| 1 | <head> |
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| 2 | <title>SEQUENCE</title> |
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| 3 | <!-- Changed by: Chris ISELIN, 27-Jan-1997 --> |
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| 4 | <!-- Changed by: Hans Grote, 30-Sep-2002 --> |
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| 5 | </head> |
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| 6 | |
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| 7 | <body bgcolor="#ffffff"> |
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| 8 | |
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| 9 | <center> |
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| 10 | EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH |
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| 11 | <IMG SRC="http://cern.ch/madx/icons/mx7_25.gif" align=right> |
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| 12 | <h2>Beam Line Sequences</h2> |
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| 13 | </center> |
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| 14 | |
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| 15 | MAD-X accepts two forms of an accelerator definition: sequences and |
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| 16 | <a href="line.html">lines</a>. However, the sequence definition is the |
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| 17 | only one used internally; lines are converted into sequences when they |
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| 18 | are USEd. Consequently, only sequences can be saved (written onto a file) |
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| 19 | by MAD-X. |
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| 20 | <p> |
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| 21 | The corresponding sequence of statements defining a sequence is |
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| 22 | <pre> |
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| 23 | name: SEQUENCE,REFER=keyword,REFPOS=name,LENGTH=real,ADD_PASS=integer,NEXT_SEQU='seq_name' |
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| 24 | label: class,AT=real{,attributes} | class,AT=real | sequ_name, AT=real |
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| 25 | ... |
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| 26 | ENDSEQUENCE |
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| 27 | </pre> |
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| 28 | where "real" means a real number, variable, or expression. |
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| 29 | <p> |
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| 30 | The first line gives the sequence name, a REFER flag (entry, centre, |
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| 31 | or exit) which specifies at which part of the element its position along |
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| 32 | the beam line will be given (default: centre), a REFPOS argument used |
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| 33 | for sequence insertion, and the total length. |
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| 34 | The ADD_PASS=integer option specifies a number of additionnal passes (max. 5) |
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| 35 | through the structure; in case of an RBEND the angle will be overwritten in |
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| 36 | survey using the i-th component (1 <= i <= <tt>add_pass</tt> <= 5) of its <i>array_of_angles</i> |
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| 37 | (see <a href="bend.html">RBEND<a>). |
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| 38 | The NEXT_SEQU='seq_name' concatenates the sequence with name 'seq_name' |
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| 39 | to the end of the specified sequence. |
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| 40 | <p> |
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| 41 | Inside the sequence ... endsequence bracket three types of commands |
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| 42 | may be placed: |
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| 43 | <ul> |
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| 44 | <li>an element declaration as usual, with an additional "at" attribute |
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| 45 | giving the element position relative to the start of the sequence; |
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| 46 | CAUTION: an existing definition for an element with the same name will |
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| 47 | be replaced, however, defining the same element twice inside the same |
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| 48 | sequence results in a fatal error, since a unique object (this element) |
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| 49 | would be placed at two different positions.</li> |
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| 50 | <li>a class name with a position; this causes an instance of the class |
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| 51 | to be placed at the position given. For uses inside ranges, instances |
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| 52 | of the same class can be accessed with an occurrence count.</li> |
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| 53 | <li>a sequence name with a position; this causes the sequence with that |
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| 54 | name to be placed at the position indicated. The entry, centre, or exit |
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| 55 | of the inserted sequence are placed at the position given, UNLESS a "refpos" |
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| 56 | (the name of an element in the inserted sequence) is given, in which case |
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| 57 | the sequence is inserted such that the refpos element is |
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| 58 | at the insertion point.</li> |
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| 59 | </ul> |
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| 60 | When the sequence is expanded in a |
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| 61 | <a href="../control/general.html#use">USE</a> command, |
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| 62 | MAD generates the missing drift spaces. At this moment, overlapping elements |
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| 63 | will cause "negative drift length" errors. |
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| 64 | <p> |
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| 65 | For efficiency reasons MAD-X imposes an <b>important restriction</b> |
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| 66 | on element lengths and positions: once a sequence is expanded, the element |
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| 67 | positions and lengths are considered as fixed; in order to vary a position |
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| 68 | or element length, a re-expansion of the sequence becomes necessary. |
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| 69 | The MATCH command contains a special flag "vlength" to |
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| 70 | <a href="../match/match.html">match element lengths</a>. |
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| 71 | <p> |
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| 72 | <a name=example>Example:</a> |
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| 73 | <pre> |
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| 74 | ! define a default beam (otherwise fatal error) |
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| 75 | beam; |
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| 76 | ! Define element classes for a simple cell: |
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| 77 | b: sbend,l=35.09, angle = 0.011306116; |
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| 78 | qf: quadrupole,l=1.6,k1=-0.02268553; |
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| 79 | qd: quadrupole,l=1.6,k1=0.022683642; |
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| 80 | sf: sextupole,l=0.4,k2=-0.13129; |
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| 81 | sd: sextupole,l=0.76,k2=0.26328; |
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| 82 | ! define the cell as a sequence: |
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| 83 | sequ: sequence,l=79; |
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| 84 | b1: b, at=19.115; |
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| 85 | sf1: sf, at=37.42; |
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| 86 | qf1: qf, at=38.70; |
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| 87 | b2: b, at=58.255,angle=b1->angle; |
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| 88 | sd1: sd, at=76.74; |
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| 89 | qd1: qd, at=78.20; |
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| 90 | endm: marker, at=79.0; |
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| 91 | endsequence; |
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| 92 | </pre> |
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| 93 | |
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| 94 | <address> |
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| 95 | <a href="http://www.cern.ch/Hans.Grote/hansg_sign.html">hansg</a>, |
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| 96 | June 17, 2002 |
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| 97 | </address> |
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| 98 | |
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| 99 | </body> |
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