2. Overview of Needs for SixTrack on-line Aperture Check
R. De Maria, M. Giovannozzi, A. Mereghetti, S. Redaelli
8th Mar 2017
A.Mereghetti

3. Outline Introduction; Available Functionalities: Road-map;
Native in SixTrack;
In SixTrack for collimation studies;
In the Fluka-SixTrack coupling;
Road-map;
8th Mar 2017
A.Mereghetti

4. The on-line Aperture Check for SixTrack
SixTrack is a single particle tracking code for beam dynamics based on symplectic integration; it is widely used at CERN for studies of dynamic aperture and collimation;
Aperture check: verify that tracked particles stay within the mechanical aperture of the machine;
 fundamental functionality for predicting losses;
…on-line: the aperture check is performed during tracking;
Loss Map, B2H, 6.5TeV
8th Mar 2017
A.Mereghetti

5. Functionality Presently Available in SixTrack
Check against a general aperture:
Shapes: RECTANGLE;
WHAT(4) and WHAT(5) of 4th row in ITER block;
defaults: 1m;
Check against local apertures:
Shapes: RECTANGLE, CIRCLE, ELLIPSE;
Input: LIMI block (name of single element, aperture type, 2 specifiers);
Liabilities:
performed only if the previous check finds at least a particle out of aperture;
Bugged parsing of LIMI block (e.g. RECTANGLE is actually treated as ELLIPSE and ELLIPSE is treated as general RECTANGLE);
Conflict on use of kp(j) (storing aperture type) with H/VMON (ORBI block);
Limited pool of aperture types;
 Need for revision!
8th Mar 2017
A.Mereghetti

6. Functionality Presently Available for Collimation Studies
Check against local apertures:
Shapes: whichever type that can be generalized as RECTANGLE;
smoothing of aperture model via linear interpolation of aper. parameters (Ds=10cm);
Done as post-processing step by a package external to SixTrack:
Trajectories dumped at every element and turn, and compared to the aperture model;
Actual point of loss is determined via back-tracking particle coordinates until they match the local aperture (linear fitting, Ds=10cm);
Aperture model of entire machine (external to lattice description used at run time):
.tfs table (thick lens) with the description of the aperture (including a tilt angle for some apertures);
Survey file describing aperture offsets;
Hard-coded
8th Mar 2017
A.Mereghetti

7. Functionality Presently Available for Collimation Studies (II)
Liabilities:
Being performed at post-processing, particles out of the machine aperture are still tracked:
Other quantities computed on-line may be over-estimated;
Need for cleaning on-line quantities!
handling of a very large file:
large disk space use;
Intensive I/O;
 not possible to port to BOINC;
OCTAGON aperture not treated;
 Room for improvement!
8th Mar 2017
A.Mereghetti

8. Functionality Presently Available in the Fluka-SixTrack Coupling
Tracking through lattice structure performed by SixTrack;
Once the location of a collimator is reached, the beam is transferred to Fluka for the interaction with beam-intercepting devices, and surviving particles are given back to SixTrack, to continue tracking;
Mechanics done at every collimator found and in every turn;
First applications: endep in SPS scrapers, LHC ion collimation;
Aperture check:
Fundamental to have it on-line for the application to the SPS scrapers, in order not to over estimate losses/endep in blade;
Functionality obtained merging assets of what was already available in native SixTrack and in SixTrack for collimation studies;
8th Mar 2017
A.Mereghetti
P.D.Hermes, 211th CWG

9. Functionality Presently Available in the Fluka-SixTrack Coupling (II)
Check against local apertures:
Shapes: whichever type that can be generalized as RECTELLIPSE + RACETRACK + OCTAGON;
smoothing of aperture model of machine via linear interpolation of aperture parameters - Ds=ad libitum;
Performed on-line during tracking (no handling of big files):
input: LIMI block;
particle trajectories are compared to local aperture model;
actual point of loss is determined via back-tracking particle coordinates via bi-section method;
full aperture model of machine (including offsets and tilt angles) is embedded in the lattice description used by SixTrack at run time:
same input info as the one for collimation studies can be fed;
merging done as pre-processing step;
Liabilities:
Back-tracking performed assuming only DRIFTs (as for collimation version);
Broken original mapping through nlostp(j) (important for DA calculations);
 Good starting point!
8th Mar 2017
A.Mereghetti

10. Road-map
Import the functionality presently available in the Fluka-SixTrack coupling:
Check performed on-line;
Aperture shapes: RECTELLIPSE-like, OCTAGON, RACETRACK;
Smoothing of aperture model + estimation of actual loss point via bi-section method;
including fixing of broken mapping!
Extension to thick-lens tracking: extend back-tracking to (sliced/split) dipoles and quads;
With care, as in coupled simulations there might nuclear interactions leading to more than a particle worth following the tracking (e.g. ion fragmentation)
8th Mar 2017
A.Mereghetti

11. Generation of SixTrack Input via MADX
Input to Fluka-SixTrack coupling:
LHC: complex aperture model generated not directly with MADX, merging .tfs aperture file and a survey file;
SPS: generation can be already done with MADX (only ELLIPSEs and RECTANGLEs);
 Using MADX only would make things more robust, self-consistent and clean;
SixTrack input files are (mostly) automatically generated by MADX via the sixtrack command;
aperture flag of sixtrack command: allows to dump the respective aperture marker immediately after each lens in the lattice structure – “present” (modulo planning/timing of pro release) implementation:
Only aperture types available in native SixTrack (RECTANGLE, ELLIPSE, CIRCLE);
Optimisation of unique definitions of aperture markers (single elements);
8th Mar 2017
A.Mereghetti

12. Generation of SixTrack Input via MADX (II)
In order to provide the on-line aperture check with complete information via MADX, some updates are required;
Some work already done: MADX ticket 351 (closed):
Update format of LIMI block automatically generated: 4 aperture specifiers + 2 offsets + 1 tilt;
(Fixing units);
Still needed:
When slicing a sequence, add an aperture marker at beginning / end of the active length of the thick element  code by H.Burkhardt  MAKETHIN module?
Offsetting aperture (separation/recombination dipoles) with reasonable longitudinal resolution (set by user, as done in the APERTURE module);
Optimise sequence of aperture markers, e.g. avoid repetitions over the same piece of DRIFT;
…can we make for the next pro release?
8th Mar 2017
A.Mereghetti

13. Conclusions
Some mechanisms for aperture checking are available in different versions of SixTrack (native / for collimation / coupling to Fluka);
to converge is synergic to all involved applications / studies:
Merging the assets of all the approaches and overcoming liabilities;
Ease of code maintenance;
Road map was shown, including requirements on the MADX side (input generation);
8th Mar 2017
A.Mereghetti

14. Spare Slides
8th Mar 2017
A.Mereghetti