# Presentation: 089038

## X-Ray Free Electron Lasers

- J.S. Wurtele
- UCB and LBNL

- Davidson Symposium
- PPPL
- June 12, 2007

- X-Ray
- FELs

**Notes:**
Thank you for the opportunity to speak here at the postech meeting.. I enjoy the opportunity to visit Korea, and interact with my colleagues in KERI on intense lasers and to find out about new developments in your la boratory. 

Today's talk will present an overview of research on advanced accelerator concepts, starting first with the question: why is it important to pursue work on accelerators, and exploring some of the ideas currently under investigation. While part of this talk will be on neutrino factories and muon colliders, which are really tools used by particle physicists (so far, at least), the other ideas are quite general, and are likely to find their first applications in fields other than particle physics.

Specially important have been Andy Charman, Ryan Lindberg and Gregg Penn.
Along the way, I hope to show you that the ideas that we have to address in this work are fundamental ones.

## Two Livingston Plots    

- Particle accelerators

- Light Sources

- Panofsky

- EU FEL

**Notes:**
the famous Livingston curve, where the energy of accelerators is plotted on a log scale—for high energy physics and light sources-- as a function of the date of operation. Careful examination will show that high energy physics has fallen off the curve. The next x-ray sources will not be off the curve, but scale and cost lead to concern for the future.

First, these machines are, with the exception of the SLC, storage rings. 

Second, there are fewer Hardron machines than there are e-p machines--Hadron machines were already getting big by 1970, but e-p could still built in many places, as they were not yet large. With LEP, and the LC, this of course, is no longer true.
At each fall off of a technology a new one has come to the rescue. The winning approach for colliders has yet to be found (muons??). For light sources, I believe the scale and cost is low enough and the demand high enough that we will see significant progress over the next couple of years. 

There many ways that advanced accelerators physics are contributing to other fields.

## Evolution of synchrotron radiation sources

**Goals:**
**High average flux**
**High peak power **
**Temporal coherence**
**Spatial coherence**
**Attosecond pulses**
**Synchronization **
**Flexibility**
**Implications (current technology): Large machines, GeV Energies**
**Critical Physics**
**Optical  manipulation of phase space**
**High brightness beam generation and preservation**
**Wiggler technology**

- _Evolution of synchrotron radiation sources_

**Notes:**
It was realized by the designers of early accelerators that synchrotron radiation will put a fundamental limit on the
Energy obtainable in circular machines. The basic facts—that a clump of particles radiates as Nxsingle particle so long as the
Clump, or bunch, as is commonly used, is long, was known many in the fourties. It was not until the 60s, however, that people realized that this radiation could be a fundamental tool for exploring matter. This began a 50 year tradition of condensed matter and more recently bio-physicists, benefiting from the hand-me-downs of HEP.

## X-ray sources expand

| Current | ~3.5kA |
| --- | --- |
| Energy | ~13.6GeV |
|  Repetition rate | ~120Hz
 |
| Peak X-Ray Power | ~8GW |
|  |  |

- _EU XFEL [DESY]_

**Notes:**

Finally, these machines are colliders. That is, two counter moving beams are collided together. In proton machines, such as the tevatron, beams circulate for more orbits than the earth has made around the sun. The physics questions that arose in the design of these machines motivated, and the scientists involved contributed to, our understanding of the long-time stability of dynamical systems.

As you certainly have felt here in Texas, the large accelerators that have pushed high energy physics 
forward for close to a century are now approaching a scale that is beyond what society may be willing to support. 

At the same time, light sources, which originated as high-energy hand-me downs, are continuing this tradition, with the building of the LCLS X-ray (.17nm) FEL at SLAC, and a similar facility planned in DESY. They too cannot get much bigger! 

Let’s take a took at the next big accelerator that the high-energy community would like to build.

## Vision for a future LBNL light source

**Vision for a future LBNL light source**

**ALS**

## Vision for a future light source facility at LBNL A HIGH REP-RATE, SEEDED, VUV — SOFT X-RAY FEL ARRAY

**Vision for a future light source facility at LBNL**** ****A HIGH REP-RATE, SEEDED, VUV — SOFT X-RAY FEL ARRAY**

**Low-emittance, **
**high rep-rate electron gun**

**Array of configurable FELs**
**Independent control of wavelength, pulse duration, polarization**
**Configured with an optical manipulation technique; seeded, attosecond, ESASE **

**Laser systems, timing & synchronization**

**Beam manipulation and conditioning**

**Beam distribution and individual beamline tuning**

## FEL BASICS

- Spread in this term is harmful!

**Notes:**
charged particles interact with their electromagnetic environment. It is this
interaction that of course allows us to accelerate them---we couple into the field that they naturally
will tend to produce--stimulated emission. But, we must beware--beams
are self-destructive as they interact with their environment.

## What drives X-ray FELs towards large energy electron beams? 

- 1. Coherent emission--bunching at X-ray wavelengths

- 2. Limits on our ability to create and propagate
- high brightness electron beams

- 3. Limits on our ability to build short wavelength wigglers

## We are limited by our inability to 

- z

- z

**Dephasing from transverse motion**

- •  allows relaxed emittance requirement in FEL--but we do not know how to produce required conditioning in a practical system (yet)

- ΔE/E

- We are limited by our inability to
- make high quality beams

## SASE FEL: amplification of fluctuations

- SASE spectrum and temporal shape has spikes--
- poor longitudinal coherence

**Notes:**
Desy A pulsed beam of atoms and clusters was illuminated with ~100-fs-long FEL pulses at 98 nm wavelength focussed into a small spot (20 μm diameter) and a power density of up to 7*1013W/cm2. Xenon atoms and clusters are chosen because they can be ionized even by single 12.7 eV photons (the ionisation potential of Xe atoms is 12.1 eV). The resulting ions are detected with a time-of-flight (TOF) mass spectrometer.

## Seeded FEL ENHANCED CAPABILITIES FOR CONTROL OF X-RAY PULSE

- Electron beam is 1.5 GeV, energy spread 100 keV, 250 A current, 0.25 micron emittance; laser seed is 100 kW at 32 nm; undulator period 1 cm

**SASE**

**25 fs seed**

**500 fs seed**

**Seeded FEL close to**
**transform limit**

***No monochromator***

## Phase space manipulation

- Manipulate beam phase space can have many advantages:
- Enhanced gain
- Seeding radiation pulse for harmonic cascades
- Attosecond pulses
- Synchronization
- Relax beam quality constraints (conditioning)
- Lower energy for given wavelength
- Many of these ideas are realized by laser interactions with the electron beam prior to the radiation generation. Some examples...

## Lasers manipulate longitudinal phase space

**e-beam  ~ 100 fs**

- during interaction in wiggler

- This cartoon is realized by manipulation of beam phase space with short pulse lasers. The idea is to condition and select specific slices of electrons to radiate differently (in direction, frequency,  intensity, *etc.). For synchrotron sources this has already been accomplished: *Zholents & Zoloterev (1996); Schoenlein, et al, 2000; Khan, Part. Acc. Conf. 2005. For FEL see Zholents et al (2003-2007)

## Extends energy reach, lower power

- High Gain Harmonic Generation (HGHG) – seed with a laser pulse and radiate at a harmonic

**L.-H. Yu et al, Science 289 932-934 (2000)**
**L.-H. Yu et al, Phys. Rev. Let. Vol 91, No. 7, (2003)**

**Notes:**
Follow with cascade with comp theory/exp

## Cascaded harmonic generation scheme

**Seed laser pulse**
**T****bunch  ****>> T****MO**
**P****MO  **>>  **P**shot

**FEL modulator**
**L****W**** < L****SAT****Strong bunching**

**3rd - 5th****  ****harmonic **
**radiator**

**3 - 5th harmonic **
**FEL modulator / low gain amplifier**
**L****W**** < L****SAT**

**3rd - 5th****  **
**harmonic **
**radiator**

**Delay bunch in micro-orbit-bump (~50 ********m)**

**Low **** electron pulse**

**Unperturbed electrons**
- **E  ****~ ****E **(0)

**seed laser pulse**

- tail

- head

- radiator

- radiator

- modulator

- modulator

**disrupted region**

## HHG laser seed--an alternative to harmonic cascades

- HHG laser seed--an alternative to harmonic cascades

- Example with seed at 30 nm, radiating in the water window
- First stage amplifies low-power seed with “optical klystron”
- More initial bunching than could be practically achieved with a single modulator
- Output at 3.8 nm (8th harmonic)

**300 MW output at 3.8 nm (8****th**** harmonic) from a **
**25 fs FWHM seed**

**1 GeV beam**
**500 A**
**1.2 micron emittance**
**75 keV energy spread**

- Gullans et al. (2007)

**Modulator**
**30 nm, L=1.8 m**

**Modulator**
**30 nm, L=1.8 m**

**Radiator**
**3.8 nm****, L=12 m**

**100 kW**
******=30 nm**

- Or, X-ray laser seed

## FEL performance is governed by beam brightness:

*Brightness = # electrons/6D-phase space volume*
- This number will NOT get larger---determined by
- gun physics and can grow through various instabilities

## RCD circa ‘89

## Plasma-based Electron Linac

**Notes:**
This is the basic idea that drove plasma accelerators for many years, and has been quite challenging
to deliver on. Plasmas are not  not metal. Look at the top structure, carefully machined, polished,
vacuum baked to reduce outgassing, and, if cared for, will provide years of good service.

Now what is this plasma? It not so easy to control---no fusion! It is a hot ionized gas, and one
cannot form it like a piece of metal. Or can one? John Dawson, who originated and pushed these
ideas along Toshi tajima from UT, understood that by doing things fast, before
the ions move, or by controlling their motion, 
that some structures in plasmas can be built. BUT they will not lsat. Ions will move at micons/ns,, so dont waste anytime.

But this story has a good turn--while the cartoon above has not been
realized, recent experiments by group at LBL (Leemans group), Malka in France and 
a group at RAL have recently seen well formed 100MeV beams of a feww hundred pico coulombs 
emerging from their experiments. I tell you about this. First how do we make ssructures

## Performance comparison PARAMETER SPACE COMPLEMENTS OTHER FACILITIES

**Performance comparison**** ****PARAMETER SPACE COMPLEMENTS OTHER FACILITIES**

## Many people within LBNL contribute to new light source

## Extras

## Potential for attosecond x-ray production 

**spectral broadening and pulse compression**

**e-beam**

**harmonic-cascade FEL**

**one period wiggler tuned for FEL interaction at 800 nm**

**2 nm light from FEL**

**2 nm modulator**

**chicane-buncher**

**1 nm radiator**

- dump

- end
- station

**1 nm coherent radiation**

- e-beam

- end
- station

**time delay chicane**

**Potential for attosecond x-ray production**** **

**e-beam**

- Zholentz and Fawley PRL 2004

## X-rays from plasma sources

- Already demonstrated—beams make x-rays
- More elaborate ideas based on ion channels [Whittum; E157SLAC]

- Many groups worldwide are working on this
- Plasma yield naturally short pulses, but hard to reach
- FEL intensities with spontaneous emission [N vs N^2]

## The Advanced Photoinjector Experiment – APEX*) 

- *) J. Staples, F. Sannibale, S. Virostek, CBP Tech Note-366, October 2006

- Frequency		65 MHz
- Field			12-25 MV/m
- RF power at 20MV/m	70 kW
- Peak wall power density	8 W/cm2
- Vacuum			10-11 Torr
- 200 MHz is also under consideration

- High repetition rate RF photocathode gun