# Presentation: 103843

## Dynamic Range Issues for Magnets & Machines

- Existing Machines
- Basic Issues
- Injection Energy Scaling
- Design Issues (or how bad can you get and still work ?)
- Solutions
- Better Magnets
- Better Machines
- So what is the (correct) dynamic range for a 50 Tev Ring ?

## Existing Machines

- Definitions:
- Machine: ratio between operating/injection energy
- (Magnets: ratio between highest/lowest field within specs)
- Examples
- Tevatron		7:1	fields	0.66T-> 4.5T
- RHIC		9:1	fields	0.39T-> 3.45
- HERA		20:1	fields	0.23T -> 4.5T (5.5T)
- LHC		16:1	fields	0.54T -> 8.4T
- (n.b.		AGS Booster  95:1          Main Ring  60:1)
- Differences at the factor of ~3 level.  Most choices heavily influenced by existing injector
- General assumption is that more dynamic range is good

## Basic Issues

- For a given machine design
- Injection energy set by lowest usable field level which is, in turn, set by field quality distribution

- Effective field quality distribution is determined by geometric field + dynamic effects: persistent currents &  eddy currents -> snapback, ramp rate dependance etc....  These effects have systematic/allowed & random/ non-allowed harmonics.  Ends v’s bodies etc..

## Basic Issues cont.

- Generally the definition of lowest usable field is the issue which poses the interesting accelerator design questions.  Quench characteristics determined by magnet technology, tends to be less flexible.
- Machine sets injection energy
- Magnets set top energy

- Operating energy determined by quench current distribution

## Injection Energy Scaling

- Magnetic field quality gets worse with decreasing excitation: persistent currents
- Some machine parameters get worse with decreasing energy i.e. beam size.  Aperture defined in terms of beam sigma
- Instabilities generally worse with decreasing energy  1/B
- Some machine parameters get better with decreasing energy i.e. injection lattice.  Relaxing focussing at injection can remove the IR magnets from consideration (less sensitivity).

- This results in dynamic range defined by the arc magnets

## Design  Issues (or How bad can you get and still work ??)

- Linear aperture, non-linear aperture, dynamic aperture

- RHIC at injection

## Design  Issues (or How bad can you get and still work ??)

- Tracking results in qualitative agreement (70%) of calculated

- Tracking predictions v’s machine measurements

- SPS with tune modulation

- HERA at injection

## Design  Issues (or How bad can you get and still work ??)

- Operating Conditions
- Dwell time -> injection lifetime
- Beam tolerances -> emittance growth (non-linear aperture), beam loss sensitivity (dynamic/physical aperture)
- Aperture requirements
- This issue gets more complicated as the injection energy increases and the beam size gets smaller
- Aperture = constant + n sigma (n = ~7-8)
- If sigma = 2mm then this is easy, if sigma = 0.1mm ??

## Solutions - build better magnets !

- Persistent currents proportional to Jc x effective filament diameter, hence reduce filament diameter
- Tevatron 7, RHIC 6, HERA 16, LHC 7
- Filament diameters down to  before Jc degradation, SSC R&D cable at 2.5.  Issue of cost & yield
- Or reduce Jc
- Reduces quench performance (hence dynamic range)
- Increase magnet aperture to reduce sensitivity to field harmonics (LHC 50->56mm, SSC 40->50mm)
- Cost implications

## Solutions - build better magnets cont.

- Better inherent field quality: doesn’t help much since dynamic effects dominate at low excitation
- Iron dominated magnets: minimal dynamic effects & random harmonics, high field point limited to 2T but magnet dynamic range can be expected to be better (for these magnets other issues dominate related to beam size)
- Higher fields:  Nb3Sn effective filament size is 50, Jc improves by factor of 7 -> persistent currents increase by ~50 !

- RHIC D0 Dipole

## Solutions - build better machines !

- Machine Design: Lattice
- Generally speaking fiscal considerations for large machines cause the arc lattice to be constructed so that field quality is _always_ the low energy issue.  In addition large arcs give more optics sensitivity to a unit of magnetic field harmonics (LHC 52:1, RHIC 5:1, chromaticity v dipole b1)
- Machine Design : Parameters
- Minimise impact of short injection lifetime e.g. rapid cycling injectors, single turn injection
- Maximise allowable tolerances e.g. 20% beam loss at injection, 100% emittance growth
- Operate at the highest possible energy e.g. localised temperature control, magnet selection/location
- Smaller beams reduce aperture demand for fixed beam sigma

## Solutions - build better machines ! cont.

- Machine Operations: minimize impact
- Optimize ramp (everyone does this)
- Fix cycling criteria e.g. 3 ramps to top energy then dwell (everyone does this)
- Feed-forward from previous operating cycles (Tev)
- Feed-back from reference magnets (HERA)
- Feed-back from online model (LHC)
- High bandwidth, distributed correction systems (All to some degree)
- Machine Design: innovations
- Passive correctors
- Iron dominated magnets at low field
- HTS ??

## Solutions - build better machines ! cont.

- Machine Design: better calculations (allow us to run closer to the ‘edge’ )
- More computing power
- More precise field measurements
- On-line data bases
- How well do we really know the input conditions ?

## So what’s the (correct) dynamic range for a 50 Tev Ring ?

- Very large machines tend to exhibit characteristics that work against a large dynamic range
- Small apertures, high fields(Nb3Sn), arc cell length
- Without any innovation then a dynamic range similar to the LHC looks about right.  This by itself would require incremental improvements.  For 50 Tev then ~3 Tev injection energy (= ~0.5mm, ~40 mm coil separation) is reasonable to adopt initially without any other technical information.  The high field approach can tolerate emittance dilution and hence a slightly less conservative approach
- If we can make the dynamic effects go away ( low-field magnet, technical innovation ) and there is sufficient physical aperture then 50:1 dynamic range is not unthinkable (= ~ 1 mm )