING AND ALTHOUGH SOME EXPERIENCE HAS BEEN GAINED ON THE SLC AND THE TTF, THERE IS AN ALMOST COMPLETE LACK OF ACTUAL EXPERIENCE WITH SYSTEMS OF THIS SIZE AND COMPLEXITY.  I THEREFORE FEEL THAT BOTH PROPONENTS ARE OPTIMISTIC ON TIMESCALES FOR INITIAL SETUP AND PROBABLY ALSO FOR OBTAINING GOOD AND RELIABLE LUMINOSITY.  AT THIS STAGE BOTH TECHNOLOGIES SEEM FEASIBLE, THE “NATURAL” ADVANTAGE OF THE LARGER BEAM OF THE COLD OPTION IS SOMEWHAT OVERCOME BY THE MORE CONTINUOUS AND SIMPLER INSTRUMENTATION OF THE WARM OPTION.

gek 06/0417 Usefull Linac Parameters (500 GeV) TESLAGLC RF Frequency (GHz) Design Luminosity (10**33) Linac Rep. Rate (Hz) No. of Particles/bunch at IP (10**10) No. of bunches/pulse Bunch separation (nsec) Bunch train length (microsec)