PXIE Home page

Functions of LEBT                                                                                         Link to LEBT folder on Sharepoint

1.    Beam transport and matching from the ion source output to the RFQ                           

2.    Beam chopping                                                                                                           

3.    Beam diagnostics

4.    Machine protection                                                                                                                  ACNET devices names for IS & LEBT

 

            Link to the PXIE Elog
                 
(requires credentials to make entries)

 

 

Project X LEBT: Functional Requirements Specification (also applicable for PXIE)

 

 

‘Reference’ design (stable since 3/14/13)

The main feature of the current LEBT design is the transport of a non-neutralized high perveance beam over a ~1m section in front of the RFQ.

Typically, the beam exiting an ion source is rapidly neutralized (> 90%) and remains so until it is chopped in front of the RFQ. Neutralization greatly reduces the beam space charge effects, most importantly, the emittance growth. However, a chopper removes the neutralizing particles and the beam becomes uncompensated. Consequently, the chopper is placed as close as possible of the RFQ to limit the length of the non-neutralized transport.

For Project X/PXIE, the 5-mA beam space charge is comparatively low; several times lower than, for instance, the beam at SNS. Simulations showed that a scheme where the beam would be uncompensated in a part of the LEBT is possible, and such scheme was devised. This scheme should reduce the transient effects during chopping in the LEBT and allows for more space after the chopper for the absorber and diagnostics. On the other hand, the beam emittance may increase.

Practically, it led to designing a scheme with 3 solenoids (instead of 2 originally) and adding a set of electrodes to enforce and maintain non-neutralization of the beam where needed.

Another feature of the design is the presence of a bending/switching magnet after the first solenoid in order to accommodate a second ‘leg’ with another ion source. Such redundancy is necessary to minimize downtime due to failures or simple maintenance of one of the ion sources (e.g.: ion source filament replacement).

Below is a sketch of the beam line showing the overall configuration of the LEBT and the main elements that composes it. Note that it is not intended to show design choices for any of the components.

 

Mechanical sketch of the beam line (12/18/12)
Earlier sketches/designs are here

Note: As opposed to the Project X front-end, for PXIE, only one source will be used. However, the switching/
bending dipole magnet will be part of the PXIE beam line.

 

The drawing is to scale. Total length » 2.2 m.

Some comments:

-   The nominal beam pipe diameter is 3”, which fits into the 80-mm bore diameter of the solenoids

-   It is envisaged to reduce the beam pipe diameter right after the chopper (for differential pumping)

-   Solenoids yoke length = 146 mm

-   Emittance scanner is located in the (redesigned) ion source vacuum chamber

s  As a result, the DCCT has been moved upstream of the switching magnet (it was located right after the chopper)

-   The isolated diaphragms and other features of the beam line downstream of the dipole switching magnet are necessary for uncompensated transport

 

 

PXIE LEBT configuration for RFQ commissioning

For PXIE, only one ion source will be used. However, it is planned to include the dipole switching magnet into the final configuration of the PXIE LEBT as shown on the sketch above. On the other hand, the risk associated with the design, construction and operation of the switching magnet, which is quite standard, is minimal. Thus, given the available resources, it was chosen to postpone the design and fabrication of this dipole for later in the program. Consequently, a first configuration of the LEBT will consist in a straight beam line from the ion source to the RFQ. The corresponding layout is shown below.

 

Mechanical sketch of the beam line in the straight configuration (as of 10/03/13)

 

 

Note that the LEBT in its full configuration (i.e. with the bending magnet) is not required to study space-charge dominated transport and the dynamics of neutralization, important and unproven features of the adopted scheme. Actually, a preliminary setup was proposed to allow conducting such studies while the various elements of the beam line were being designed and fabricated (Phase I). While it was not realized, the beam line presented on the sketch is merely a more complete version of the one that was planned for these space-charge compensation studies. Also, the lattice has not been changed since the inception of the 3-solenoid design. In particular, the location of the solenoids is fixed, which in turns set stringent spatial restrictions for the design of the other beam line components (e.g.: chopper). Finally, theoretically, fully neutralized beam transport solutions exist with the beam line described here.

 

 

Beam transport

An important constraint for the design of the optical lattice is the ability to efficiently extinguish the beam, whether for pulsed mode operation during commissioning or when the LEBT chopper is used for machine protection purposes during normal operation. Early assumptions for the design of the LEBT kicker and absorber, in particular the maximum deflecting voltage that could be applied, lead to envelope solutions requiring the beam to be very small at the location of the absorber (see previous optics solutions). While this is not necessarily an issue for neutralized transport, it results in an unacceptable emittance growth for the space-charge dominated transport approach. This was verified with PIC-like codes (e.g.: TraceWIN).

The present conceptual design of the chopper is such that the beam size in the chopper vicinity can be larger than previously sought for. Mainly, it is achieved by allowing the maximum deflecting voltage to be greater than what was initially anticipated. Because a larger kick can be applied, the beam footprint can also be increased while still being completely diverted onto the absorber. For the optics solution below, the minimum deflecting voltage needs to be 4.3 kV (i.e. the voltage at which 100% of the particles is lost to the absorber). The kicker driver currently anticipated will be capable of delivering up to 5 kV.

 

Optics design solution (as of 10/03/13)

The plot below shows the 2.5s beam envelope in the LEBT along with the rms normalized emittance. These calculations are performed using a MathCAD spreadsheet written by Valeri Lebedev, a.k.a. VACO. While the code is not quite a ‘real’ PIC code, it calculates the evolution of macro-particles along the LEBT and is capable of applying space charge kicks.

 

 

In this calculation, the beam current is 10 mA and the magnetic field is 3.25, 4.62 and 4.52 kG, for each solenoid respectively. While the emittance growth is not negligible, ~75%, the final emittance is within the LEBT specifications (< 0.25 mm mrad, rms, normalized). Note that for a 5 mA beam, the space charge effect is less and the emittance growth is ‘only’ 45%.

 

 

Fabrication/Installation Status

Installation status report presentations can be found here.

 

Below are some highlights (as of 10/03/13)

-   In spring 2013, the ion source was brought from LBNL to the CMTF building at Fermilab. The ion source has been under vacuum since.

-   The part of the enclosure where the ion source will be located has been completed. Installation of the CMTF building infrastructure (e.g.: power, water) is underway.

-   A stand for the ion source and first solenoid has been built and assembled at CMTF. The ion source assembly (i.e. ion source proper and its vacuum chamber) has been mounted on it.

-   HV rack installation (for ion source) is being reviewed.

-   A Faraday cup was designed and built.

-   One solenoid has been procured.

-   One isolated diaphragm prototype is being fabricated.

-   One Allison emittance scanner probe is being built at Oakridge.

 

 

 

 

Elements specifications & Designs                            
documentation                                           

Solenoids                                                                          

Switching magnet (dipole)

Chopper

Beam scraper (entrance of RFQ)

DCCT/Toroid

Emittance scanner (Allison type)

Isolated diaphragms (within solenoids)

LEBT/RFQ Interface

Faraday Cup (for commissioning)

 

 

 

 

 

‘Integrated’ accelerator systems papers                           Links to topical papers & presentations

 

Facilities                                                                                 Vacuum (Systems, Outgassing…)

Chopping/Chopper                                                                   Sputtering at low energy

LEBT issues                                                                            Ion beam induced desorption/secondary-emission

                                                                                                Neutralization/H minus ‘destruction’

                                                                                                Blistering at low energy

                                                                                                Emittance growth

 

 

 

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